Third-Order Nonlinear Optical Properties of Oligomers of Thienyleneethynylenes and Thienylenevinylene

Abstract

Third-order optical nonlinear properties of four thienyleneethynylenes and two thienylenevinylenes containing from two to five thiophene units were measured. Third-order nonlinear susceptibilities x (~) ( -3w;o,o,o) at the fundamental wavelength of 1064 nm were measured by third harmonic generation (THG) for thin films of the oligomers in poly(methy1 methacrylate) (PMMA) using the Maker fringe technique. The data were reduced to the equivalent molecular second hyperpolarizabilities y ( -3w;w,o,w). Theoretical calculations of the static (zero-frequency) ( y ) values were performed at the semiempirical Austin model one/finite field (AM1/ FF) and intermediate neglect of differential overlap/single and double configuration interaction (INDO/SDCI) sum-over-states (SOS) levels and at the nonempirical valence effective Hamiltonian (VEH)-SOS level. The evolution of y as a function of the torsion angle between the thiophene rings was investigated via the AM1/ FF method. With the INDO-SOS technique, the frequency dependence of y( -3o;o,o,w) was also analyzed. For the ethynylenes, the experimental THG values of y lie between 63 x and 2300 x esu and increase with the number of heterocycle units of the oligomers. These values are affected by three-photon resonance. The resonant THG y-values of the ethynylenes lie within 10% of the corresponding values of the vinylenes. After a simple correction for three-photon resonance, the dependence of y-values on the number of n-electrons is fitted to power laws and compared with the theoretical calculations. The theoretical static y-values indicate a higher response in the vinylene derivatives than in the corresponding ethynylene derivatives. However, the experimental THG results provide the reverse trend: this can be explained by a stronger resonance enhancement of the THG y-values in the ethynylene compounds, as is demonstrated by the results of the frequency-dependent calculations.

Theoretical study of the size confinement effect in linear π-conjugated oligomers

“…Fig. 3, left panel, shows the dependence of the maximum of the p-p * absorption band on the size of the OTVs (dots) [8]. The solid line represents the theoretical curve based on Eq.…”

“…Typically, the pp * absorption band undergoes a progressive red shift with increasing length of the conjugated oligomer measured by the number of monomer repeat units. Several series of conjugated oligomers consisting of repeat units based on thiophenes, phenylenes or mixed building blocks (in the following also denoted as monomers) have been synthesized and their photophysical properties analyzed [4][5][6][7][8][9][10][11][12][13]. A general non-linear dependence of the absorption energy on the number of repeat units is observed for all these systems.…”

“…The dots mark the experimental data taken from the literature[8], while the solid line connecting the dots represents the theoretical curve based on Eq. (10) (k = 1, and ''-'') with x 1 = 5.65 eV (thiophene chromophor), x 2 = 10.82 eV (vinylene chromophor), and J = 2.69 eV, R 2 = 0.9980.…”

Effective conjugation length and UV/vis spectra of oligomers

“…1ene)s 2 [13-151 and 3 [24] and the oligo(2,5-thienyle-neetheny1ene)s 4 [25] in comparison to 1 [26]. Irrespective of the principal convergence caused by the perturbation, which was interpreted at the beginning as a result of the alternating bonds (or building blocks), it is obvious that the mode of convergence can be quite different.…”

Electrochemical synthesis and characterization of the electronic structure of new conjugated polymers from dithienylene vinylene derivatives

“…1(b)) also shows a fine structure, as already observed by Geisleretal. [18].Based on the above-mentioned arguments, we also propose it to correspond to a vibronic progression, in contrast to the previous interpretation [ 18]. The relative intensity of the components of the vibronic progression is related to the degree of geometrical relaxation of the molecules in the excited state.…”