Stereoisomerism and spectroscopic properties of α,ω-diphenylpolyenes. Part 1.—Theoretical study on the shape of the long-wavelength u.-v. band

Abstract

As a first step in the interpretation of the spectroscopic changes connected with stereoisomerization of a,w-diphenylpolyenes, a " quantitative " justification for the diffuse spectrum of stericallyhindered isomers in the near u.-v. region has been attempted taking trans-and cis-stilbene as test molecules. Potential energy curves have been obtained for two low-frequency vibrational modes, namely the twisting of the essential single bonds and the torsion of the central double-bond, in both the initial ('A) and final ('B) electronic states. The corresponding sets of vibrational energies and wavefunctions have been obtained by expressing the potential functions as power series and by expanding the wavefunctions in a basis of harmonic oscillator eigenstates. Franck-Condon distributions calculated for the the two vibrational modes have shown that the " twisting " rather than the " torsion " mode is responsible for the blurring of the fine structure in the conjugation band of cisstilbene.* the choice of stilbene as the first term of this series is supported by some peculiar similarities between the u.-v. spectra of stilbenes and those of a,w-diphenylpolyenes regarding the longestwavelength band.

Stilbene. A critical commentary and some new calculations on its structure and isomerization

“…The cis isomer was incorrectly calculated to be more stable than the trans isomer in the ground state. Similar calculations for butadiene give the same erroneous result.68 Momicchioli et al,32,34 who used an intermediate approximation for Ec, obtained asymmetric potentials. Their ground state surface exhibits minima at ( , ) = (0,0) and ca.…”

“…Momicchioli et al's -electron model predicts that the = 90°conformation is an energy maximum for both isomers. 32 In this conformation, the -electron systems of the phenyl rings are completely decoupled at the oneelectron level from that of the ethylene fragment. The energy barriers to rotation through the = 90°conformation were calculated to be ~22 and 6 kcal/mol in the trans and cis isomers, respectively.…”

“…Potential energy surfaces for the c-t isomerism of stilbene have been calculated in a number of semiempirical MO models. 26,28,29,31,32,34,41 Most of these26, 31,32,34,41 are restricted to the x electrons and therefore neglect the effect of hyperconjugation, which is expected to be greatest in the region of the perpendicular configuration. In all cases, a C2 axis of symmetry has been maintained during the course of the reaction.…”

Optical and photophysical properties of anisole- and cyanobenzene-substituted perylene diimides

“…52 In a topic case, the cis stereoisomers of α,ωdiphenylpolyenes, this interpretation gained theoretical support by explicit calculations of the S0 and S1 potential energy curves and Franck-Condon distributions. 53 On the other hand, blurring of the vibronic structure in the spectra of PDI has been closely associated with distortion of the perylene core with large twist angles between its naphthalene units. 54 is markedly bathochromically shifted with respect to the corresponding band of a similar tetraphenoxy-substituted PDI (maximum at 582 nm in dichloromethane).…”

Semi-empirical calculations of chemical shifts in partially paratropic anions

“…All rings are considered to be regular polygons with sides of 1-39 A-t (exception : the central ring of TxH-, with Rcs= 1.72 A, is assumed to have its vertices on a circumscribed circle). For DbcHand Dba-it appeared necessary (see w 3.2) to include double-bond fixation in the central ring: the ethylenic C=C bond has been given a length of 1-35 A and the two adjacent single bonds a length of 1.47 A (these lengths have been estimated from investigations on stilbene [36,37]). The exponential factor introduced by Pariser and Parr [29] is employed to calculate the corresponding resonance integrals : -2.47 eV for the shorter and -1.90 eV for the longer bond.…”