EXCITED STATE STRUCTURE OF METAL COMPLEXES: VIBRATIONAL
SPECTROSCOPY AND DENSITY FUNCTIONAL THEORY STUDIES OF 1,10PHENANTHROLINE AND ITS COMPLEXES
S. Howell1* and K. Gordon1
1
Department of Chemistry, University of Otago, Union Place, Dunedin, New Zealand;
Email [email protected]
Keywords: polypyridyl complexes, MLCT excited state, DFT, transient resonance Raman
Abstract: Transient resonance Raman spectroscopy has been utilised in concert with density
functional theory (DFT) calculations (B3LYP/6-31G(d) and B3LYP/6-311+G(d,p)) to study the
metal-to-ligand charge-transfer (MLCT) excited states of polypyridyl complexes such as
[Cu(phen)(PPh3)2]+ (Fig. 1). The perdeuterated analogue was also studied as this aids in spectral
assignment and the understanding of the modes, as well as testing the robustness of the
computational method used.
+
N
N
Cu
Ph3P
PPh3
Fig. 1. [Cu(phen)(PPh3)2]+
Polypyridyl complexes, such as those containing 1,10-phenanthroline (phen), have been studied
for decades. Interest in these complexes arose due to their uses in areas such as nanotechnology,1
photocatalysis2 and solar cell devices.3 The key species for use of these complexes in such
applications is their MLCT excited state, in which the metal is formally oxidised and one of the
ligands in the complex formally reduced to a radical anion.4 We are interested in establishing the
structural changes that occur in metal complexes with polypyridyl ligands upon photoexcitation.
The use of B3LYP/6-31G(d) calculations is sufficient to model the structures and vibrational
spectra of phen and its perdeuterated analogue: d8-phen. These calculations predict all of the
observed bands in the IR and Raman spectra. The frequencies lie within 5 cm-1 of the observed
values for 15 of the 22 modes in the 1000 – 1700 cm-1 region. The shifts observed for each mode
upon deuteration are reproduced successfully in the calculated spectra. Calculations on phen were
also carried out using the 6-311+G(d,p) basis set. This produces almost identical results.
[Cu(phen)(PPh3)2]+ was also modelled using B3LYP/6-31G(d), with the shifts in the positions of
phen modes upon complexation well predicted.
The excited state spectra of polypyridyl complexes may often be approximated by considering
the spectra of the radical anion of the polypyridyl ligand due to the MLCT nature of the excited
state.5 Calculations were carried out on phen.- using the 6-311+G(d,p) basis set as this correctly
predicted phen.- to be a 2B1 state. These calculations are in good agreement with previously
collected resonance Raman spectra of phen.-.6 The excited state spectrum of [Cu(phen)(PPh3)2]+
was collected using transient resonance Raman spectroscopy with a ns pulsed excitation source.
The use of a range of photon fluxes and comparison with spectra taken using continuous-wave
excitation of near identical excitation wavelength make the identification of excited state features
relatively straightforward. The spectrum shown in Fig. 2 is thus attributed to the excited state:
[Cu(phen)(PPh3)2]+*.
A significant number of bands observed in the spectra of
[Cu(phen)(PPh3)2]+* and [Cu(d8-phen)(PPh3)2]+* are virtually identical in wavenumber and relative
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intensity. These similarities between the spectra of the deuterated and non-deuterated complexes
are attributed to PPh3 modes. This is confirmed by comparison to the resonance Raman spectra of
[Cu(PPh3)4]+. The residual bands in the Cu complexes' spectra are then attributed to phen.- and d8phen.- species. One of the advantages of comparing the [Cu(phen)(PPh3)2]+* and [Cu(d8phen)(PPh3)2]+* spectra is that the electronic properties of the isotopomers are equivalent and their
resonance Raman spectra may be directly compared. Thus the PPh3 modes in the spectrum of
[Cu(d8-phen)(PPh3)2]+* possess the same intensity pattern in the [Cu(phen)(PPh3)2]+* spectrum.
This allows for the more ready identification of the phen.- and d8-phen.- modes.
1559
1469
1193
1093
d
1000
1427
1585
For the complexes studied it was found that the assignment of phen.- bands based on the
B3LYP/6-311+G(d,p) calculation was ambiguous. Incorporation of the {Cu(PH3)2}+ moiety to the
calculation, hence calculating the reduced state of [Cu(phen)(PH3)2]+, allowed for a greater level of
prediction.7
1469
1552
S
c
1585
1453
1511
Raman Intensity
S
b
1000
1587
S
a
1512
S
1200
1400
1600
-1
Raman Shift / cm
Fig. 2. Resonance Raman spectra of complexes in CH2Cl2 (10 mmol dm-3): (a)
[Cu(phen)(PPh3)2]+, λexc = 356.4 nm, 60 mW, (b) [Cu(phen)(PPh3)2]+ *, λexc = 354.7 nm, 2.5 mJ
per pulse, (c) [Cu(d8-phen)(PPh3)2]+, λexc = 356.4 nm, 60 mW, (d) [Cu(d8-phen)(PPh3)2]+ *, λexc =
354.7 nm, 2.5 mJ per pulse. s denotes solvent bands.
Acknowledgements:
Support from the University of Otago, MacDiarmid Institute for Advanced Materials and
Nanotechnology, New Economy Research Fund and the Foundation for Research Science and
Technology is gratefully acknowledged.
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