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The fluorescence properties of ion pair charge transfer (IPCT) complex with bithiophene group were studied by steady-state and time-resolved measurements in various solvents. The photophysical properties of 5,5′-di(4-pyridyl)-2,2′-bithiophene (DPBT) were identical to the unsubstituted bithiophene except for the peak wavelength. The absorption of polymeric 5,5′-di(4-pyridiniumyl)-2,2′-bithiophene (PBT2+) salts showed both the locally excited state (LE) and intramolecular charge transfer (ICT) bands in all solvents studied. In less-polar solvents such as cyclohexane and toluene, absorption spectra showed an IPCT band in addition to the LE and ICT bands. The fluorescence spectra of polymeric PBT2+ salts showed dual emission which originated from the LE and ICT states in polar solvents. The ICT fluorescence lifetime strongly depends on the solvent polarity. The fluorescence from the PBT2+ salts was effectively quenched by their counter ion in less-polar solvent. These results indicate that the counter halide anion affects on the excited-state relaxation process.

Viral vectors that can infect neurons transsynaptically and can strongly express foreign genes are useful for investigating the organization of neural circuits. We previously developed a propagation-competent rabies virus (RV) vector based on a highly attenuated HEP-Flury strain (rHEP5.0-CVSG), which selectively infects neurons and propagates between synaptically connected neurons in a retrograde direction. Its relatively low level of transgene expression, however, makes immunostaining necessary to visualize the morphological features of infected neurons. To increase the transgene expression level of this RV vector, in this study we focused on two viral proteins: the large protein (L) and matrix protein (M). We first attempted to enhance the expression of L, which is a viral RNA polymerase, by deleting the extra transcription unit and shortening the intergenic region between the G and L genes. This viral vector (rHEP5.0-GctL) showed increased transgene expression level with efficient transsynaptic transport. We next constructed an RV vector with a rearranged gene order (rHEP5.0-GML) with the aim to suppress the expression of M, which plays a regulatory role in virus RNA synthesis. Although this vector showed high transgene expression level, the efficiency of transsynaptic transport was low. To further evaluate the usability of rHEP5.0-GctL as a transsynaptic tracer, we inserted a fluorescent timer as a transgene, which changes the color of its fluorescence from blue to red over time. This viral vector enabled us the differentiation of primary infected neurons from secondary infected neurons in terms of the fluorescence wavelength. We expect this propagation-competent RV vector to be useful for elucidating the complex organization of the central nervous system.