Environment on singlet-triplet transitions wo classes of solvent being attributed to van der W aals-London interaction petween aromatic rings in the solute and solvent molecules); (2) the intensity •edistribution is in the same direction but is much larger in glassy solutions at •educed temperatures, the effect increasing from aliphatic hydrocarbons to ilcohols to glycerol as solvent, and in this case is attributed to interaction between itilbene's phenyl groups and the -OH of solvent

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... (3) similar results are >btained when resonance through the molecule is increased by increasing the lumber of ethylenic double bonds (in the diphenylpolyene series) or by substitu tion, e.g. with dimethylamino groups, at the 4-and 4'-positions in stilbene, and the opposite effect is observed when resonance is reduced by substitution causing steric hindrance to planarity; (4) almost no vibrational structure has been observed in the spectrum of fraws-stilbene vapour (Professor D. W. G. Style, private com munication), where torsion about the ethylenic double bond would be nearly unrestricted.