The relative energies of stationary points on the potential energy surface of trans-stilbene have been accurately determined using Hartree–Fock, second and third-order Møller–Plesset (MP2, MP3), as well as Coupled Clusters theories with single and double excitations (CCSD), together with a perturbative estimate of connected triple excitations [CCSD(T)], in conjunction with basis sets of increasing size, containing up to 1130 basis functions. A focal point analysis has been carried out in order to determine how the energy differences and rotational barriers approach convergence, enabling extrapolation of the CCSD(T) results to a near-complete basis set. The investigated saddle points pertain to independent rotations of the phenyl rings about the single C–C bond, and to pedalling motions described by a twofold rotation of the central ethylene bond about the longitudinal axis of the molecule. The benchmark calculations presented in this study lead to the conclusion that, in the nonrelativistic limit and within the frozen core approximation, trans-stilbene in vacuum is a strictly planar molecule in its absolute energy minimum form, in sharp contrast with many previous theoretical studies. This point has been ultimately confirmed by an MP2 geometry optimization using the aug-cc-pVDZ basis set. At last, the energy of cis-stilbene relative to the trans-isomer is accurately evaluated.

No abstract

In the present study, we investigate the influence of temperature on the width and location of bands in the UV-visible absorption spectra of oligomer chains of poly(p-phenylene vinylene) (PPV). These spectra have been computed by means of molecular dynamics (MD) simulations along with the classical MM3 force field, in conjunction with (Z)INDO/S-CIS calculations of vertical excitation energies and transition moments. In addition, the MD(MM3) computations enable consistent insights into the average structures of cis-and transstilbene, at temperatures ranging from 0 to 500 K. For trans-stilbene and larger PV-n oligomer chains converging to PPV, thermal motions at room temperature in the vacuum result in a constant broadening of bands by 24 nm (0.20 eV) at half the maximum. Compared with the trans-isomer, thermal broadening intensifies to 33 nm (0.44 eV) for cis-stilbene, due to enhanced steric effects. When accounting in addition for the outcome of vibronic broadening, the width of the first absorption band increases at 300 K to 33 nm (0.48 eV) for trans-stilbene and ∼46 nm (0.40 eV) for the largest PV-n chains. At last, upon studying a large molecular cluster reproducing the crystalline structure of trans-stilbene, it has been found that, although they strongly impede the rotations of phenyl rings and out-of-plane distortions, packing effects and steric hindrances in the solid phase lead to a limitation of the width of the first absorption band by 6 nm (0.07 eV) only at 300 K.

ABSTRACT:The ultraviolet (UV)-Visible absorption spectrum of trans-stilbene (tS) is computed at different temperatures by couplin g molecular dynamic s (MD ) simulations with the classical MM 3 force fiel d to ZINDO/S-CI S calculation s of vertical excitation energies and transition dipol e moments. The selection of a large number of structures along the MDtrajectories enables a consistent treatment of temperature effects in the vacuum , whereas the ZINDO/S-CI S calculation s permit a reliable treatment of electron correlation and relaxation, taking account of multistat e interactions in the fina l state. Thermal motions are found to alter very differentl ythe width and shape of bands. Structural alterations such as the stretching and the torsion of the vinyl single and double bonds very stronglyinfluenc e the appearance of the fi rst valence state, pertaining to the highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO ) transition. At temperatures less than 400 K, these are found to yield a merelyGaussian and very pronounce d thermal broadenin g of the related band (A), up to nearly 30 nm, together with a minor blue shift of its maximum λ max . In contrast, a red shift by several nanometers occurs due to thermal motions for the remainin g three valence bands. As can be expected, the broadenin g intensifie s at higher temperatures, and for the A-band , becomes markedly asymmetric when T exceeds 400 K. The combinatio n of MD(MM3 ) and ZINDO/S-CIS computation s enables also consistent calculation s of hot bands, which are forbidden by symmetryat 0 K.

No abstract