| المستخلص: |
The role of solvent polarity on the optical properties of triphenylamine (TPA), phenylcarbazole (PCB), 4-(N-carbazole) benzonitrile (CBN) and 4- (N, N diphenlamino) benzonitrile (DPABN) where investigated using various techniques. The large stock's shift of fluorescence of TPA (~4000 cm-1) with respect to its absorption in hexane or acetonitrile (ACN) reveals the role of diphenylamoino group as acceptor and its emission is attributed to be from a CT state (in which the lone pairs of N-atom electrons are delocalized with the two phenyls during the excited state). This role disappeared by bridging the two phenyl groups of TPA leading to PCB which emit structured fluorescence being mirror image to its absorption. This results and reasoning are confirmed by the fast conversion of TPA to PCB through excitation by electromagnetic radiation (emr). Introducing a CN withdrawing group at one of the phenyls of TPA flips the role of diphenylamino group to become donor in DPABN. This is clear from the large red shift of its fluorescence, being from a PICT ~480 nm, when present in ACN or alcohols solvents. This is different from the CT band of TPA appearing ~365 nm in the same solvents. Moreover introducing the CN on TPA leading to DPABN made its absorption spectrum closely similar to that of CBN. The fluorescence of CBN and DPABN are closely similar in ACN and alcohols, however, those of DPABN appear at longer wavelengths and are attributed to be from PICT state. The lack of LE emission of both CBN and DPABN in polar solvents is attributed to very fast conversion from LE →ICT and hence a large fluorescence quantum yield and rate constant of fluorescence from PICT state. Their fluorescence in non-polar solvents like hexane is however different. For CBN the LE emission overwhelm that of CT and the opposite is observed for DPABN (CT fluorescence overwhelm that of LE). Moreover, we investigated the fluorescence properties of CBN and DPABN in water- methanol mixture and found the disappearance of PICT emission by increasing % of water in the mixture. Due to low solubility of these probes in water we believed that emission observed in the region of LE is attributed to aggregates (with nano structure) to be enhanced in 99% water- methanol mixture. This suggestion was confirmed by comparing to fluorescence of solids samples of CBN and PABN and gave closely similar fluorescence and life times. We were able to dissipate the aggregates by either heating or dissolving in CD aqueous solutions, were the PICT returns to appear, and that of aggregates to disappear. All our results and interpretation are confirmed with excitation and time resolved spectra.
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