Intensities of Electronic Transitions in Molecular Spectra VII. Conjugated Polyenes and Carotenoids

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Abstract
The observed absorption spectra of conjugated polyenes and their derivatives are compared with the results of theoretical calculations on various models of butadiene, octatetraene, and β‐carotene. In each case the unsaturation electrons give rise to a series of N→V electronic transitions which spread over an increasing range of the spectrum with increase in the number of conjugated double bonds. According to the theory, the greater part of the whole N→V intensity is concentrated in its longest wave‐length member N→V1, and the latter gets more and more intense as it shifts toward longer wave‐lengths with increase in the number of conjugated double bonds. Exactly this behavior has been observed experimentally in the spectra of the conjugated polyenes and their derivatives. Thus we obtain a satisfactory theoretical explanation of the existence of strong color in polyene pigments and related compounds, i.e. in the carotenoids. Quantitatively, the theory (when the rough Hückel molecular orbital approximation is used) requires empirical adjustment or calibration; new experimental work on the spectra of the lower polyenes would be valuable in this connection. The relations between absorption and fluorescence spectra (N→V1 and V1→N) in the substituted polyenes are discussed. An explanation is given of the ``gap'' between the longest wave‐length absorption band and the shortest wave‐length fluorescence band. According to the theory, the shape of the molecule strongly affects the distribution of intensity and somewhat affects the total intensity in the N→V spectrum. The more elongated the molecule, the greater the total intensity, and the greater the preponderance of intensity in the longest wave‐length transition N→V1. The α‐furyl polyene aldehydes and acids may be cited as affording probable illustrations of this effect. Here the introduction of the furyl group, although it contains two conjugated double bonds, increases the N→V1 intensity much less than if the same grouping – HC=CH–CH=CH – had been added to the polyene chain. The explanation given here is that the two double bonds increase the over‐all length of the molecule much less when they are bent around as in the furyl ring than when they are stretched out in a zigzag chain as apparently they are in the simple polyenes and polyene aldehydes and acids. According to the theory, the N→V1 transitions in conjugated polyenes and their derivatives are polarized approximately along the long axis of the molecule. This should give rise to a high polarizability of such molecules along that direction, which may help explain their reactivity, in particular their tendency to polymerize.