Pyridinium Salts: Luminescent Spectroscopy and Photochemistry

The electronic absorption and emission properties of pyridinium cations are summarized and rationalized in terms of their molecular structure.Photochemical processes of pyridinium salts are surveyed vith particular reference to cyclization reactions and the effect on reaction modes and rates of structural variations.The various photophysical and photochemical processes are formulated in a general scheme of photoinduced excitation and relaxations.There has been great interest recently in both the fundamental and the applied aspects of the photochemistry of heterocyclic cations.Investigations of the excited states of n-iso-electronic pyrylium and pyridinium cations have led to fundamental and detailed conclusions on photoinduced processes in heteroaromatic compounds.The photochemical synthesis has been achieved of unusual heteroaromatic systems, often beyond the reach of traditional methods, which include new and unique 1 luminescent materials.The uv-speetroseopy of pyrylium salts has been previously reviewed in detail, but the spectroscopy of pyridinium salts is not yet summarized, and the photochemistry of pyridinium cations has been covered only partially in more general r e v i e v ~. ~' ~ The photochemistry, and especially the luminescent spectroscopy, of pyridinium cations possess their o m intrinsic interest: ve have therefore summarized and critically analyzed the available results.The first two parts of the present review are devoted to the absorption and emission of pyridinium cations in relation to their molecular structure.The third part summarizes photochemical investigations of pyridinium salts.The final section deals with the general scheme of photoinduced processes in aryl-substituted pyridinium cations, the compounds most completely investigated.1. ELECTRONIC ABSORPTION SPECTBA AND HOLECUULR STRUCTURE OF PYRIDINIUH SALTS.The electronic absorption bands of the parent pyridinium cation (PC) were assigned by comparisons of the n-iso-electronic aromatic system, benzene, pyridine and pyryliun ~a t i o n .~-~ 1 The nn* absorption bands of benzene ( La and lLb) also occur in the azaheterocycle The lover molecular symmetry of the azines (C2,) in comparison vith benzene ( 0 ) renders the S1-So 6h transition more permitted.The intensity of the long-wavelength lLb band, due to the symmetry forbidden Alg-B2" transition,6 increases considerably in the azines, and even more in the case of the pyridinium cation.There is also a noticeable decrease of the vibronie structure of the pyridinium cation absorption spectrum5 in comparison with those of benzene and pyridine. 7Theoretical calculations of the pyridinium cation electronic absorption energies8-lo are in 1 good agreement with experimental values for the two long-wavelength transitions: l ~c l ~l and Lh-'A~.'The pyridinium cation electronic distribution in the ground state is predicted by theoretical analysis to localize the positive charge mostly on the 2-, 4-, and 6-carbon atoms.The electronic density distributions in singlet excited states have been also investigated.10