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H-bond interactions of methyl cellosolve (MCS) with chlorobenzene (CBZ) have been studied using FTIR spectroscopy and Time Domain Reflectometry (TDR) studies. ( M C S ) O − H ⋯ π ( C B Z ) , ( M C S ) O − H ⋯ C l ( C B Z ) , ( M C S C H 2 ) C − H ⋯ π ( C B Z ) , ( M C S C H 2 ) C − H ⋯ C l ( C B Z ) H-bond interactions may exist in the MCS-CBZ binary solutions. The shifts in the ν ( C − O − C ) and ν ( C − O H ) stretching bands on mixing are due to the proximity effect arising out of the involvement of both C H 2 and O − H hydrogens, respectively, in heteromolecular H-bond. The role of molecular interactions and molar volumes become relatively less significant in determining the relaxation times of the solutions. Instead, the viscosity plays a more dominant role in determining the relaxation times of the MCS-CBZ solutions. Parallel orientation of dipoles is favoured in MCS rich solutions while in CBZ rich solutions the dipoles prefer antiparallel alignment. The excess values of the dielectric constant and free energy show negative and positive deviations, respectively.

Molecular structure, ground state and triplet–triplet electronic absorption spectra were predicted for the title compound using the time-dependent density functional theory calculations. These predictions reproduce well the experimental results, making allowance for the contribution of the solvent interactions, disregarded in the present calculations. Spectral and photophysical properties were compared to those of other alloxazine derivatives, and rationalized based on the proximity effect theory. Triplet quenching rate by oxygen was determined, along with the resulting singlet oxygen yields.

Electronic structures and transitions were calculated for a series of methyl derivatives of 10-methyl alloxazine using the time-dependent density-functional theory. Comparison of quantum chemical results to experimentally determined spectral and photophysical properties of these compounds indicates that these properties may be rationalised on the basis of the proximity effect, whereby two closely located n ,π* and π,π* lowest excited states are vibronically coupled by out-of-plane modes. This theory allows explaining the substituent, temperature, and solvent dependence of the non-radiative decay rates, which affect the fluorescence lifetimes and quantum yields.

Experimental and theoretical studies concerned with radiationless decay in aromatic molecules with non-bonding electrons are presented. Particular attention is focused on N-heterocyclic systems. Evidence for the important role of vibronically active out-of-plane modes in non-radiative transitions is cited. These results are analyzed within the theoretical framework of the quantum-statistical-mechanical approach to relaxation phenomena.