Complete triplet—triplet absorption spectra (2000–10000 Å) were measured in alcoholic solution at 113°K with naphthalene, anthracene, tetracene, and a few methylated derivatives. Several new bands were observed. Calculations of the higher ππ* triplet states energies with the LCAO—MO—SCF Pariser—Parr—Pople method were improved by the use of large configuration interactions. These include many doubly excited configurations with respect to both the ground singlet state S0 and the lowest triplet state T1, thanks to the systematic use of all monoexcitations with respect to S0 plus all monoexcitations with respect to T1. The comparison of all experimental and theoretical results for the three polyacenes allows one to modify a few previous assignments and to give a general tentative assignment for all observed transitions.

Abstract— Model studies including quantum chemical calculations and the measurement of infrared and ultraviolet spectra are presented as contributions to the elucidation of the nature of the photochemical step of vision. The importance of the hydrogen bond in which the protonated nitrogen of the retinal Schiff base is involved is stressed as well as that of the perturbation of the β‐ionone ring by negative groups. It is suggested that by combining these two perturbations the low excitation energy of rhodopsin can be obtained without actual protonation of the Schiff‐base prior to photon absorption. The variation of rhodopsin's color from one species to another could also be related to this. Protonation could be a consequence of photonabsorption and the higher basicity of the excited state. This, in turn, leads to the suggestion that the protonated species is actually bathorhodopsin, not rhodopsin. Comments are made on the identity of the (ππ*) state which is involved.

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