Fluorescence and two-photon absorption of push—pull aryl(bi)thiophenes: structure—property relationships

Photophysical and TPA properties of series of push—pull aryl(bi)thiophene chromophores bearing electron-donating (D) and electron-withdrawing (A) end-groups of increasing strength are presented. All compounds show an intense intramolecular charge transfer (ICT) absorption band in the visible region. Increasing the D and/or A strength as well as the length of the conjugated path induces bathochromic and hyperchromic shifts of the absorption band as reported for analogous push—pull polyenes. Yet, in contrast with corresponding push—pull polyenes, a significant increase in fluorescence is observed. In particular, chromophores built from a phenyl—bithienyl conjugated path and bearing strong D and A end-groups were found to combine very large one and two-photon brightness as well as strong emission in the red/NIR region. These molecules hold promise as biphotonic fluorescent probes for bioimaging.