K. Matczyszyn et al. / Journal of Molecular Structure 565±566 (2001) 53±57
57
The azobenzene dyes with different electron donor
and acceptor group exhibit large positive solvato-
chromic shift for the lowest p±pp transition (Az1,
Az2 and Az3), which is in good agreement with our
experimental results. This is connected with the fact
that on going to polar solvents, dipolar character
increases in this excited state (i.e. the ground state is
characterised by the formula DA, while D2A1 char-
acterises the excited state) [10]. Obviously, this would
produce a large positive difference of dipole moment
between the electronic ground state and the charge
transfer (CT) excited state Dm. On the other hand,
as the solvent polarity increases, the lowest n±pp tran-
sition moves towards higher energy (blue shift). In
water, the computed blue shift of transition is equal
nmax < 700 cm21 for Az1, Az2 and Az3 molecule.
The results of our calculations of the vectorial part
of the hyperpolarisability indicate that the following
sequence holds for molecules in the gas-phase (and in
solvents for Az1, Az2 and Az3): bvec(Az-1) .
be considered. Encouraging results were obtained also
for Az3 molecule and further experiments will be
carried out in placing it in LC matrix. The solvato-
chromic effect occurring in Az1 and Az3 will be
studied in detail. No signi®cant in¯uence on elec-
tronic spectra and kinetics of the thermal cis±trans
reaction was found in Az4 and Az5 molecules.
The results of calculations show that the in¯uence
of changes of the substitution and solvent on b are
very important aspects to be considered when
searching for compounds with optimised structure
with highly ef®cient nonlinear optical effects.
Acknowledgements
The research was supported by the Technical
University of Wroclaw, KBN grant 3 T09A 117 18
and NSF CREST grant #9805465&9706268. Calcula-
tions have been carried out in Wroclaw (Quantum
Chemistry Group) and Mississippi Center for Super-
computing Research (MCSR). The authors thank
Prof. J. Sworakowski for help and comments, Prof.
J. Lipinski for discussion and Dr Z. Galewski for the
gift of the material.
b
vec(Az-3) . bvec(Az-2) . bvec(Az-5) . bvec(Az-4).
It should be noted that the value of bvec for Az1 is
comparable with that for DANS molecule (a well-
known optically nonlinear molecule) [6]. The calcu-
lated values of the static hyperpolarisabilities are
given in Table 1.
A pronounced in¯uence of solvent on the character
of bvec and its dispersion has been observed. For
example, the values of bvec(22v;v,v) for Az1 in
water at frequencies between 0 and 1.17 eV are larger
by a factor of 1.8±2.5 as compared to the gas phase.
The calculated value bvecꢀ22v; v; v 101:5 £
10230 esu (in chloroform) for Az1 is in a good agree-
ment with the experimental value 87 £ 10230 esu
obtained by Cheng et al. [15] from EFISH measure-
ments at v 0:650 eV (in dioxane). Unfortunately,
there are no experimental data for the remaining
molecules investigated here.
References
[1] G.H. Brown (Ed.), Photochromism Wiley/Interscience,
New York, 1971.
[2] H. DuÈrr, H. Bouas-Laurent (Eds.), Photochromism. Molecules
and Systems Elsevier, Amsterdam, 1990.
[3] M. Irie (Ed.), Photoreactive Materials for Ultrahigh Density
Memories Elsevier, Amsterdam, 1994.
[4] Z. Galewski, Liquid Crystalline Polymorphism of Schiff Bases
and Azobenzenes, University of Wroclaw, Wroclaw, 1999.
[5] N. Nishimura, T. Sueyoshi, H. Yamanaka, E. Imai, S. Yama-
moto, S. Hasegawa, Bull. Chem. Soc. Jpn 49 (1976) 1381.
[6] H. Rau, in: H. DuÈrr, H. Bouas-Laurent (Eds.), Photochromism.
Molecules and Systems, Elsevier, Amsterdam, 1990 (chap. 4).
[7] Ch. Reichardt, Solvents and Solvent Effects in Organic Chem-
istry, VCH, Weinheim, 1988.
Â
[8] J. Sworakowski, J. Lipinski, è. Zioøek, K. Palewska,
Ï
S. Nespurek, J. Phys.Chem. 100 (1996 ) 12288.
4. Conclusions
Ê
[9] J. Lipinski, W. Bartkowiak, J. Phys. Chem. A 101 (1997) 2159.
[10] W. Bartkowiak, J. Lipinski, J. Phys. Chem. A 102 (1998) 5236.
[11] J. Lipinski, W. Bartkowiak, Chem. Phys. 245 (1999) 263.
[12] B.J. Orr, J.F. Ward, Mol. Phys. 20 (1970) 513.
The results presented in this paper indicates that
Az1 should be a promising photoactive medium
when put in non-polar environment. It is characterised
by quite low energy of activation of thermal cis±trans
isomerisation and very good nonlinear properties.
However, the in¯uence of environment should always
[13] J. Lipinski, Int. J. Quantum Chem. 34 (1998) 423.
[14] D.R. Kanis, M.A. Ratner, T.J. Marks, Chem. Rev. 94(1994)195.
[15] L.-T. Cheng, W. Tam, S.H. Stevenson, G.R. Meredith,
G. Rikken, S.R. Marder, J. Phys. Chem. 95 (1991) 10631.