Dynamics of the fluorescence quenching of 1,4-dihydroxy-, 1-amino-4-hydroxy- and 1,4-diamino-9, 10-anthraquinones by aromatic hydrocarbons

Abstract

The quenching of fluorescence from 1,4-dihydroxy-9, 10-anthraquinone (Quinizarin, QZ), 1-amino-4-hydroxy-9, 10-anthraquinone (AHAQ) and 1,4-diamino-9, 10-anthraquinone (DAAQ) by aromatic hydrocarbons, namely, benzene (BZ), toluene (TOL), p-xylene (XY), mesitylene (MES), naphthalene (NP), tetramethylbenzene (TMB), pentamethylbenzene (PMB), hexamethylbenzene (HMB), pyrene (PY) and anthracene (AN) has been attributed to charge-transfer (CT) or electron-transfer (ET) interactions between the excited-state fluorophore (acceptor) and the ground-state quencher (donor). In cyclohexane, quenching proceeds via the formation of a CT-type exciplex, the emission energies of which have been correlated with the ionisation and oxidation potentials of the donors. Steady-state and time-resolved fluorescence measurements at different temperatures (10-50 ° C) and in solvents of different polarity have been employed to calculate the kinetic parameters associated with the exciplex formation and decay, wherever possible. In acetonitrile, very weak exciplex emission has been observed only with a few of the quenchers having high ionisation potential (weak donors). With strong quenchers (low ionisation potential) there is no observable exciplex emission indicating that the ET process is the principal quenching mechanism. The quenching constants in acetonitrile have been correlated with the change in free energy for the electron-transfer reaction following Marcus and Rehm-Weller relationships, the former giving a better correlation between the experimental and the theoretical data.