CHARACTERIZATION OF A NEW SOLVENT-SENSITIVE 2-PHOTON-INDUCED FLUORESCENT (AMINOSTYRYL)PYRIDINIUM SALT DYE

A newly synthesized hemicyanine dye, l-N-(hydroxyethyl)amino]styryl]-N -methylpyridinium tetraphenylborate (ASPT), has recently been shown to be an excellent dye for both one- and two-photon-pumped lasing and fo r applications in optical power limiting. We report on the absorption, one- and two-photon-induced emission, quantum yield, and the excited- state lifetime of ASPT in different solvents of varying polarity. We a lso present the spectral characteristics of ASPT within a reverse-mice lle and beta-cyclodextrin cavity. Our results demonstrate that ASPT is one of the few dyes which exhibits highly solvent-polarity dependent fluorescence in the useful orange-red region of the spectrum (560-620 nm). Therefore, it offers many applications in the biomedical field as a fluorescent probe molecule. In addition, ASPT is fluorescent upon e xcitation with an infrared (IR) source (two-photon-induced emission) m aking it even more attractive, since the background fluorescence from biological fluids upon IR excitation is highly reduced compared to ult raviolet or visible excitation. The solvatochromic effect of ASPT is p roposed to be due to a change in the dipole moment and reduced hydroge n bonding effects of the dye upon excitation. On the basis of our resu lts, we propose that ASPT exists in two mesomeric forms. One is predom inant in the ground-state and the other in the excited-state. We obser ve a low quantum yield of the dye (<10%) in all the investigated solve nts. The excited-state lifetime of ASPT is also observed to be solvent -dependent (30-400 ps). The calculated radiative decay rate is observe d to remain constant but the nonradiative decay rate increases in solv ents of increasing polarity. Finally, we report on the spectral featur e of ASPT within a reverse-micelle cavity with varying water loadings and upon binding to a beta-CD cavity.