M. F. Granville

High‐resolution one and two photon excitation spectra of trans, trans‐1,3,5,7‐octatetraene

One and two photon excitation spectra for the lowest energy singlet transition (2 1Ag←1 1Ag) of trans, trans 1,3,5,7‐octatetraene in n‐octane at 4.2 K(0–0 at 28 56l cm−1) have been measured and analyzed. The one photon spectrum is found to be vibronically induced while the two photon spectrum is electronically allowed. The contribution of the higher lying 1 1Bu←11Ag transition (0–0 at approximately 32 100 cm−1) to the observed two photon excitation spectrum is found to be negligible allowing vibrational overtones of the 2 1Ag←1 1Ag system to be observed. On the bases of frequency, intensity, and one or two photon allowedness, 128 measured lines are assigned as combinations of 4 antisymmetric and 11 symmetric normal modes.

Franck–Condon analysis of the 1 1Ag→1 1Bu absorption in linear polyenes with two through six double bonds

The vibronic intensity distribution at room temperature and 77 °K of the 1 1Ag→1 1Bu transition for the series of linear polyenes with two through six conjugated double bonds has been measured and analyzed. These inhomogeneously broadened solution spectra can be described reasonably using only two harmonic normal modes of vibration: single and double bond stretches at ∼1200 and ∼1600 cm−1, respectively. Excited state normal coordinate displacements were determined by a least‐squares fitting procedure. For both the double and single bond modes the estimated change in bond length upon excitation decreases as the length of the polyene chain increases. The bandwidth of these spectra decreases with increasing polyene length. This is well described by assuming that in solution a range of conformations differing by very small torsions about the terminal single bonds has been frozen in. The Franck–Condon analysis indicates that under these conditions all of the polyenes are approximately planar in the 1Bu state, ...

Absorption and emission of 2,12‐dimethyltridecahexaene

We have obtained well resolved optical spectra of a simple, methyl substituted hexaene at 4.2 K. These spectra provide detailed information on the vibronic levels associated with the ground (1 1Ag) and first excited (2 1Ag) singlet electronic states. The strongest vibronic feature, due to a totally symmetric CC double bond stretch, shifts from 1576 cm−1 in the 1 1Ag state to 1779 cm−1 in the 2 1Ag state. This shift, not accounted for by current theoretical descriptions of polyene electronic states at the crude adiabatic level, may be rationalized as a manifestation of vibronic coupling between the ground and first excited singlet states.

Resonance enhanced Raman spectrum of all-trans anhydrovitamin A.

The Raman spectrum of all-trans anhydrovitamin A in hexane at 77 degrees K is presented. The similarity of the Raman spectra of anhydrovitamin A and the protonated Schiff base of retinal is striking. The implications of this for visual pigment studies and bacteriorhodopsin are discussed. Tentative assignments of geometry for four cis-trans isomers of anhydrovitamin A are made on the basis of the observed room-temperature absorption spectra.