Photophysics of aromatic thiourea derivatives and their complexes with anions. Fast and ultrafast spectroscopic investigations

In order to collect detailed information on the interaction mechanism between fluorescent thiourea derivatives and anions, 9-[4-(trifluoromethyl)phenylthioureidomethyl]anthracene (1) and the corresponding 10-cyanoanthracene derivative (2) were synthesized and investigated in DMSO and MeCN by using absorption and emission steady state techniques, both in the absence and in the presence of different anions (AcO−, H2PO4−, HSO4−, and Br−). A wide examination of the mechanism of anion recognition was also performed by time-resolved transient absorption spectroscopy, with nano- and femto-second time resolution. A complete picture of the excited state deactivation pathways of 1 and 2, where the main operative processes were fluorescence, intersystem crossing and internal conversion, was obtained. Even if steady-state measurements suggest that 1 and 2 selectively interact in the ground state with the anions H2PO4− and AcO−, time-resolved investigations demonstrate that the substrates are able to complex all the four anions. The photophysics of such complexes was fully characterized. The anions mainly modify the lifetime of the lowest excited singlet state and, especially in the cases of H2PO4− and AcO−, the efficiencies of fluorescence emission and triplet formation. In particular, no evidence was found of further deactivation processes such as photoinduced electron transfer, photodissociation, and photoionization.