Res. Chem. Intermed.
, Vol. 29, No. 3, pp. 293– 305 (2003)
Also available online - www.vsppub.com
Effect of viscosity on the singlet-excited state dynamics
of some hemicyanine dyes
ZEENA S. PILLAI, P. K. SUDEEP and K. GEORGE THOMAS
Photosciences and Photonics Division, Regional Research Laboratory (CSIR),
Trivandrum 695 019, India
Received 10 January 2003; accepted 27 January 2003
Abstract—Photophysical properties of hemicyanine dyes (1–3) were investigated in solvents of
varying polarity and viscosity. Hemicyanines possess relatively low uorescence quantum yields
1%) in polar solvents. A signi cant increase in uorescence quantum yield and lifetimes was
observed with increase in viscosity of the solvent medium. The radiative, as well as nonradiative decay
channels from the singlet-excited state were investigated by varying the viscosity of the medium. The
viscosity-dependentradiationless relaxation observed in hemicyanine dyes is suggestive of a restricted
rotor motion in the singlet excited state.
: Hemicyanines; singlet excited state; viscosity; polarity; nonradiative rate contstants;
radiative rate constants.
Interest in the design and studies of chromophoric systems has increased signi -
cantly in recent years, due to their potential applications in optoelectronic devices
(for e.g. optical recording systems , thermal writing displays  and laser print-
ing systems ) and in biomedical procedures [4 – 6]. Most of the chromophoric
systems used for such applications contain strong donor– acceptor groups (e.g.
polymethine dyes) . Polymethine dyes with heterocyclic acceptors and nonhete-
rocyclic donor groups in terminal positions are referred to as hemicyanine dyes. Re-
cently amphiphilic hemicyanines (e.g. (dibutylamino)stilbazolium butylsulfonate)
have been suggested as voltage-sensitive uorescent probe to record the fast changes
in the electrical membranes potential (molecular voltmeter) in neurons [8 –10]. The
viscosity dependent photophysical properties of hemicyanine dyes offer applica-
tions in polymer sciences, nonlinear optics and molecular electronics [11– 13]. The
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