Excited singlet (S-1) state interactions of 6,11-dihydroxy-5,12-naphthacenequinone with aromatic hydrocarbons

Interaction in the excited singlet state (S-1) of 6,11-dihydroxy-5,12-napht hacenequinone (DHNQ) with aromatic hydrocarbons (AH) has been investigated using steady-state (SS) and time-resolved JR) fluorescence quenching measur ements. In both nonpolar (cyclohexane; CH) and polar (acetonitrile; ACN) so lvents, the fluorescence quenching of DHNQ by AHs is accompanied with the a ppearance of exciplex emissions. The emission maxima of the exciplexes corr elate linearly with both ionization potentials (IP) and oxidation potential s {E(AH/AH(+))} of the quenchers (AH), indicating the charge transfer (CT) type of interaction between the S-1 state of DHNQ (acceptor) and the ground state of the AHs (donor). The kinetic details of the exciplex formation ha ve been evaluated by analyzing the SS and TR fluorescence quenching results at different temperatures following a suitable mechanistic scheme. Picosec ond laser flash photolysis (LFP) studies on the DHNQ-AH systems show a majo r transient absorption band in the 530 to 630 nm region along with a weak l ong-wavelength absorption tail. The transient lifetimes for the 530-630 nm absorption band are very similar to the exciplex lifetimes estimated from t he fluorescence quenching results. At the long wavelength absorption tail, the transient lifetime could not be estimated due to very weak absorption. It is inferred that the 530-630 nm transient absorption band is mostly due to the S-1 --> S-n transition. The long wavelength absorption tail has been attributed to the anion radical of DHNQ, drawing an analogy with the anion radical absorption spectrum of 1,4-dihydroxy-9, 10-anthraquinone (quinizar in; QZ), a lower analogue of DHNQ. The picosecond LFP results largely corre late with the results obtained from the fluorescence quenching studies.