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With the 1-aminonaphthalenes 1N5 and 1DMAN a fast radiationless process occurs in n-hexane, diethyl ether, and acetonitrile, which is shown to be internal conversion (IC). The IC reaction is slower with 1N4 and much less efficient with 1MAN and 1AN. This IC process is thermally activated and slows down with increasing solvent polarity, due to a larger IC activation energy. In the ground state S0, the amino twist angle θ relative to the naphthalene plane increases in the order 1MAN, 1AN, 1N4, 1N5, 1DMAN, as derived from absorption and fluorescence spectra, 1H NMR spectra, ground-state dipole moments, and ab initio calculations. For the five 1-aminonaphthalenes in the equilibrated S1 state, the twist angle and the radiative rate constant have similar values. The different IC efficiences of these molecules are therefore determined by the structural differences (amino twist angle) between S1 and S0. A correlation is found between the IC efficiency in these molecules and the twist angle θ. The IC process to S0 starts from the equilibrated S1 state, which is vibronically coupled with S2 due to a small energy gap ΔE(S1,S2). It is therefore concluded that the extent of vibronic coupling and the magnitude of the twist angle θ are the determining factors in the IC process.

Photocatalytic splitting of water was investigated in a heterogeneous system consisting of micro-crystallites of oxotitanium tetraphenylporphyrin deposited on fused silica plates, immersed in water and excited within the visible range of their absorption spectra. The water photolysis was evidenced by the spectroscopic detection of hydroxyl radicals generated in the reaction. The experimental results confirm the mechanism of water splitting and generation of OH˙ radicals proposed theoretically by Sobolewski and Domcke [Phys. Chem. Chem. Phys., 2012, 14, 12807] for the oxotitaniumporphyrin-water complex. It is shown that photocatalytic water splitting occurs in pure water, and neither pH-bias nor external voltage is required to promote the reaction.

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We study absorption and emission spectra of optically nonlinear single crystals of 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (DCNP) at 5 K. We argue that fluorescence has a complex origin, it is emitted from the excitonic band, with the bottom at ∼18,115 cm(-1), and from trap states, and the two main traps have depths of ∼875 and ∼2465 cm(-1). The excitonic origin of the emission is confirmed by the vibrational structure of fluorescence, closely resembling vibrations observed in the Raman scattering spectrum (recorded for DCNP crystals at 295 K) and by very short decay time of the excitonic emission, as a consequence of exciton migration and trapping at deep traps.

The results of experimental studies of fluorescence and phosphorescence of acridine in the low-temperature inert neon matrix, at 7 K, are reported. It is found that the low-temperature inert matrix of neon affects the energy levels of acridine molecules very weakly even as compared. with nonpolar (aprotic) and non-reactive solvent (e.g. hexane) and that there are different sites for acridine molecules in the neon matrix. However, the observed fluorescence spectra are strongly dependent on the excitation wavelength and besides the different (monomer) sites other emitting species are also contributing to the observed fluorescence emission of acridine in the neon matrix. Clear-cut evidence of the formation of singlet excimers of acridine in the neon matrix demonstrates itself as a very broad and structureless fluorescence spectrum with a relatively large shift from the origin of monomer (site) fluorescence which is characterized by a very distinct vibrational structure. The phosphorescence emission was observed only for the monomers. The observed differences in the low-energy part of excitation spectra of phosphorescence and fluorescence are discussed in terms of the close-lying excited singlet states of n, π* and π, r* character (mixed by the vibronic coupling) and tentatively interpreted as dne to the formation of resonance acridine dimers. Their fluorescence spectrum is slightly shifted toward lower energies from the origin of monomer (site) fluorescence.