Knowledge Bank

We have obtained highly resolved optical spectra of several dimethyl polyenes in n-alkane hosts. Studies of the undecapentaene-nonane system have been completed and reveal several interesting features of polyene excited states. The transitions between the ground and first excited singlet states are induced by low frequency, nontotally symmetric vibrations in agreement with Herzberg-Teller expectations for ``forbidden” transitions. This finding provides strong support for the assignment of the lowest excited singlet state as $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Ag$ Vibrational analysis of the absorption into the excited $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Ag$ state indicates increases in carbon-carbon double bond stretching frequencies. This is somewhat unexpected given the reduction in double bond orders calculated from Pariser-Parr-Pople and other treatments. The photochemical implications of these findings also will be discussed.