Quasilinear spectra of para-substituted stilbenes at 77�K

Abstract

Spectra of Para-Halogen Substituted Stilbenes. The stilbene molecule possesses a unified system of 14 Tr-eteetrons and is the first member of the diphenylpolyene series. The spectral properties of stilbene vapor [1], solutions [2][3][4][5], and crystals [6, 7], and also of stilbene impurities in various crystalline solids [6, 8] over a wide temperature range have been the subject of numerous investigations.Use of the quasilinear spectra method [9], which permits highly accurate measurements of the O -O transition frequency and the molecular frequencies of complex organic compounds to be made not only in the ground (So) but also in the excited ($1) states, makes it possible to consider the detailed vibrational structure of the stilbene molecule [10, 11], as well as those of some other members of the diphenylpolyene series [10] in the S o and S~ states. Introduction of a substituent causes perturbation of the v-electron cloud, changes the state of the sigma bonds, and increases the molecular mass. In an earlier study [5] absorption and fluorescence spectra were measured at room temperature of heptane and toluene solutions of various stilbene derivatives. However the diffuse nature of the spectra only allowed an approximate estimation of the shift of the band maxima to be made. Investigation of the absorption anf fluorescence spectra of stilbene and of its para-, mono-F-, CI-, and Br--derivatives present as impurities in dibenzyl at 20~K [8] showed that under these eonitions only the spectra of stilbene and 4-F-stilbene were sufficiently sharp for carrying out vibrational analysis.Quasilinear electronic-vibrational spectra of molecules are able to yield information regarding changes in the first electronic transition energy (O-O frequencies), molecular vibrational frequencies in the ground and excited states, caused by introduction of a substituent and depending on the properties of the original molecule and of the substituent. The quasilinear spectra method is particularly sensitive to different intramolecular interactions [12]. In the present work this method has been used to study the effect of a substituent in the para-positionof the stilbene molecule, on the frequency of the first electronic transition and the molecular frequencies in the ground and first excited states. The accuracy of the frequency determinations for most of the spectral lines (~ S0~) was 5-10 cm -~. The frequency interval ~ 200 cm -~ combining with the other frequencies which appear as overtones, was determined with an accuracy of 2-5 cm -1.We have carried out measurements of the absorption and fluorescence spectra of different n-alkane (from hexaae to undecane) solutions of 4-F-, 4-C1-, 4,4' -diCl-, 4-Br-, and 4-I-derivatives of stilbene at 77~The concentrations of the solutions were ~10 -4 g/ml.The spectra were recorded on a DFS-8 spectrograph with an inverse linear dispersion of 6.25 A/mm. For fluorescence excitation the ~313 nm region of the DKSSh-1000 lamp emission was used. The same lamp was used for measuring the absorption spect...

Structural Effects on the Isomerization Dynamics of trans-Stilbenes: IVR, Microcanonical Reaction Rates, and the Nature of the Transition State

“…The low-frequency modes ν 24 , ν 25 , ν 36 , and ν 37 in S 1 are similar for the two conformers, with all but ν 37 being somewhat smaller than for TS, as expected from inertial considerations for modes entailing motion of the para ring position. The displacement of 25 0 1 to lower frequencies upon methoxy substitution in comparison with TS is in agreement with early work on MS in solution. , …”

“…The change from TS is again qualitatively consistent with that observed in the fluorescence of MS and DMS in solution. 54,55 Similarly, from inertial considerations and the involvement of the para position in these motions, a successive lowering can be expected in MS and DMS of ν 24 ′′, ν 72 ′′, and ν 36 ′′ from their values in TS. The assignments in Tables 4 and 5 have been made accordingly (see Table 2).…”

“…The displacement of 25 0 1 to lower frequencies upon methoxy substitution in comparison with TS is in agreement with early work on MS in solution. 54,55 The assignment of the 161 cm -1 band of syn-MS as 72 0 1 , however, appears less certain. In TS, the calculated value of ν 72 ′ is slightly below 100 cm -1 , 16,50,56 and a frequency of 200 cm -1 has been assigned to 72 0 2 based on experimental isotopic shifts.…”