Characterization of a New Solvent-Sensitive Two-Photon-Induced Fluorescent (Aminostyryl)pyridinium Salt Dye Upvan Narang, Chan F. Zhao, Jayant D. Bhawalkar, Frank V. Bright, and Paras N. Prasad* Photonics Research Laboratory and Department of Chemistry, Natural Science and Mathematics Complex, State UniVersity of New York at Buffalo, Buffalo, New York 14260-3000 ReceiVed: NoVember 15, 1995 X 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-photon- pumped lasing and for 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 also present the spectral characteristics of ASPT within a reverse-micelle and -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 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 -CD cavity. Introduction Hemicyanine dyes are well known for their large second- order hyperpolarizability and the ability to form stable Lang- muir-Blodgett mono- and multilayers for second harmonic generation. 1-4 We recently reported on the synthesis of a new hemicyanine dye, trans-4-[p-[N-ethyl-N-(hydroxyethyl)amino]- styryl]-N-methylpyridinium tetraphenylborate (ASPT), 5 which has shown promising one- and two-photon-pumped lasing. 5-8 Previously, we also reported on the application of this dye as an efficient optical power limiter. 7 Because of these exciting prospects offered by this dye, there is need to understand in detail its spectroscopic features. In an effort to understand the spectroscopic behavior of ASPT, we have investigated its absorption, one- and two-photon- induced emission, quantum yield, and excited-state lifetime in solvents of varying polarity. We observe a negative solvato- chromism in the absorption (blue-shift in absorption with increasing polarity of the solvent), a positive solvatochromism in the emission (red-shift in the emission with increasing polarity of the solvent), low quantum yield (<10%), and a variable excited-state lifetime (30-400 ps) for ASPT in various solvents. The calculated change in the dipole moment upon excitation of ASPT is ∼10 Debye. On the basis of these 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 also observe a strong solvent-polarity-dependent fluo- rescence (em max ) 559 (benzene) - 619 nm (DMSO)) of ASPT in the orange-red region of the spectrum. Despite numerous applications in the biomedical diagnostics area, there are very few dyes which are solvent-sensitive in this more practical spectral region. In addition, a majority of the biological samples have intrinsic fluorescence due to molecules which are inher- ently fluorescent and absorb in the ultraviolet and visible region. Therefore, for minimum background fluorescence and maximum sensitivity, an ideal fluorescent probe will absorb in the IR (little/ no absorbance by the biological samples) and emit efficiently in the visible region of the spectrum. ASPT offers both these advantages. It can be excited with an IR source through the two-photon absorption process, and it emits in the visible region (560-620 nm) of the spectrum. In addition, ASPT is observed to be solvent-sensitive; therefore, it has tremendous potential for application in the biomedical diagnostics area as a fluorescent probe. Experimental Section Materials. trans-4-[p-[N-ethyl-N-(hydroxyethyl)amino]styryl]- N-methylpyridinium tetraphenylborate (ASPT; Figure 1) was * Author to whom all correspondence should be sent. X Abstract published in AdVance ACS Abstracts, February 15, 1996. Figure 1. Molecular structure of trans-4-[p-[N-ethyl-N-(hydroxyethyl)- amino]styryl]-N-methylpyridinium tetraphenylborate (ASPT). 4521 J. Phys. Chem. 1996, 100, 4521-4525 0022-3654/96/20100-4521$12.00/0 © 1996 American Chemical Society