Structure and Dynamics of 9(10H)-Acridone and Its Hydrated Clusters. III. Microscopic Solvation Effects on Nonradiaive Dynamics

摘要

As the final part of the series, onradiative dynamics and energy-level structure of relevant electronic excited states in 9(10H)-acridone (AD) and its hydrated clusters have been studied by various spectroscopic methods. Time-resolved fluorescence measurements on their ~1(#pi#,#pi#~*) origin excitation have revealed that the fluorescence decay is very fast (approx=10 ps) for bare AD but drasticaly lengthened (> ns) in AD-(H_2O)_n (n = 1-6 and higher). Bare AD has been observed clearly by mass-selective delayed ionization and sensitized phosphorescence detection, which indicates the eficient formation of molecules in triplet manifold after the ~1(#pi#,#pi#~*) excitation. Several weak peaks have been identified around each ~1(#pi#,#pi#~*) vibronic band of bare AD, and they are attributed to ~3(n,#pi#~*) transitions which borrow intensity from the nearby ~1(#pi#,#pi#~*) bands through the direct spin-orbit coupling. Such satellite bands completely disappear in the fluorescent cluster spectra with n >= 1. All of the experimental observations indicate that the dominant nonradiative path way in bare AD 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 y the energy-level invesion between the S_1 and T_2 states inducedby 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-bonding networks. Owing to the increasing S_1-T_2 separation, the fluorescence quantum yield becomes larger for the higher clusters, which is approaching to the bulk solution value, I.E., #PHI#_f approx= 1. The (#pi#,#pi#~*) and (n,#pi#~*) shift at each cluster geometry have been calculated as (N)HOMO-LUMO gaps in DFT orbital energies to support the picture on the energy-level structure. Most importantly, a small falloff in the fluorescence decay constants 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 already been established in papers I and II. The delayed ionization has identified new spectral features that are completely absent in the fluorescence excitation spectrum. They are assigned to the N-H bonded isomer(s) with n <= 3, which is at least as stable as the C=O bonded conformer-(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 experimental findings demonstrate the site-specific solvation effects on the electronic energy-level structure and the resultant nonradiative dynamics in the hydrated clusters of a bifunctional molecule.