Examination of the 1MLCT and 3MLCT Excited States of Ditungsten Quadruply Bonded
Paddlewheel Complexes
Undergraduate Research Thesis
Presented in Partial Fulfillment of the Requirements for Graduation “with Honors Research
Distinction in Chemistry” in the Undergraduate Colleges of The Ohio State University
By
Sean McDermott
The Ohio State University
May 2016
Project Advisors: Professor Malcolm H. Chisholm, Department of Chemistry
Professor Claudia Turro, Department of Chemistry
Table of Contents
ABSTRACT
4
CHAPTER 1: INTRODUCTION
6
1.1 METAL-METAL QUADRUPLE BONDS
6
1.2 DITUNGSTEN PADDLEWHEEL COMPLEXES
8
1.3 HIGHLY CONJUGATED CARBOXYLATE LIGANDS
9
1.4 METAL TO LIGAND CHARGE TRANSFER STATES
11
1.5 TIME RESOLVED LASER SPECTROSCOPY
13
CHAPTER 2: THE HOMOLEPTIC COMPLEX
15
2.1 INTRODUCTION
15
2.2 SYNTHESIS
15
2.3 CHARACTERIZATION
16
2.4 GROUND STATE RESULTS
20
2.5 EXCITED STATE RESULTS
25
CHAPTER 3: THE BIS-BIS COMPLEX
28
3.1 INTRODUCTION
28
3.2 SYNTHESIS
29
3.3 CHARACTERIZATION
29
3.4 GROUND STATE RESULTS
33
3.5 EXCITED STATE RESULTS
36
2
CHAPTER 4: DISCUSSION
38
CHAPTER 5: FUTURE DIRECTIONS
43
INDEX OF FIGURES AND TABLES
46
ACKNOWLEDGEMENTS
47
REFERENCES
48
3
Abstract
Around the world today, the utilization of fossil fuels to provide humankind with the
requisite magnitude of energy to maintain a technologically advanced society is becoming
antiquated. Renewable processes that do not contribute to the alteration of the climate can help to
ensure that Earth will remain recognizable for generations to come. Photons from the sun
represent an excellent starting point due to the endless, free supply available in most places of
the world. An obstacle that inhibits the transition to greater solar energy use is the need for more
reliable, efficient, and inexpensive photon harvesting systems to be discovered and implemented.
Charge transfer complexes are of particular interest due to their high absorption intensities
throughout the visible region and the efficiency that these complexes can provide when
converting photon energy to usable electricity. Such complexes, when comprised of a metal
center surrounded by organic ligands (metal-organic complexes), exhibit a metal to ligand charge
transfer (MLCT) transition upon excitation by absorption of a photon. This process induces a
spatial transfer of electron density towards the periphery of the complex, which allows the
charge to be used in many applications.
Two complexes composed of a quadruply bonded ditungsten center coordinated by four
carboxylate ligands were investigated in order to examine their structure, as well as their two
long-lasting, prevalent excited states. One of the complexes (Complex 1) has four identical 9anthracene carboxylate (AnCOO-) ligands bonded to the bimetallic core. The other complex
(Complex 2) has two AnCOO- ligands coordinated in a trans fashion with respect to each other,
as well as a pair of pivalate ligands in the other coordinating positions. Based on electronic
absorption data, computed molecular orbital electron densities, and previous studies of similar
4
compounds, both of these complexes undergo excitation into an MLCT state upon absorption of
radiation from the visible region of the electromagnetic spectrum.
Complexes 1 and 2 have relatively long-lived 1MLCT excited state with lifetimes of 26
ps and 25 ps, respectively, when compared to previously reported coordination compounds.
Additionally, 1 goes on to populate a 3MLCT excited state from the 1MLCT state, by means of
intersystem crossing, with lifetimes between 3 and 10 ns. In contrast, it appears as though the
triplet excited state for 2 may be a metal-centered 3δδ* state. The lack of functional group that
strongly, and uniquely, absorbs infrared radiation makes it difficult to make a concrete
conclusion on the localization of charge between the ligands in these excited states.
Finally, the crystal structures of both complexes were obtained. In order to gain a better
understanding of the electronic conjugation between the metal center and the ligands while in the
solid state, the torsion of the dihedral angle between the carboxylate group and anthracene rings
was determined from the crystal structures. Ligands that were trans to each other were twisted
out of conjugation to the same extent. For 1, this dihedral angle is 40.92º (57.1% relative
conjugation) and 81.49º (2.2% relative conjugation) for the two pairs of trans ligands. In
Complex 2, the dihedral angle of the anthracenecarboxylate ligands was determined to be 52.73º,
or 36.7% relative conjugation.
5
Chapter 1: Introduction
1.1 Metal-Metal Quadruple Bonds
In 1964, F. Albert Cotton first discovered the possibility of quadruple bond formation
between two metal atoms as his team studied various rhenium compounds. A group of Russian
chemists had previously reported that two ReCl42- ions would form the dimeric ion [Re2Cl8]4complex with a Re-Re molecular bond.1 Cotton’s team performed a new synthesis of this ion
discovering that the compound was in fact a dianion, as opposed to the tetraanion the Russian
team had published.2 Showing that the rhenium atoms were in fact trivalent, he explained how
having eight total d-orbital electrons allowed for the formation of a quadruple bond. The crystal
structure of this compound (Figure 1.1) also showed that the chlorine atoms were eclipsed and
not staggered, even though a staggered conformation would provide less steric hindrance.
Figure 1.1 The first reported [Re2Cl8]2- structure.3 Note the eclipsed chlorine atoms.
Cotton deduced that the dz2 orbital on each rhenium atom overlapped to form a σ bond,
the dxz and dyz orbitals on each rhenium atom overlapped to form two π bonds, and finally the dxy
6
orbital on each rhenium atom overlapped to form a δ bond.4 These four bonds complete the
quadruple bond between the two rhenium atoms (Figure 1.2). Remaining are the s, px, py, and
dx2-y2 orbitals, which form a set of hybrid molecular
orbitals leading to the Re-Cl bonds. The large
involvement of the dx2-y2 orbital on each rhenium atom to
bond with the chlorine locks the chlorine atoms on the xaxis and y-axis of the molecule, forcing an eclipsed
conformation (reference the Cartesian plane made
available in Figure 1.1). Rotating these chlorine atoms to
a staggered conformation inherently rotates the dxy orbital
on the rhenium atom leading to a reduction of δ bond
character (Figure 1.3). This does not mean that all such
compounds resemble the perfectly eclipsed model, but
instead that thermodynamic preference is given more
Figure 1.2 A qualitative look at the dorbital overlaps forming molecular
bonding and antibonding orbitals in a
metal-metal bond
heavily to this model in order to retain the energetic
stabilization provided by the δ bond.5
Figure 1.3 The fully eclipsed model (a) vs.
the fully staggered model (b) of [M2Cl8]n-.
There is no δ bond character in (b) making
conformations closer to that of (a) more
energetically favored.
7
The presence of four bonding molecular orbitals between the two metal centers in Figure
1.2 implies that if there are eight total d electrons present in the dimetal center of the complex, all
eight will go into bonding orbitals leading to a metal-metal bond order of four. This creates the
quadruple bond, represented with its eight electrons in an electron configuration σ 2 π 4 δ 2 , which
is at the heart of this study and much of the work done in the Chisholm group. Additionally,
when chelating ligands instead of chlorine atoms are bonded to the metal center forming a bridge
between each metal atom, such as the carboxylate ligands in complexes 1 and 2, the bonds must
be eclipsed. The fact that this formation is the most favorable to maintain the quadruple bond
makes the synthesis of these complexes possible.
1.2 Ditungsten Paddlewheel Complexes
Quadruply bonded dimetal complexes with four ligands coordinated such that each forms
a ‘bridge’ between the metal atoms are considered to have a paddlewheel structure (Figure 1.4).
This nomenclature stems from the structure of these complexes being ostensibly analogous to
that of the paddles “characteristically found at the rear of steamboats”.2 This project has focused
on two different types of carboxylate paddlewheel complexes, both with tungsten forming the
metallic core. The carboxylate paddlewheel complexes that have the formula M2(O2CR)4, where
all four bridging ligands are identical in structure are referred to as homoleptic complexes. On
the other hand, if the complex has the formula M2(O2CR1)2(O2CR2)2, it is referred to as a bis-bis,
heteroleptic complex where R1 and R2 represent two different functional groups. Bis-bis
complexes have two pairs of carboxylate ligands that may be arranged either in a cis or trans
disposition with respect to each other, leading to a range of electronic properties. Comparing the
8
homoleptic complex to the bis-bis complex will help to shed light on the electronic properties
present in these complexes upon photoexcitation.
It is crucial to understand these properties in order to properly use similar molecules in
real world applications. Knowledge of the localization of excited electron density, the time frame
at which these excited states last, and of the ability of these molecules to transfer that electron
density to another molecule all impact what sort of application may be viable in electronics and
photon harvesting.
Figure 1.4 The paddlewheel molecular structure (pseudo D4h symmetry). M = W in this study
while R represents aryl organic groups.6
1.3 Highly Conjugated Carboxylate Ligands
When a hydrocarbon has a conjugated π-system containing n atoms, there are n
overlapping atomic orbitals and therefore n molecular orbitals.7 Hückel Theory, proposed by
Erich Hückel in 1930 and derived from the secular determinant, shows that all n of these
molecular orbitals are confined within a fixed energy range. Following Hückel theory, the
greater number of molecular orbitals involved in the π electron system results in a smaller gap
between the energy of the highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) (Figure 1.5). The electronic implications of this property
are that higher conjugation in organic molecules requires lower energy electromagnetic radiation
to promote an electron from the HOMO into the LUMO. The polynuclear aromatic compound
9
anthracene, with a carboxylate group at the ninth position, 9-anthracene carboxylic acid
(AnCOOH), was chosen as the chelating ligand for the present work because it acts as an
electron acceptor and has a π* LUMO that is lower in energy than the δ* orbital of the dimetal
center.
Figure 1.5 A highly conjugated π-electron system decreases the energy of the LUMO
significantly.
Since anthracene has a LUMO lower in energy than the δ* orbital of the dimetal center
and anthracene is a good electron acceptor, it is proposed that photoexcitation of the complex
would cause a high-energy electron in the complex to move into an antibonding orbital centered
on the ligands. This idea is contingent on the HOMO for the complex being centered on the
quadruple bond, as well as the presence of an electronic route from the center out to the
periphery. Using a carboxylate ligand to induce coordination provides this route through the π
orbitals present on the oxygen atoms leading to the hydrocarbon centered orbitals. Another
requirement for maintaining strong conjugation is that the bond between carbon 9 and carbon 11
(Figure 1.6) cannot be twisted to the extent at which conjugation is broken between the ligands
and the metal center. As the bond twists, the conjugation decreases with the square of the cosine
of the angle between the π-bonding orbitals of carbon 9 and carbon 11.8
10
O
O
11
1
9
8
2
7
3
6
4
5
10
Figure 1.6 Skeletal formula for 9-anthracene carboxylate (AnCOO-) with each carbon labeled.
1.4 Metal To Ligand Charge Transfer States
Generally, electronic excitation of a complex causes an electron to leave the HOMO or
other occupied orbital and populate a higher energy, unoccupied orbital. In general, the lowest
energy possible electronic transition would be from the HOMO to the LUMO. As previously
discussed, if the organic ligand coordinated around the dimetal center is conjugated enough, this
LUMO becomes a π* orbital that is centered on the ligand. In contrast, coordination of a simple
alkyl hydrocarbon would leave the LUMO as a δ* orbital centered on the dimetal center. The
former opens up possibilities of relatively low energy electromagnetic radiation inducing a
spatial movement of an electron from the HOMO. In such a scenario, upon excitation, the
electron would change positions from initially being localized on the metal center to being
localized within orbitals on the ligands. An electronic state of this type is called a metal to ligand
charge transfer state, or MLCT state.
When a ground state complex in the singlet state is excited into an MLCT state, the
multiplicity remains unchanged, such that the spin, S, is zero and the multiplicity (2S+1) is one
in the excited state (1MLCT). From here, the state can relax by adopting the electron
configuration of the ground state, thus re-populating the ground state directly, or may take an
indirect route by undergoing a spin flip into a triplet state, where S = 1 and the multiplicity of the
11
excited state complex is now three (3MLCT). The 1MLCT state is higher in energy than the initial
ground state and the energy of the complex decreases if it crosses over to a 3MLCT. The
ultimate electronic state of the complex is always the ground state, which is the lowest in energy
out of the three (Figure 1.7).
Figure 1.7 High-energy electron configurations for (a) singlet ground, (b) excited singlet, and (c)
excited triplet states.
Many of the transition metal complexes that have been extensively investigated for their
possible use in solar energy conversion have 1MLCT states that are extremely short lived, on the
order of femtoseconds. This means that the elucidation of the properties of the 1MLCT state
through direct observation is extremely difficult. Thus, studies of these complexes, such as
Ru(bpy)32+, focus strictly on the longer-lived 3MLCT state, which has a lifetime of ~600
nanoseconds.9 In contrast, complexes with dimetal quadruple bonds have relatively longer lived
1
MLCT states that can last up to around 20 picoseconds (~1000 times longer than Ru(bpy)32+).10
Additionally, the lifetimes of some of the 3MLCT states in these quadruply bonded complexes
have been shown to be forty-fold longer than that of Ru(bpy)3.11 High-energy electrons residing
on the ligand for lengthy periods of time in both the singlet and triplet states make both states
more easily studied and more applicable to the physical world.
12
For instance, if the goal is to transfer the excited electron density from the complex onto
another molecule or into some electronic system, this process becomes more efficient the longer
that excited electron density remains on the outside of the complex, readily available for a
reduction reaction. If the excited state relaxes too quickly back to the ground state, there will not
be enough time to adequately transfer electrons as desired. Having both the 1MLCT and 3MLCT
last for substantial periods of time, a reduction of another molecule can occur from either
pathway. This is where these molecules could be extremely useful in photon harvesting and
electronic conduction systems.
1.5 Time Resolved Laser Spectroscopy
The main way to visualize ultrafast states is through time resolved spectroscopy. In this
type of spectroscopy, the molecule is initially excited with a short laser pulse at a specific
wavelength, “pumping” the molecule into the excited state of interest. Another laser pulse passes
through the solution shortly after to “probe” the molecules in the excited state. This pumping and
probing is repeated over a series of time intervals to investigate how the excited system behaves
during ultrafast times (Figure 1.8). Two types of time resolved laser spectroscopy will be utilized
in this study, transient absorption (TA) spectroscopy and time resolved infrared (TRIR)
spectroscopy. The TA instrument is the ultrafast spectroscopy analog to UV-Vis ground state
spectroscopy while the TRIR instrument is the ultrafast spectroscopy analog to IR ground state
spectroscopy. The data obtained from these instruments are graphed as the difference in
absorption in the excited state from absorption in the ground state. Changes in absorption found
using these techniques help to decipher lifetimes of the excited states of molecules and can guide
in answering the question about localization in excited state complexes.
13
Figure 1.8 Depiction of three different pumps followed by a probing of the system over three
different time intervals.
14
Chapter 2: The Homoleptic Complex
2.1 Introduction
The synthesis of a ditungsten homoleptic complex with four carboxylate ligands
surrounding the core initially is significantly more difficult than the corresponding preparation
using a molybdenum center. Dimolybdenum carboxylate complexes, for example, generally can
be synthesized with two equivalents of molybdenum carbonyl and four equivalents of the desired
ligand as a carboxylic acid simply through a high temperature reflux.13 The first success in the
synthesis of a homoleptic carboxylate compound with a W24+ core occurred at -20°C and was a
multistep synthesis involving W2Cl6(THF)4, sodium amalgam, and sodium trifluoroacetate.14 The
product, W2(O2CCF3)4, laid the foundation for synthesis of other tetracarboxylates, such as
ditungsten tetraacetate. While this additional step adds to the difficulty of synthesis, the real
challenge arises from the ease at which compounds of the form W2(O2CR)4 can be oxidized by
air. The amount of oxygen in the ambient environment serves to decompose these complexes in a
matter of seconds. The following synthesis of this compound (R = 9-anthracene), while
seemingly trivial, was significantly altered until a method was found that repeatedly produced a
product with high purity and good yield.
2.2 Synthesis
W2(Piv)4 (Piv – pivalate) and
approximately fifteen equivalents of 9-anthracene
carboxylic acid (AnCOOH) were mixed in toluene, which resulted in a purple mixture. Under a
nitrogen atmosphere, the solution was stirred for three days at room temperature. The toluene
supernatant was decanted after centrifugation and the resulting solid was washed three times in
toluene and twice in methanol. The deep purple solid was dried under vacuum on a Schlenk line
15
producing an ~30% yield of W2(AnCOO)4, complex 1. Great care had to be taken in order to
prevent both air and water from contacting the product or reaction mixture, which quickly results
in the decomposition of the W24+ quadruple bond, and the production of a light brown mixture of
decomposition products.
This reaction occurs through the exchange of pivalate anions from the ditungsten core
and subsequent bonding of a 9-anthracenecarboxylate anion to the quadruply bonded center,
reforming the bridge between the two tungsten atoms, and releasing pivalic acid. Fifteen
equivalents of 9-anthracene carboxylic acid are used in the reactants, as opposed to four
equivalents, in order to push the reaction to completion towards the products because the
reactants and products are in equilibrium. The methanol washes are crucial to remove all of the
excess 9-anthracenecarboxylic acid molecules due to its insolubility in toluene.
2.3 Characterization
Nuclear Magnetic Resonance (NMR)
The 1H NMR of complex 1 dissolved in d6-DMSO was taken on a Bruker DPX 400 MHz
instrument (Figure 2.1). All proton chemical shifts were referenced to the protio impurity in
DMSO at δ = 2.507. Solvent chemical shifts were observed at (δ, ppm): 2.507 (s, CH3 of
DMSO), 3.310 (s, H2O). Complex 1 exhibits resonances due to the aromatic protons of the
AnCOO- ligands with chemical shifts at (δ, ppm): 7.102 (t, 8H), 7.517 (t, 8H), 8.210 (d, 8H),
8.563 (d, 8H), 8.791 (s, 4H).
16
Figure 2.1 The complete 1H NMR spectrum for complex 1.
Matrix-Assisted Laser Desorption Ionization Time-Of-Flight (MALDI-TOF)
The MALDI-TOF mass spectrum of 1 was obtained using a Bruker Microflex mass
spectrometer with dithranol used as a matrix, in positive mode (Figure 2.2). The calculated most
abundant isotopic molecular weight for W2O8C60H36 (M+) was 1252.14 g/mol and the observed
molecular weight was at m/z = 1252.155, consistent with the expected mass of the +1 ion
generated within the mass spectrometer. The calculated isotopic distribution is overlaid in the
figure, showing the indubitable presence of this molecule in solution.
17
Figure 2.2 The MALDI-TOF spectrum for complex 1, ranging from (M-4)+ to (M+5)+. The
actual peaks are in blue, the calculated peaks are overlaid in red.
Single Crystal X-ray Diffractometry
Several milligrams of complex 1 were dissolved into a minimal amount of THF in a
small vial and the cap was loosely screwed on. This vial was then placed into a larger vial that
was approximately a third filled with hexanes. The cap for this larger vial was tightly screwed
on. This vial duplex was left isolated for one week to allow diffusion of hexanes into the small
vial, diffusion of THF out of the small vial, and ultimately a decrease in solubility of the solute in
the small vial occurred. Small purple crystals formed on the walls of the vial. Fluorinated oil was
layered over the crystals to allow transportation to a Nonius Kappa CCD diffractometer with Mo
Kα x-ray radiation. The structure was solved from the resulting data with SHELXT-14 (Figure
2.3).15 The crystallization incorporated several highly disordered THF molecules within the unit
cell but these were removed from the crystal structure for clarity.
18
Figure 2.3 Molecular structure of complex 1 drawn at 50% probability. Solvent and hydrogen
atoms are removed for clarity. Key: black = tungsten; red = oxygen; gray = carbon.
Bond
Distance (Å)
W-W
2.2029(3)
W-O(1)
2.078(2)
W-O(2)
2.102(2)
W-O(3)
2.075(2)
W-O(4)
2.076(2)
Table 2.1 Selected bond distances between a central tungsten atom and its neighboring atoms.
Chemical Formula
Formula Weight
Temperature (K)
Space Group
a (Å)
b (Å)
c (Å)
C68H52O10W2
1396.79
150(2)
Triclinic, P-1
9.7403(2)
11.5682(1)
16.2666(3)
o
98.749(1)
α()
o
β()
91.342(1)
o
102.843(1)
γ ()
3
1763.12(5)
V (Å )
3
1.316
Dcalculated (Mg/m )
Crystal Size (mm)
0.23 X 0.15 X 0.12
-1
3.309
µ (mm ) [Mo, Kα]
2
Goodness-of-fit on F
1.179
Table 2.2 Selected data collection parameters for the crystal structure of Complex 1.
19
There are two dihedral angles at which the ligands align themselves around the
ditungsten center. A 0º dihedral angle would lead to 100% conjugation between trans ligands
while a 90º dihedral angle would lead to 0% conjugation between the ligands and the metal
center. One of the pairs of trans ligands is twisted with respect to the carboxylate group at a
40.92º angle. This corresponds to a π-system conjugation with a magnitude of 57.1%. The other
pair of trans ligands is twisted at an 81.49º angle, corresponding to a π-system conjugation with a
magnitude of 2.2%.
These values are truly only indicative of the molecule when in solid-state, but show that
in this state there only exists extended conjugation across one pair of ligands trans with respect to
each other. When comparing this to the solution phase testing done with 1H NMR, it is not
possible to discern these two unique ligand torsions. This does not mean that the ligands are all
aligned in a homogenous fashion, but maybe rather that they interconvert at a frequency that is
not registered on the NMR timescale at room temperature.
2.4 Ground State Results
Electronic Absorption
The electronic absorption spectrum of Complex 1 was collected on a Perkin Elmer
Lambda 900. Absorption of electromagnetic radiation in this energy range generally results in
electronic excitation within the complexes being sampled. For the bimetallic complexes studied
in the Chisholm group, there are typically two types of electronic excitations observable through
UV-Vis spectroscopy. First, there is a π-π* electronic transition of electrons centered on the
ligands. This transition usually occurs upon absorption of ultraviolet radiation, but with highly
conjugated π electrons the absorption can breach into the visible region of the electromagnetic
20
spectrum. Complex 1 absorbs strongly, due to these π-π* transitions at wavelengths below 400
nm.
Figure 2.4 Electronic absorption spectrum of 1 dissolved in THF with 1MLCT maximum at
λMLCT = 637nm
The second form of electronic absorption is the MLCT, or a Mδ-Lπ* transition, where
the electron is leaving a metal based molecular orbital and entering an antibonding orbital
centered on the ligands. These transitions require lower energy radiation to induce excitation
than the ligand-centered π-π* counterpart. For visualization of why this transition is lower
energy, see the molecular orbital diagram for Complex 1 in the following section. Complex 1
exhibits a broad MLCT absorption peak that covers nearly the entire visible region of the
electromagnetic spectrum, with a maximum absorption value at 637 nm. This value was utilized
when exciting the complex in order to do subsequent excited state studies.
21
MolecularOrbital(MO)Diagram
The electronic structure calculations where performed using Density Functional
Theory with a 6-31G* basis set. The molecular orbitals nearest in energy to the HOMOLUMOgapwereusedtobuildafrontiermolecularorbitaldiagram.Relativeenergylevels
for each molecular orbital could then be used to construct a theoretical picture of the
electroniceventsthatmaybeoccurringduringexcitationofthecomplex(Figure2.5).The
optimizedgeometryshowsthathavingallfourligandstwistedpartiallyoutofconjugation
withthemetalcenter(toasimilarextent)isthepredictedminimumenergystructure.
By looking at the electronic structure of the HOMO, it is clear that almost the
entirety of the electron density is localized around orbitals on the ditungsten center and
the lateral oxygen atoms. Earlier, it was discussed how an organic ligand with a highly
conjugated π electron system lowers the ligand π* orbitals below that of the dimetal δ*
orbitals,asisevidentbytheelectronicstructuresoftheLUMOandLUMO+1inFigure2.5,
whicharedegenerate.Theelectrondensityinbothofthesemolecularorbitalsislocalized
onligandsthatarepositionedtranstoeachother.Thereforeintheory,anMLCTexcitation
processwouldbethelowestenergyelectronicexcitationandthiswasshownwithUV-Vis
spectroscopy.
22
Figure 2.5 Frontier MO diagram for complex 1. Selected orbitals shown are close in energy to
the HOMO and LUMO. The optimized geometry has all four ligands partially twisted out of
conjugation to a similar extent.
23
ComputationalInfraredSpectroscopy
Ground state frequency analysis was also performed using Density Functional
Theorytoestimatetheenergiesofradiationthatwouldbeabsorbedtoinduceavibrational
excitation.Thisanalysiswasnotcompletedexperimentallyduetothelowsignaltonoise
ratio of the instrument during testing and the ambiguous results. One way to
experimentallydeterminesomeofthegroundstateIRabsorptionfeaturesisbylookingat
theTRIRspectruminChapter2.5andobservinganygroundstatebleaches.Anissuethat
arises in this scenario however is due to the obvious presence of only one ground state
bleachfeatureat1460cm-1.
Regardless, a list of important features in the infrared spectrum can be compiled
using GaussView (Table 2.3). When an electron transfers to a ligand based molecular
orbital, it enters a π* orbital. This lowers the bond order and thus lowers the vibrational
energy of the anthracene C-C stretches. The carboxylate vibrational bands, in contrast,
move to higher energy because there is less electron density on each metal atom and
thereforelessπ-backbondingoccurs.Thisbackbondingtypicallyplaceselectrondensityin
the antibonding orbitals of the oxygen so less of it would effectively increase the
carboxylate bond order. In an MLCT state, the anthracene ring vibrational bands should
lowerinenergywhilethecarboxylatevibrationalbandsshouldmovetohigherenergy.
Vibration Description
Wavenumber
CO2 asymmetric
1394.72 cm-1
Anthracene ring stretch
CO2 symmetric
1355.43 cm-1, 1286.81 cm-1, 1250.64
Anthracene ring stretch
cm-1
Table 2.3 Calculated ground state vibrational frequencies and assignments of intense IR-allowed
normal modes of 1.
24
Figure 2.6 Theoretical infrared transmittance spectrum for complex 1.
2.5 Excited State Results
TransientAbsorption(TA)Spectroscopy
Complex 1, dissolved in THF, was studied using femtosecond transient absorption
spectroscopywithanexcitationwavelengthof700nm(fwhm~300fs).Thiswavelengthis
on the low energy side of the MLCT transition, and the resulting TA spectra collected at
variousdelaytimesbetweenthepumpandtheprobeareshowninFigure2.7.Thespectra
exhibitaclearbleachat~400nmcorrespondingtothedepletionofthegroundstate,which
absorbsstronglyinthisregion.Positiveabsorptionfeaturesareevidentinthe425-550nm
range assigned to the anthracene radical anion.22 The presence of the anthracene anion
absorptionisconsistentwiththeMLCTassignmentofthisexcitedstate.
Akinetictracewascollectedat440nm,whichwasfittedtoabiexponentialdecay
andresultinginthelifetimeofthe 1MLCTstateof26±2psandanothercomponentwithτ
25
>1ns.Afterapproximately3ns,thelongesttimeframethefemtosecondTAcanmeasure,
therewerestillfeaturesindicatingthepresenceofexcitedelectrondensityontheligands.
Thisisagoodindicationoftheexistenceofa3MLCTstateduetointersystemcrossingfrom
the 1MLCT excited state. Utilizing nanosecond transient absorption, which records the
earliestmeasurement~20nsafterexcitation,therewasnoremainingsignofthis 3MLCT
feature. Combining the data from the fsTA and the nsTA, it can be concluded that the
lifetimeofthe3MLCTissomewherebetween3nsand10ns.Giventheshortlifetimeofthe
1MLCT state, it may be hypothesized that the spectrum collected at early times, 1-3 ps,
corresponds to the 1MLCT state, whereas that at decay times longer than ~100 ps
correspond to the 3MLCT state, which does not decay significantly within the ultrafast
experiment.
Figure 2.7 fsTA for complex 1 in THF. λexcitation = 700 nm. Kinetic trace is in the inset and is
measured at 440 nm.
26
TimeResolvedInfrared(TRIR)Spectroscopy
ThefemtosecondTRIRspectrumof1wasalsocollectedinTHFwithlaserexcitation
600nm(fwhm~300fs).Thisexcitationlaserisatadifferentenergythantheoneusedin
thefsTAexperimentduetothetediousnatureofswitchinglasersaswellasthefactthat
this higher energy laser will not interfere with the subsequent infrared spectroscopy,
however,bothexcitationwavelengthsareinthelow-energysideoftheMLCTabsorptionof
the complex. The results from this instrument are shown in Figure 2.8 and agree closely
withthelifetimefromthefsTA.ThefsTRIRkinetictrace,takenat1449cm-1,canbefittedto
amonoexponentialdecayandresultsinan 1MLCTlifetimeof28.3±0.6ps,consistentwith
thevalueof26±2psfromthefsTAexperiment.Itcanalsobeenseenfromthisdatathat
the 3MLCT lifetime is likely longer than 2.5 ns, since excited state features remain even
afterthelongertimeframes.DatafromtheTRIRisintegraltocomingtoaconclusionofthe
extent of electron localization, and future experiments on similar systems will use this
instrumentmoreintensively(seeChapter5).
Figure 2.8 fsTRIR for complex 1 in THF. λexcitation = 600 nm. Kinetic trace is in the inset and is
taken at 1449 cm-1.
27
Chapter 3: The Bis-Bis Complex
3.1 Introduction
In order to understand what is happening in the excited state of the homoleptic complex,
1, it is imperative to synthesize another complex as a reference. Complex 1 has four equivalent
electron-withdrawing ligands that can act as redox centers when the complex is excited into an
MLCT excited state. The replacement of two of these ligands with moieties that do not have a
conjugated π-electron system removes the possibility of a low energy MLCT transition to these
ligands. These new ligands have high-energy π* molecular orbitals. Therefore, electromagnetic
radiation in the visible region is no of longer sufficient energy for excitation to an MLCT state
where the electrons are localized on these ligands. This new complex, termed a bis-bis complex,
only has two electron-withdrawing ligands as opposed to the four coordinated to the homoleptic
counterpart.
Comparison of the homoleptic complex MLCT excited state to the analogous state in the
bis-bis complex helps to determine the localization of electron density in the excited among the
ligands. If the electron is delocalized to all four electron-withdrawing ligands in the homoleptic,
but only two in the bis-bis, then there should be a significant difference in the excited state
infrared absorption spectra of the complexes (see Chapter 4). On the other hand, if the excited
electron is either completely localized or delocalized over two of the ligands, then there will
likely be no difference in the excited state infrared radiation absorption spectra of the two
complexes.
28
3.2 Synthesis
The synthesis of 2 was carried out completely in a glove box in the absence of air and
water. A suspension of 9-anthracene carboxylic acid (AnCOOH) in toluene, due to lack of
solubility, was prepared in a test tube and added drop-wise to a solution of W2(Piv)4 in toluene at
a rate of approximately 1 mL/min while continuously stirring the resulting mixture. The yellow
W2(Piv)4 solution initially turned green after only a couple drops of the AnCOOH suspension
was added, but further addition of ~1 mL of the suspension changed the color of the solution to
blue. After allowing the reaction to proceed overnight, the solution turned a deep red. This red
mixture was left at room temperature and under a nitrogen atmosphere for three days. The
toluene supernatant was decanted after centrifugation and the resulting red solid was washed
three times with toluene and three times with hexanes. During each toluene wash, the supernatant
remained purple indicating some solubility of the product in toluene. The red solid was dried
under vacuum on a Schlenk line producing a ~20% yield of trans-W2(AnCOO)2(Piv)2 (2).
It should be noted that this reaction was attempted through bulk addition of both reactants
but each attempt resulted in an inseparable mixture of homoleptic, tris-mono, bis-bis, and monotris complexes. A steady, drop-wise addition of the insoluble ligand allows the tris-mono
complex to be formed initially, followed by a subsequent formation of the bis-bis complex.
3.3 Characterization
Nuclear Magnetic Resonance (NMR)
The 1H NMR of 2 dissolved in d6-DMSO was collected on a Bruker DPX 400 MHz
instrument (Figure 3.1). All proton chemical shifts were referenced to the protio impurity in
DMSO at δ = 2.507. Solvent chemical shifts were observed at (δ, ppm): 2.507 (s, CH3 of
29
DMSO), 3.310 (s, H2O). Complex 2 exhibits resonances due to the aromatic protons of the
AnCOO- ligands with chemical shifts at (δ, ppm): 7.569 (t, 4H), 7.643 (t, 4H), 8.144 (d, 4H),
8.225 (d, 4H), 8.728 (s, 2H), as well as protons on pivalate ligands at chemical shift (δ, ppm):
1.614 (s, 18H).
Figure 3.1 The complete 1H NMR spectrum for complex 2.
Matrix-Assisted Laser Desorption Ionization Time-Of-Flight (MALDI-TOF)
The MALDI-TOF mass spectrum of 2 was obtained using a Bruker Microflex mass
spectrometer with dithranol used as a matrix, in positive mode (Figure 3.2). The calculated most
abundant isotopic molecular weight for W2O8C40H36 (M+) was 1012.14 g/mol and the observed
molecular weight was at m/z = 1012.1586 consistent with the parent +1 ion. The calculated
isotopic distribution is overlaid in the figure and matches well with the observed pattern.
30
Figure 3.2 The MALDI-TOF spectrum for complex 2, ranging from (M-4)+ to (M+5)+. The
actual peaks are in green, the calculated peaks are overlaid in red.
Single crystal X-ray diffractometry
Single crystals were grown using the same technique as described for 1 and the structure
was determined using X-ray diffractometry in the same manner as for 1. The crystallization of 2
also incorporated several highly disordered THF molecules within the unit cell but these were
removed from the crystal structure for clarity shown in Figure 3.3. The pivalate moieties in the
crystallization where also highly disordered and thus this ligand was modeled in two positions to
impose some restraints on the final structure. One of these positions is shown in Figure 3.3,
where the other position would be have the methyl groups on the pivalate rotated by 60º.
31
Figure 3.3 Molecular structure of complex 2 drawn at 50% probability. Solvent and hydrogen
atoms are removed for clarity. Key: black = tungsten; red = oxygen; gray = carbon.
Bond
Distance (Å)
W-W
2.1969(3)
W-O(1)
2.077(2)
W-O(2)
2.073(2)
W-O(3)
2.099(2)
W-O(4)
2.098(2)
Table 3.1 Selected bond distances between a central tungsten atom and its neighboring atoms.
Chemical Formula
Formula Weight
Temperature (K)
Space Group
a (Å)
b (Å)
c (Å)
C56H68O12W2
1300.80
150(2)
Triclinic, P-1
9.9732(2)
14.5295(3)
19.7932(3)
o
69.669(1)
α()
o
β()
88.549(1)
o
81.0740(1)
γ ()
3
2655.63(9)
V (Å )
3
1.627
Dcalculated (Mg/m )
Crystal Size (mm)
0.27 X 0.08 X 0.08
-1
4.389
µ (mm ) [Mo, Kα]
2
Goodness-of-fit on F
1.019
Table 3.2 Selected data collection parameters for the crystal structure of Complex 2.
32
As discussed in Chapter 2, the dihedral angle at which the anthracene rings are twisted
with respect to the carboxylate oxygen atoms helps dictate the extent of conjugation between the
π orbitals in the ring structures and the π orbitals in the carboxylate. The 9anthracenecarboxylate ligands, which are positioned trans to each other, are twisted at a 52.73º
angle. This corresponds to a 36.7% conjugation through the bridging carboxylate to the fused
rings. Again, this value is from the solid-state of the molecule and the extent of conjugation
could be altered in solution.
3.4 Ground State Results
Electronic Absorption
The electronic absorption of 2 was measured in a similar manner as that described for 1.
There is a strong absorption at wavelengths shorter than 400 nm corresponding to a π-π*
transition in the anthracene ligands. Additionally, there is a broad absorption covering a majority
of the visible portion of the electromagnetic spectrum that exists due to the MLCT transition in
2. The maximum absorbance value for this MLCT band is at a wavelength of λ = 651 nm, a
slightly lower energy value than that of 1. This slight decrease in energy difference between the
HOMO metal-based molecular orbital and the LUMO ligand-based molecular orbital is
supported by comparison of the band gaps in the computed molecular orbital diagrams.
33
Figure 3.4 Electronic absorption spectrum of 2 in THF with maximum at λMLCT = 651nm Molecular Orbital (MO) Diagram
Theelectronicstructurecalculationsfor2wereperformedusingDensityFunctional
Theorywitha6-31G*basisset.Again,theelectrondensityfiguresoftheorbitalsnearestto
the HOMO and LUMO were configured into a frontier molecular orbital diagram (Figure
3.5). In contrast to the frontier molecular orbital diagram of 1 (Figure 2.5), the ligandbased π* molecular orbital is no longer degenerate and the gap between the HOMO and
LUMOhasdecreasedby0.07eV.Amajorityofthisdecreaseisduetoanincreaseinenergy
of the HOMO attributed to the fact that there are fewer electron-withdrawing ligands
surroundingtheW2core,increasingtheelectrondensityatthemetals.SincetheHOMOhas
electron density based on the metal center, this would cause the relative energy of the
HOMOtoincrease.Asdiscussedabove,thisdecreaseisevidentintheelectronicabsorption
spectrumwithlowerMLCTmaximumfor2 ascomparedto1.Itshouldalsobenotedthat
in all of the molecular orbitals selected for this diagram, there is no electron density
34
present on the pivalate ligands. This predicts that any low energy MLCT state that exists
willtransfertheelectrondensityonlytotheanthraceneligands.
Figure 3.5 Frontier MO diagram for complex 2. Selected orbitals shown are close in energy to
the HOMO and LUMO.
35
ComputationalInfraredSpectroscopy
Similarlytocomplex1,thetheoreticalinfraredtransmittancespectrumwas
obtainedusingDensityFunctionalTheory.StrongIR-activeabsorptionsrelatingto
vibrationsintheanthraceneringsarerecordedinTable3.3andthecalculatedspectrumis
displayedinFigure3.6.
Vibration Description
Wavenumber
CO2 asymmetric
1397.12 cm-1, 1429.64 cm-1
Anthracene ring stretch
CO2 symmetric
1357.38 cm-1, 1286.47 cm-1, 1249.81
Anthracene ring stretch
cm-1
Table 3.3 Vibrational assignment and computed energy level of intense vibrational IR
absorptions
Figure 3.6 Theoretical infrared transmittance spectrum for complex 2.
3.5 Excited State Results
A femtosecond transient absorption analysis in the UV-Vis could not be run for 2 in this
study due to the instrument being down at times when the complex was synthesized and pure.
However, TRIR was available, which provided valuable data for assignment of the excited state
of the complex.
36
Time Resolved Infrared (TRIR) Spectroscopy
The femtosecond TRIR for 2 was taken collected in THF and an excitation pump
laserwavelengthof650nm(fwhm~300fs),andtheresultingtracesareshowninFigure
3.7.ThefsTRIRkinetictrace,takenat1546cm-1,wasfittedtoabiexponentialdecaywhich
resultsina3.1±0.9pscomponentassignedtovibrationalcoolinganda 1MLCTlifetimeof
25±2ps.Unlikecomplex1,itisunclearfromthefsTRIRifthetripletstateisinfactMLCT
in nature. The short-lived features on the anthracene rings appear to disappear after the
singletstatedecays.Somefeaturesbetween1400cm-1and1450cm-1remainafteralong
time(>1ns),butthislonepeakisknowntocorrespondstothecarboxylatebondsandhas
beenpreviouslyobservedwhenthetripletstateis3δδ* in nature.
Figure 3.7 fsTRIR for complex 2 in THF. λexcitation = 650 nm. Kinetic trace is in the inset and is
taken at 1546 cm-1.
37
Chapter 4: Discussion
The Chisholm group, in 2005, published a direct observation of the 1MLCT state in the
dimolybdenum analog to 1.16 Through this previous work, it was shown that in Mo2(9anthracenecarboxylate)4, an MLCT excited state is populated upon absorption of visible
radiation. This initial 1MLCT state was determined to have a lifetime of 10 ps. Decay to the
ground state from the 1MLCT state happened at least partially through fluorescence, but there
was an alternate pathway this molecule took to reach the ground state. Intersystem crossing – the
process of the excited electron undergoing a spin flip – gives rise to a 3MLCT state. When
studying the complex with a femtosecond TRIR, it was clear that this state had not relaxed after
the duration of the experiment (50 ps). Utilizing nanosecond TRIR, it was then determined that
the lifetime of the 3MLCT excited state was 76 µs.
The long lifetimes of these MLCT excited states are still of great interest to chemists
studying the electronics of these complexes. Having a complex where the excited electron
density remains on the peripheral ligands for an extended period of time gives ample opportunity
for subsequent reduction reactions, or electron transfers from the ligand to another molecular all
together. The implications of this photoexcited state in the physical world range from photon
harvesting to conductivity.17 Probably the most extensively studied
3
MLCT state in a
coordination complex is that of Ru(bpy)32+. This 3MLCT excited state lasts approximately 600
ns.9
With Ru(bpy)32+ and similar complexes that have easily accessible MLCT states by
means of photon absorption, the utilization of the excited electron in specific applications can
only occur from the triplet state. This is because the 1MLCT state lasts for a very short period of
time (40 ± 15 fs or less).18 With the quadruply bonded dimolybdenum complexes, as
38
demonstrated by the Chisholm group, the singlet and triplet MLCT states last approximately 250
and 100 times longer than Ru(bpy)32+ respectively. The sheer increase in duration of both MLCT
states lends itself to the potential for engineering the previously mentioned systems in a more
efficient manner.
The study that is discussed in this paper examined the similarities and differences
between 1 and its dimolybdenum analog, as well as between 1 and 2 (where the number of
electron withdrawing ligands differ). With the dimolybdenum complex, it was seen that the
1
MLCT excited state had a lifetime of approximately 10 ps, which is slightly shorter than the
approximately 25 ps lifetimes of the 1MLCT states in 1 and 2. Yet, upon comparison of the
3
MLCT states it is clear that the lifetime of this state for Complex 1 is significantly shorter than
not only that of Mo2(9-anthracenecarboxylate)4 but also Ru(bpy)32+. The shorter lifetime in
ditungsten compounds can be attributed to two possible factors. One reason is due to an increase
in spin-orbit coupling afforded by the third-row transition metal, thus increasing the rate constant
for spin flip from the excited triplet state to the singlet ground state.11 The second reason is the
Energy Gap Law, which states that the rate of nonradiative decay is dependent on the energy
difference of the initial and final states.19 Since tungsten molecular orbitals are higher in energy
than molybdenum molecular orbitals, the energy gap between the ground state and the 3MLCT
state is smaller and therefore decay back to the ground state happens relatively more quickly.
In light of this information, it is important to confirm that the triplet state being studied is
in fact MLCT in nature, as opposed to being a 3WWδδ* state. The possibility of 1 being a
3
WWδδ* state can easily be ruled out since these states have previously been reported to
typically last on the order of microseconds.10 In addition, the fsTRIR shows electron density
remaining on the anthracene ligands. This is shown because the features that are present after a
39
short period of time for the 1MLCT, appear to still remain after a couple nanoseconds. In
contrast, the fsTRIR for 2 appears to indicate that the electron density no longer remains on the
anthracene ligands after the 1MLCT state has decayed. This could indicate the presence of a
3
W2δδ* state instead of a 3MLCT state. Between 1400 cm-1 and 1450 cm-1, a feature remains
present after these longer time intervals. This absorption is related to the bond energies of the
carboxylate group, showing that these energies are still impacted in the triplet state. Based on
previous studies, this commonly occurs in a 3WWδδ* state. A δδ* singlet or triplet state would
hold the high-energy excited electron within the core of the molecule, making in much less
accessible for further reactions or applications.
As discussed in Chapter 2, the fsTRIR data for both 1 and 2 do not provide enough
conclusive evidence to describe the excited electron density as being delocalized over all four
ligands or localized to a greater extent (i.e. delocalized over two of the ligands). The answer to
this could be found by introducing a covalently-tethered IR functional group to the complex (see
Chapter 5). Regardless, a comparison of the fsTRIR figures (Figures 2.8 and 3.7) makes evident
a collection of similar features between the two complexes (Table 4.1). Note the proximity in
energy at which these peaks are located at short time intervals and this could indicate that the
electron density is being delocalized over the same number of ligands in each complex, which
would likely be two given the number of anthracene-containing ligands in complex 2.
Complex 1
Complex 2
-1
1273 cm
1271 cm-1
1288 cm-1
1287 cm-1
1312 cm-1
1318 cm-1
-1
1439 cm
1428 cm-1
1446 cm-1
1433 cm-1
Table 4.1 Comparison of energy values for similar peaks from fsTRIR
40
If one of the complexes had electrons more greatly delocalized than another complex, this
means the coupling of the ligands is greater and thus lowers the potential energy well of the
ligands. This would decrease the energy gap between the ground state and the 1MLCT state,
likely shortening the lifetime (as discussed with the Energy Gap Law earlier). This lends itself to
the idea that the electron density is delocalized over the same number of ligands in each
complex, since the 1MLCT lifetimes of 1 and 2 differ by only ~1 ps.
In Chapter 2.4, it was discussed how the vibrational energy of the bonds within the ligand
would shift when the complex is in an MLCT state. One of the downfalls of studying a complex
where the main IR absorption bands are carboxylate and ring based vibrations is that these
absorptions are weak. If the ligands contained an IR reporter, such as a nitrile or an ethynyl
functional group, the shift in vibrational energy is explicit and to a greater extent. The change is
vibrational energy could be as great as approximately 250 wavenumbers.20 Such a substantial
shift makes it possible to determine the extent of excited electron density localization.
Ultimately, it should be noted that both 1 and 2 appear to behave similarly upon
photoexcitation with respect to the transfer of the excited electron onto a coordinated electron
withdrawing ligand. This charge remains on the ligand in the singlet excited state for both
complexes. Looking at the dihedral angles of the complexes while in the solid state, one pair of
trans ligands in the homoleptic complex is twisted so greatly as to maintain only 2.2%
conjugation. Theoretically, such torsion would prevent the two ligands from being strong
electron withdrawing moieties. Electronically, 2 then looks very similar to 1 while in the solid
state.
While the electronics of both 1 and 2 do not have any striking differences, the utilization
of 1 in applications that require excited state electron transfer would likely be more useful than 2
41
because the latter appears to only maintain the excited electron density on the ligands while in
the singlet excited state. This greatly decreases the time this high-energy electron density is
accessible to be used in a charge transfer application. The difference between ~10 ns and 25 ps (a
400 fold decrease in duration) is expected to have a significant impact on what that high-energy
electron is able to do before relaxation back to the ground state. Additionally, the vulnerability of
these complexes to degradation in an atmosphere containing oxygen or water has a detrimental
impact on the potential for physical, real-life applications. Exposure of 1 to air leads to
widespread decomposition after approximately an hour. In comparison, 2 shows decomposition
after just minutes. The four bulky anthracene molecules that surround 1 act to protect the
ditungsten center from oxidation. This not only makes the synthesis and preparation of 1 much
simpler, and makes it more feasible for such a molecule to be put into use in solar cells or other
electronic applications.
42
Chapter 5: Future Directions
Firstly, an investigation of mixed valency should be performed. Such a study will provide
insight to the coupling between ligands that are coordinated to the dimetal center. This coupling
is related to the extent of localization of the electron in the MLCT excited state. A system where
all four ligands are highly coupled can partially lead to the excited electron density being
delocalized over all of the ligands. On the other hand, minimal coupling could cause the electron
to stay localized on a single ligand.
Anion calculations, where the charge on the molecule is -1 and the molecule is a doublet,
were attempted but were never successfully completed. A continued effort on these calculations
will help to predict what happens when a charge is delocalized over all four ligands. The
calculation automatically sets up the anion in this manner and results can be compared to the real
molecule in a MLCT excited state. Large differences between the anion calculations and the
excited state results (i.e. TRIR vibrational modes) would lend evidence that the excited charge is
not completely delocalized in the system.
Furthermore, emission data should be collected, analyzed, and compared for both
complexes. If the complex actually enters a 1MLCT state upon excitation, it would be expected
to see weak fluorescence from this state in the near infrared region of the electromagnetic
spectrum. Then, depending on whether the complex progresses to a 3MLCT state or a 3δδ*, the
phosphorescence emission will differ. A 3W2δδ* state is much longer-lived – on the order of
microseconds – and occurs around 800 nm.21 This emission study would help to clarify what
kind of triplet excited state the complex (specifically 2) populates after it decays from the
1
MLCT state.
43
The infrared radiation transmittance spectra were not collected satisfactorily as the
instrument was not able to produce a strong enough difference between the signal and the noise.
This point should be addressed, in order to determine the vibrational energies of the ground state
complex. The resulting data can be compared to the computational infrared absorptions in order
to assign specific vibrations to each energy level, and it should also be compared to the TRIR
data to determine how these vibrations change in a MLCT state.
Another way to study the mixed valency is through synthesizing a new ligand that
contains a functional group that can act as an IR reporter. A proposed ligand is suggested in
Figure 5.1. When the MLCT excited state of a complex is populated, electron density enters the
π antibonding orbital of the ligand causing a decrease in bond order and thus a decrease in bond
vibrational energy. This decrease should be significant and can be measured explicitly through
comparison of an fsTRIR spectrum and a ground state IR spectrum. Both the bis-bis and the
homoleptic versions of this compound should be synthesized and compared.
HO
O
Figure5.19-anthracenecarboxylatewithanethynylterminalgroupbondedtothe10
positionoftheanthracene.
44
Once the extent of localization is determined through IR and fsTRIR, a second
interestingstudywouldbetoinvestigatehowthatchargeisdistributedontheligand.Two
potentialligandsareproposedinFigure5.2tocompletethisstudy.
HO
O
N
HO
O
N
N
Figure5.2Proposedpositioningofethynylandnitrilegroupsonanthracenemoleculeto
studyelectroniceffectofdistance.
Ontheleft,thereisanethynylgroupclosetothedimetalcenterandanitrilegroup
thatisbondedtothe10positionoftheanthracenemolecule(seeFigure1.6).Ontheright,
there is also an ethynyl group close to the dimetal center and two nitrile groups that are
bondedtothe3and6positionsoftheanthracene.IftheelectrondensityintheMLCTstate
isnotdistributedequallythroughtheligand,thentheethynylgroupclosesttothedimetal
center will undergo a vibrational shift with a magnitude different to the nitrile groups
bonded in various places on the anthracene molecules. The reason for using an ethynyl
group in one area of the ligand, and nitrile groups on the other side of the ligand is that
thesetwoIRreportershavedifferentvibrationalenergies.Twotriplybondedcarbonatoms
typicallyabsorbinfraredradiationbetween2100cm-1and2260cm-1whileacarbonatom
triplybondedtonitrogentypicallyabsorbsbetween2220cm-1and2260-1.
45
Index of Figures and Tables
Title
Figure 1.1
Figure 1.2
Figure 1.3
Figure 1.4
Figure 1.5
Figure 1.6
Figure 1.7
Figure 1.8
Figure 2.1
Figure 2.2
Figure 2.3
Table 2.1
Table 2.2
Figure 2.4
Figure 2.5
Table 2.3
Figure 2.6
Figure 2.7
Figure 2.8
Figure 3.1
Figure 3.2
Figure 3.3
Table 3.1
Table 3.2
Figure 3.4
Figure 3.5
Table 3.3
Figure 3.6
Figure 3.7
Table 4.1
Figure 5.1
Figure 5.2
Page
6
7
7
9
10
11
12
14
17
18
19
19
19
21
23
24
25
26
27
30
31
32
32
32
34
35
36
36
37
40
44
45
Description
Crystal structure of [Re2Cl8]2Metal-metal bonding d-orbital overlap
Eclipsed vs. staggered conformations of [M2Cl8]nGeneric paddlewheel molecular structure
Orbital energy splitting in highly conjugated π systems
Skeletal formula for 9-anthracenecarboxylate
High energy electron configurations
Depiction of pump-probe spectroscopic technique
1
H NMR spectrum of complex 1
MALDI-TOF spectrum of complex 1
Crystal structure of complex 1
Selected bond lengths in complex 1
Selected data collection parameters for crystal structure of complex 1
Electronic absorption spectrum for complex 1
Frontier Molecular Orbital diagram for complex 1
Computed vibrational assignments for complex 1
Theoretical infrared spectrum for complex 1
fsTA for complex 1 in THF (λex = 700 nm) – kinetic trace in inset
fsTRIR for complex 1 in THF (λex = 600 nm) – kinetic trace in inset
1
H NMR spectrum of complex 2
MALDI-TOF spectrum of complex 2
Crystal structure of complex 2
Selected bond lengths in complex 2
Selected data collection parameters for crystal structure of complex 2
Electronic absorption spectrum for complex 2
Frontier Molecular Orbital diagram for complex 2
Computed vibrational assignments for complex 2
Theoretical infrared spectrum for complex 2
fsTRIR for complex 2 in THF (λex = 650 nm) – kinetic trace in inset
Comparison of energy values of fsTRIR peaks
Proposed ligand structure for mixed valency future study
Proposed ligand structures for charge distribution future study
46
Acknowledgements
This thesis was written in loving memory of Professor Malcolm H. Chisholm, who was
one of my first and most influential chemistry professors at The Ohio State University and who
always made sure I had everything I needed in my undergraduate research career.
Thank you to Professor Claudia Turro for taking over as my project advisor for my final
undergraduate semester and taking the time to work with me so I could still finish my thesis.
Many thanks to Dr. Philip Young who took me under his wing and taught me everything
I would need to know to be successful in the Chisholm group. Everything. And once Phil
graduated, Changcheng Jiang helped to guide me to completion of this project. Also, thanks to
Phil and Changcheng for collecting the fsTA and fsTRIR data for the complexes I was studying.
Additional thanks to Dr. Christopher Durr for assisting me with growing crystals of my
complexes and performing the single crystal x-ray diffraction for both complexes to obtain the
crystal structure. And of course thanks to Will Kender, Lisa Nguyen, Chris Ziehm, and the rest
of the crew for the consistent support, assistance, and guidance. Especially to Will for helping
me edit my thesis.
And finally, of course, thanks for all of the love and support from my friends and family
over these past years.
Defense Committee
Professor Claudia Turro, Department of Chemistry
Professor Terry L. Gustafson, Department of Chemistry
Professor David Terman, Department of Mathematics
47
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