Fluorescence spectra of bithiophene and terthiophene single crystals and of their isolated molecules in cyclodextrin

In order to examine the effect of subsumption space of beta- and gamma-cyclodextrin (CyD) on the photophysics of oligothiophenes, 2, 2'-bithiophene (BT) and 2, 2': 5', 2-terthiophene (TT), the fluorescence spectra were compared with those of the single crystals (SC) at 15, 77 K and room temperature (RT). Both the numbers of BT included in beta- and gamma-CyD are twin (BT2). The numbers of TT included in beta- and gamma-CyD are unit (TT1) and twin (TT2), respectively. Electronic excitation of BT encapsulated in beta- and gamma-CyD gives similar fluorescence spectra, showing bathochromic shift compared with that of BT single crystal, (BT)SC. The observation that the fluorescence spectra of encapsulated BT2 are similar to the spectra of its THF solution suggests the configuration of BT2 in beta- and gamma-CyD should be face-to-face configuration (BT2)(parallel to). On the contrary, TT in beta- and in gamma-CyD afford quite different fluorescence spectra. Encapsulated TT in beta-CyD exhibits the hypsochromic shift of fluorescence maxima compared to that of TT single crystal, (TT)(SC). While the bathochromically shifted fluorescence spectra of TT2 in gamma-CyD is also ascribed to the face-to-face configuration (TT2)(parallel to) as in the case of BT2 in gamma-CyD. Fluorescence spectra show the excited ground state complex of BT2 and TT2 in gamma-CyD. (C) 2004 Elsevier Inc. All rights reserved.