Theoretical and experimental studies of the opto-electronic properties of positively charged oligo(phenylene vinylene)s: Effects of chain length and alkoxy substitution

Abstract

In this paper a combined experimental and quantum chemical study of the geometry and opto-electronic properties of unsubstituted and dialkoxy-sustituted phenylene-vinylene oligomers ͑PV's͒ is presented. The optical absorption spectra for PV cations with different chain lengths and substitution patterns were measured using pulse radiolysis with time-resolved spectrophotometric detection from 1380 to 500 nm ͑0.9 to 2.5 eV͒. The geometries of the PV's studied were optimized using density functional theory ͑DFT͒ for both the neutral and singly charged molecule. The spectra for the PV radical cations were then calculated using singly excited configuration interaction with an intermediate neglect of differential overlap reference wave function method together with the DFT geometry. The agreement between experimental and theoretical absorption energies is excellent; most of the calculated radical cation absorption energies are within 0.15 eV of the experimental values. The pattern of dialkoxy-substitution is found to have a large effect on the optical absorption spectrum of the cation. Using the calculated charge distribution it is shown that the degree of delocalization of the charge correlates with the energy of the lowest absorption band. If alkoxy side chains are present on some of the rings the positive charge tends to localize at those sites.

Charge transport along phenylenevinylene molecular wires

“…The influence of alkoxy substituents on the charge transfer integrals between the LUMOs is smaller than found for the HOMO orbitals. This is consistent with previous calculations where it was shown that the presence of alkoxy substituents has a considerable localizing effect on positive charges on PV chains, [43] whereas the effect on the distribution of a negative charge is much smaller. …”

Polarons in semiconducting polymers: Study within an extended Holstein model

“…4 Similar observations were made for oligo͑phenylenevinylenes͒ ͑OPVs͒. 5,7 In stark contrast to this, calculations for oligomers based on ͑parametrizations of͒ Hartree-Fock ͑HF͒ show a clear localization of the charge in the middle of the oligomer. 2,[4][5][6][7] All DFT calculations for charged oligomers mentioned earlier were based on the local density approximation ͑LDA͒, some including the generalized gradient approximation ͑GGA͒.…”

“…1 Within the framework of ab initio electronic structure theory, studies of self-trapped carriers have been performed by taking a finite polymer segment ͑oligomer͒ of N units and analyzing the change of the atomic equilibrium configuration upon charging the system, for large N. [2][3][4][5][6][7] The general conclusion drawn from such calculations based on the density-functional theory ͑DFT͒ is that no clear self-trapping, which should always occur in a onedimensional system, 8 is observed for the studied oligomers of computationally feasible sizes. [3][4][5]7 In Ref. 3 oligothiophenes were studied up to 16 thiophene units.…”

Theoretical Investigation of Excited States of Oligothiophene Anions

“…This was confirmed with theoretical investigations at various semiempirical [36][37][38][39] and ab initio levels, 34,40 and with density functional theory (DFT). [41][42][43] DFT orbital energy plots do not show symmetrically positioned energy levels in the gap, and the electronic transitions are only in some cases dominated by a single electron configuration. 43 Since the polaron-bipolaron model and diagrams like that in Scheme 1 are therefore confirmed neither by experiment nor by theory, the question arises of how electron-hole symmetry can be rationalized in PT.…”