Photoacoustic Studies of Non-Radiative Energy Transfer in the Flexible Styrylcyanine-Rhodamine B Dye System

Abstract

We have employed (PAS) photoacoustic spectroscopic technique to demonstrate the ability of the styrylcyanine (dye 1) to act as an energy donor via non-radiative transfer of excitational energy process to a suitable acceptor molecule such as rhodamine B (Rh B). Considering the low yield of fluorescence for dye 1 which is characterized by very large Stokes shift (λ _a = 485 nm and λ _f = 613 nm), it follows that the lowest singlet planar charge-transfer state (S_1,ct) of dye 1 relaxes almost entirely by internal conversion. The non-radiative’ energy deactivation process can be thought of as having two sources: (i) the Stokes heating (resulting from long range intramolecular geometrical relaxation to yield a weakly fluorescent TICT-like S_1,ct state) and (ii) the vibrational relaxation. It is observed that addition of Rh B to the ethanol solution of dye 1 at room temperature completely quenches its PA spectrum. This observation agrees qualitatively with the conclusion that energy transfer process takes place from the planar S_1,ct state of dye 1.