Structure and dynamics of 9(10H)-acridone and its hydrated clusters. III. Microscopic solvation effects on nonradiative dynamics

As the final part of the series, nonradiative dynamics and energy-level str ucture of relevant electronic excited states in 9(10H)-acridone (AD) and it s hydrated clusters have been studied by various spectroscopic methods. Tim e-resolved fluorescence measurements on their (1)(pi,pi*) origin excitation have revealed that the fluorescence decay is very fast (approximate to 10 ps) for bare AD but drastically lengthened (> ns) in AD-(H2O)(n) (n = 1-6 a nd higher). Bare AD has been observed clearly by mass-selective delayed ion ization and sensitized-phosphorescence detection, which indicates the effic ient formation of molecules in triplet manifold after the (1)(pi,pi*) excit ation. Several weak peaks have been identified around each (1)(pi,pi*) vibr onic band of bare AD, and they are attributed to (3)(n,pi*) transitions whi ch borrow intensity from the nearby (1)(pi,pi*) bands through the direct sp in-orbit coupling. Such satellite bands completely disappear in the fluores cent cluster spectra with n greater than or equal to 1. All of the experime ntal observations indicate that the dominant nonradiative pathway in bare A D is the S-1(pi,pi*) --> T-2(n,pi*) intersystem crossing (ISC) followed by the T-2(n,pi*) --> T-1(pi,pi*) internal conversion. This direct ISC process becomes prohibited by the energy-level inversion between the S-1 and T2 st ates induced by the H bonding to the C=O site. Thus the relaxation pathway is "switched" to the second-order ISC [S-1-(pi,pi*) --> T-1(pi,pi*)] in the fluorescent hydrated clusters, where the carbonyl site is involved in H-bo nding networks. Owing to the increasing S-1-T-2 separation, the fluorescenc e quantum yield becomes larger for the higher clusters, which is approachin g to the bulk solution value, i.e., Phi(f) approximate to 1. The (pi,pi*) a nd (n,pi*) shifts at each cluster geometry have been calculated as (N)HOMO- LUMO gaps in DFT orbital energies to support the picture on the energy-leve l structure. Most importantly, a small falloff in the fluorescence decay co nstants from n = 2 to 3 has been definitely correlated to the crossover in H-bonding topologies (the C=O bonded --> the bridged form), which has alrea dy been established in papers I and TI. The delayed ionization has identifi ed new spectral features that are completely absent in the fluorescence exc itation spectrum. They are assigned to the N-H bonded isomer(s) with n less than or equal to 3, which is at least as stable as the C=O bonded conforme r(s) with the same size. The ISC in the hydrate(s) should be as fast as in bare AD, because of the lack of the S-1-T-2 level inversion. These experime ntal findings demonstrate the site-specific solvation effects on the electr onic energy-level structure and the resultant nonradiative dynamics in the hydrated clusters of a bifunctional molecule.