Characterization of a new solvent-sensitive two-photon-induced fluorescent (aminostyryl)pyridinium salt dye

A newly synthesized hemicyanine dye, trans-4-[p-[N-ethyl-N-(hydroxyethyl)amino]styryl]-N-methylpyridinium tetraphenylborate (ASPT), has recently been shown to be an excellent dye for both one- and two-photonpumped lasing and for applications in optical power limiting. We report on the absorption, one- and twophoton-induced emission, quantum yield, and the excited-state lifetime of ASPT in different solvents of varying polarity. We also present the spectral characteristics of ASPT within a reverse-micelle and β-cyclodextrin cavity. Our results demonstrate that ASPT is one of the few dyes which exhibits highly solvent-polaritydependent fluorescence in the useful orange-red region of the spectrum (560-620 nm). Therefore, it offers many applications in the biomédical field as a fluorescent probe molecule. In addition, ASPT is fluorescent upon excitation with an infrared (IR) source (two-photon-induced emission) making it even more attractive, since the background fluorescence from biological fluids upon IR excitation is highly reduced compared to ultraviolet or visible excitation. The solvatochromic effect of ASPT is proposed to be due to a change in the dipole moment and reduced hydrogen bonding effects of the dye upon excitation. On the basis of our results, we propose that ASPT exists in two mesomeric forms. One is predominant in the ground-state and the other in the excited-state. We observe a low quantum yield of the dye (<10%) in all the investigated solvents. The excited-state lifetime of ASPT is also observed to be solvent-dependent (30-400 ps). The calculated radiative decay rate is observed to remain constant but the nonradiative decay rate increases in solvents of increasing polarity. Finally, we report on the spectral feature of ASPT within a reverse-micelle cavity with varying water loadings and upon binding to a β-CD cavity.