Ultrafast excited state absorption and charge separation in phenylene-based conjugated polymers

Abstract

Ultrafast pump-probe spectroscopy of phenylene-based -conjugated polymers give low and high energy photoinduced absorptions (PAs) to final states with very different relaxation behavior. While the low energy two-photon state that is reached in PA relaxes to the optical exciton in ultrafast times, the higher energy two-photon state violates the Vavilov-Kasha rule and instead of relaxing to the optical exciton undergoes interchain dissociation to a polaron pair state. We have calculated the excited state absorptions of long oligomers of poly-paraphenylene and poly-paraphenylenevinylene within a rigid band correlated electron Hamiltonian, using the complete -electron basis and the powerful multi-reference singles and double configuration interaction technique. Our calculated PA spectra well reproduce the experimental PA. We further show that there exists a fundamental difference in the natures of the wavefunctions of the final states that are reached in low and high energy PA, and the difference originates from the existence of multiple kinds of bands in phenylene-based systems within one-electron theory. We speculate that the violation of the Vavilov-Kasha rule and charge separation into polaron pairs upon excitation to the high energy two-photon state is a consequence of its specific electronic structure, and discuss an earlier work on wavelength dependent photoconductivity of poly-paraphenylenevinylene to support this conjecture.

Subgap Two-Photon States in Polycyclic Aromatic Hydrocarbons: Evidence for Strong Electron Correlations

“…On the other hand, lower order CI such as double-CI does not give the correct excited state ordering for large molecules. It is precisely for such large systems that the MRSDCI is used. , We have previously performed calculations as large as the ones reported here for quasi-1D molecules and oligomers. − …”

Ground and excited state nonlinear optical properties of poly(-para phenylene vinylene)

“…bands, in contrast the degenerate localized levels numbered 7,8,9 (14,15,16) form the localized band (cf. Fig.…”

“…It may be noted from earlier studies (cf. Table 1 in [16]) that excitons involving d 3 orbitals were found only in the kA g state and that too its contribution is very small (for higher oligomers than PPV4 this contribution is negligible, see below). Therefore one can safely use this kind of frozen basis set without any loss of generality so that higher oligomer lengths can be studied with such a powerful technique.…”

“…Such an effort was initiated earlier. Presence of these two different classes of two photon states were found by explicit theoretical calculations on phenyl based short oligomers [15,16]. Such different relaxation dynamics was attributed to their specific electronic nature -mA g states are superpositions of one electron-hole and two electron-hole excitations between the highest-delocalized valence-band and the lowest delocalized conduction-band states, but the kA g states have different two electron-hole contributions involving both delocalized and localized orbitals.…”

A Fortran 90 Hartree–Fock program for one-dimensional periodic π-conjugated systems using Pariser–Parr–Pople model

“…In recent years, our group and collaborators(author?) [28,29,30,31,32,33,34,35,36,37,38,39,40], along with numerous other groups(author?) [1,2,3,41,42,43,44,45,46], have used the P-P-P model to study the electronic structure and optical properties of conjugated molecules and oligomers.…”