Photophysics of Anthril in Fluids and Glassy Matrixes.

Abstract

Photophysics of 9,9'-anthril have been investigated at room temperature and cryogen phase (77 K) exploiting steady state and time-resolved emission techniques together with quantum chemical calculations. Absorption spectra, emission spectra, and emission lifetimes of anthril have been recorded and analyzed in polar ethanol and nonpolar methylcyclohexane media. Both room temperature and cryogenic experiments reveal a single emission band upon excitation at the nπ* absorption band whereas on exciting the system at the ππ* band, dual emission bands have been observed. Characterization of these two fluorescence bands to be originating from near-trans and relaxed skew conformers have been made by monitoring their differential effect on varying the polarity of solvents. Similarly, two phosphorescence bands have been assigned to trans and cis geometries by looking at the change in the emission spectra in the two rigid matrixes of different polarity. Observation of a single isoemissive point in the time-resolved area normalized emission spectroscopy (TRANES) for both fluorescence and phosphorescence emissions unambiguously validates the coexistence of the two conformers in the excited singlet and triplet states, respectively. Qualitative quantum chemical calculations indicate that the S1 and T1 states are responsible for the dual fluorescence and phosphorescence bands. Effortless transitions from the higher excited singlet states (S3 or S2) to the lowest energy excited singlet (S1) state because of their energy proximity discard any possibility of S2 emission, consistent with two other 1,2-dicarbonyl compounds like α-furil and 2,2'-pyridil, while going in contrast to the observation of S2 emissions from benzil and α-naphthil. On the basis of the vivid photophysical studies on five probes in fluid media and 77 K glassy matrixes, we conclude that exhibition of the S2 emission for aromatic 1,2-dicarbonyl compounds is truly system dependent and not a general phenomenon for all the molecular systems in the series.