Modulation of Two-Photon Absorption of Asymmetrically-Substituted Phthalocyanines by the
Position of Inner Protons (Tautomerization) and Structure of Linking Group
Aleksander Rebanea, Mikhail Drobizheva, Nikolay S. Makarova, Gema de la Torreb and Tomás Torresb
aPhysics
Department, Montana State University-Bozeman, USA
b Departamento de Quimica Organica, Facultad de Ciencias, Universidad Autonoma de Madrid, Madrid, Spain
Two-Photon Absorption Spectra
(B-band Region)
Abstract
N
C(CH3)3
N
N
N
2000
N
N
s2, GM
1500
1000
(2)
μ0  n1μ(1)
(5)
0  n2μ 0  4.6 D
2000
(2)
μ(1)
0 and μ 0 - dipole moments of two tautomers
Our additional measurements of the ratio of T1 and T2 Q(0-0) - bands in chloroform
and octane shows that in chloroform, n1 = n2 = 0.5. Substituting these numbers in (5)
and solving Eqs. (1)-(5) with the assumption that all dipole moments are either
parallel or anti-parallel, we find the following molecular parameters for T1 and T2:
1000
500
0
0
300
350
400
450
500
550
300
Transition wavelength, nm
350
0.5
hn
27
270 (880)
T2
6.9
3.3
8.6
17
100 (1200)
nCT-B (excitation), cm
-1
23800
23700
23600
-1
k = - 490 +/- 70 cm
0.6
0.8
23600
-1
k = -930 +/- 160 cm
23400
23200
0.4
Dn B  
0.6
0.8
1.0
f()
( m 0  Dm B 0 )
Df (  )
a 3 hc
m0
800
0.5
Dn S 
0.0
300
400
500
600
-0.4
700
800
ln(Ared/Ablue)
-0.6
-0.9
0.004
0.008
1/T, K
0.012
-1
0.2
Absorption
ln(Ared/Ablue)
-0.5
-0.8
2(   1 )
,
2  1
  dielectric constant,
n  const
Results:
T1
k = 153 +/- 27
50
T 1:
T2
k = 85 +/- 19
0
0.4
0.6
T 2:
0.8
Dm Q 0
2
 153 cm 1
3
a hc
Dm Q 0
3
a hc
(1)
 85 cm 1
(2)
1.0
f()
k = -123 +/- 8 K
-0.7
680
-0.9
0.004
0.008
0.012
-1
2. Q1(0-0) absorption band shift as a function of solvent polarity:
Dn Q  
700
720
740
, nm
, nm
n1/n2 = exp(-DE/kT)
4. While large changes of permanent dipole moment contribute
considerably to the resulting 2PA strength, the resonance
enhancement effect dominates in both molecules and tautomers.
-0.8
292 K
270 K
230 K
175 K
77 K
660
3. The absolute s2max values of both forms in the region of longwavelength side of B-band is large, ~ 103 GM.
2
5. Nitrophenylethynyl- and nitrostyryl-substituted Pcs show similar 2PA
strength.
15000
0.0
740
Df (  )
a 3 hc
a  molecular radius,
Arrhenius plot
1/T, K
T = 293 K
T = 216 K
T = 77 K
720
2. By using selective fluorescence detection we obtained 2PA spectra
of T1 and T2 separately.
100
2
f ( ) 
-0.6
-1.0
0.4
0.2
0.0
Dm i 0
-0.5
0.6
-0.7
1. We found that at room temperature in solutions push-pull
substituted phthalocyanines present in a dynamic equilibrium of two
spectroscopically different tautomeric forms.
1. Stokes shift as a function of solvent polarity:
1.0
k = -108 +/- 20 K
-0.4
Conclusions:
1.5
Arrhenius plot
-0.3
Absorption
8.6
nCT-B (excitation), cm
m10
Solvatochromic Measurements for NSPc in Q-bands
, nm
0.4
Since DE/k = 108K,
n1/n2 = 0.70 at 300K
4.4
We find |Dm0B | = 27 D for T1 and 17 D for T2. Substitution of these values in Eq.
(TLS) gives the “dipole” contribution to s2, see Table, last column. For both
tautomers it is several times less than the total experimental s2.
In order to obtain the magnitude of the permanent dipole moment
change, we performed the solvatochromic analysis of excitation and
fluorescence spectra of both T1 and T2 tautomers.
( m 0  Dm Q 0 )
a hc
Dm Q 0  m Q  m 0
mQ  dipole moment
of Q  state
Since DE/k = 122K,
n1/n2 = 0.67 at 300K
3
-1
700
2.0
Temperature dependence of absorption spectrum
700
2.3
through equation:
hn
Δμ10  μ1  μ0 , Γ1  linewidthof tarnsition
Df (  )
nQ (excitation), cm
600
T1 Excitation (reg = 805 nm)
T2 Excitation (reg = 699 nm)
Absorption
T1 + T2 Excitation
Fluorescence
, nm
680
T1
f()
m1
S1
2 (2 )4 L4
2
2 1

μ
Δμ
, (TLS)
10
10
2
5 (hnc)
Γ1
-1
1.0
660
s2 dipole, (s2, full)
GM
-1
max
Stokes shift, cm
T1 Excitation (reg = 805 nm)
T2 Excitation (reg = 702 nm)
Full Absorption
T1+T2 Excitation
Fluorescence (ex = 400 nm)
1.5
0.1
|Dm0B |
Debye
0.4
NPEPc
2.5
Absorption, Excitation, Fluorescence
Absorption, Excitation, Fluorescence
NSPc
0.3
a
Å
GT-2
Absorption, Excitation and Fluorescence in Octane
500
|Dm0Q |
Debye
The change of permanent dipole moment is found from solvatochromic shifts of CT
B-bands in excitation spectra of T1 and T2:
Since the molecules are strongly non-centrosymmetrical (carry large
permanent dipole moment), two-photon absorption should be allowed to
the same states as one-photon absorption. If the low-frequency part of
B-band is dominated by charge-transfer state, the 2PA to this state can
acquire a particular strength if the change of permanent dipole moment
is large.
S0
GT-1
NSPc
400
550
Transition wavelength, nm
C(CH3)3
0.0
300
500
|m0 |
Debye
23800
N
NO2
C(CH3)3
2.0
450
NSPc
form
What Is The Role of Charge Transfer in 2PA Strength?
σ2
NO2
2.5
400
These spectra are obtained in octane by selectively monitoring
fluorescence intensity either at the T1 (718 nm) and T2 (698 nm)
fluorescence maxima. 2PA cross sections were calculated, by taking into
account real number density of each tautomer in solution at room
temperature.
HN
N
Four equations (1)-(4) contain 5 unknown parameters. To solve them, we add a fifth
equation, based on independent measurement of <m0> in NSPc in chloroform [G. Rojo,
G. de la Torre, J. Garcia-Ruiz, I. Ledoux, T. Torres, J. Zyss, F. Agullo-Lopez, Chem.
Phys., 245 (1999) 27], which writes:
3000
N
N
NH
(CH3)3C
HN
NH
(CH3)3C
2PA tautomer T1
2PA tautomer T2
1PA tautomer T1
1PA tautomer T2
2PA tautomer T1
2PA tautomer T2
1PA tautomer T1
1PA tautomer T2
In the case (1), 2PA cross section is given by
N
N
NPEPc
4000
Question. Which mechanism dominates the 2PA cross section in this
region: (1) A transition in two-level system with a large change of
permanent dipole moment or (2) a transition in three-level system with
Q-band playing a role of intermediate state?
Chemical Structures
C(CH3)3
NSPc
2500
s2, GM
We have recently started a comprehensive study of twophoton absorption (2PA) properties of phthalocyanines (Pcs) in
near IR region [1]. These investigations can lead to further
development of several intriguing applications of Pcs, including
optical power limiting, 3D memory, and deeper-penetrating
photodynamic therapy.
Here we study 2PA properties of two metal-free push-pull
nitrophenylethynyl- and nitrostyryl-substituted Pcs. We have
found that in both molecules there are two unequivalent
tautomeric forms (T1 and T2) simultaneously present at room
temperature in comparable amounts. At low temperatures these
forms can be reversibly photo-transformed, which is important
property for re-writable 3D memory application.
By monitoring the fluorescence signal at a particular
wavelength, corresponding to spectral maximum of each
tautomer, we were able to obtain 2PA spectra of T1 and T2
separately. Intrinsic (femtosecond) maximum 2PA cross
section, attained near 800 nm, vary from s2 ~ 1.5 x 103 to 4 x
103 GM (1GM = 10-50 cm4 s), depending on particular molecule
and tautomer. Such large s2 values can be due to either (1)
strong resonance enhancement in three-level system (with Qstate playing a role of real intermediate state) or (2) to a 2PA
transition in two-level system with large change of permanent
dipole moment (with ground and final levels working as
intermediate states), or even to both, with possible quantum
interference between the corresponding pathways [2]. While
solvatochromic shifts imply large changes of permanent dipole
moment, ranging from 15 to 30 D, the resonance enhancement
effect dominates in excitation region from 800 to 900 nm in
both molecules and tautomers.
Extraction of Permanent Dipole Moments in the Ground State
T2
Results:
-1
k = -177 +/- 61 cm
14500
T 1:
-1
k = 80 +/- 26 cm
T1
14000
T 2:
0.4
0.6
f()
0.8
1.0
( m 0  Dm Q 0 )
3
 80 cm 1 (3)
3
 177 cm 1 (4)
a hc
( m 0  Dm Q 0 )
a hc
Acknowledgments: This work was supported by AFOSR,
and MBRCT.