Dynamics of hydrogen bond reorganization in the S1(ππ) state of 9-Anthracenecarboxaldehyde

Schematic diagram to show conventional and aromatic π-hydrogen bonding interactions between the excited (S1) state of 9-ACD and the Trifluoroethanol (TFE) solvent molecules. TRIR spectrum of the S1 state of 9-ACD in TFE at 0.3 ps delay time reveals overlapping of the IR bands of hydrogen bonded carbonyl stretch and the ring mode vibration of the aromatic anthracenyl moiety. However, at 75 ps delay time, these two spectra are resolved because the peak wavenumber of the ring mode vibration suffers a dynamic blue shift due to hydrogen bond reorganization process, which leads to strengthening of the aromatic π-hydrogen bonding interaction. [Display omitted] •Effect of solvent polarity and hydrogen bonding interaction on the excited state dynamics of 9-Anthracenecarboxaldehyde (9-ACD)have been investigated using steady state and time-resolved electronic and vibrational spectroscopic techniques. Photophysical properties and dynamics of the excited states are seen to be more sensitive to intermolecular hydrogen bonding interaction with the solvent, rather than its polarity.•FTIR studies reveal formation of complexes with the protic solvents via formation of conventional hydrogen bond at the carbonyl site of 9-ACD molecule. In strong hydrogen bond-donating solvents, namely, 2, 2, 2-trifuoroethanol (TFE) and 1, 1, 1, 3, 3, 3-hexafluoroisopropanol (HFIP), nearly all the molecules of 9-ACD in solution remain engaged in complex formation with insignificant number of molecules remaining free in solution. In aprotic solvents, the S1 state is short-lived (lifetime is only a few tens of ps) and intersystem crossing (ISC) is the major deactivation pathway (the triplet yield is near unity). However, in strong hydrogen bond donating solvents, the S1 state lifetime is long (a few ns) because the deactivation pathway for the S1 state via the triplet manifold is blocked.•Both the ultrafast time-resolved electronic and vibrational spectroscopy studies in TFE and HFIP reveal that hydrogen bond reorganization process following photoexcitation of the hydrogen bonded complex is the only relaxation process undergone by the S1 state in sub-100 ps time domain. However, the time-resolved IR (TRIR) spectroscopy studies further reveal the possibility of formation of aromatic π-hydrogen bonding and/or reorganization of the hydrogen bond at this site, which makes the major contribution towards the S1 state relaxation process. Effect of solvent polarity and hydrogen bonding interaction on th