Effect of Thermal Motions on the Structure and UV-Visible Electronic Spectra of Stilbene and Model Oligomers of Poly(p-Phenylene Vinylene)

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 trans-stilbene, 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.