POLYENE SPECTROSCOPY AND PHOTOISOMERIZATION: OCTATETRAENE, NONATETRAENE AND CYCLOOCTATRIENE.

摘要

High resolution S(,1)-S(,0) emission and excitation spectra of the following isomers of 1,3,5,7-octatetraene in n-octane host at 4.2(DEGREES)K are reported: (a) trans,cis-octatetraene; (b) cis,cis-octatetraene, and (c) a sigma-cis isomer with no inversion center. Isomerization around one or two (PI)-bonds produces only small effect on the Franck-Condon (FC) factors though it does effect the force constants of the vibrational modes. Isomerization around a sigma bond, however, changes the Franck-Condon envelope drastically and causes a large bathochromic shift (1360 cm('-1)) in the S(,1)-S(,0) transition energy.;Synthesis of the following molecules are described: (a) Nona-2,4,6,8-tetraene; (b) 3-deutero-1,3,5,7-octatetraene; (c) 3-deutero-2,4-pentadienal; and (d) trans,cis-octatetraene.;We have found that the photochemistry of 1,3,5,-cyclooctatetriene in n-hexane at room temperature, yielding stable 1,3,5,7-octatetraene isomer as one end product, is both wavelength dependent and intensity dependent (quadratic dependence). A reaction mechanism is presented to account for the observed photo-kinetics. We also show that it is possible to obtain the absorption spectrum of the transient intermediate from the measured rates of photodecay of the initial reactant and the rates of formation of the stable end product as functions of wavelengths and intensities of irradiation.;The only effect of mono-deuterium substitution in octatetraene is found to be a (10 (+OR-) 2) cm('-1) blue shift in S(,1)-S(,0) transition energy. Monomethyl substitution (as in nona-2,4,6,8-tetraene) is found to effect the FC factors of the vibronic transitions involving skeletal torsional modes. The number of spectral lines due to nonatetraene as well as cis,cis-octatetraene guest molecules vary in sets in some n-alkane host crystalline media, indicating inequivalent guest substitutional 'sites'; n-hexane through n-dodecane hosts were studied.