Singlet and triplet excitations of chiral dialkoxy-p-phenylene vinylene oligomers

Abstract

The excited state properties of a series of ␣,-dimethyl-oligo͕2,5-bis͓2-(S)-methylbutoxy͔-p-phenylene vinylene͖s (OPVns, with n the number phenyl rings͒ are investigated for nϭ2 -7 in solution at ambient temperature, under matrix-isolated conditions at low temperature, and as nanoaggregates using absorption ͑time-resolved͒, photoluminescence, photoinduced absorption, circular dichroism, and circular polarized luminescence spectroscopies. The singlet (S 1 ←S 0 ) and triplet (T n ←T 1 ) transition energies decrease with conjugation length. For the S 1 state of OPVn the lifetime strongly decreases with chain length due to enhanced nonradiative decay and radiative decay. The increase in the nonradiative decay rate constant is much more pronounced, and as a result the photoluminescence quantum yield is less for longer oligomers. Studies at low temperature afforded spectra with well-resolved vibronic fine structure. Under these conditions the Stokes' shift is very small (р0.04 eV). The Huang-Rhys parameter and relaxation energies in the excited state associated with S 1 ←S 0 and T n ←T 1 transitions and their evolution with conjugation length have been determined via a Franck-Condon analysis of the vibronic fine structure. For OPV7, the relaxation energy of S 1 with respect to S 0 (0.165Ϯ0.02 eV) is considerably larger than for T n with respect to T 1 ͑0.03 eV͒, but the latter increases for the shorter oligomers, in contrast to the relaxation energy of S 1 . The OPVns (nу5) form nanoaggregates in polar solvents such as ethanol. The changes in absorption and emission together with a strong circular dichroism and circularly polarized emission indicate the formation of chiral H-type aggregates under these conditions.

Ultrafast depolarization of the fluorescence in a conjugated polymer

“…In alkoxy-substituted PPV oligo- mers, only those with four and more repeat units have significant absorption at the excitation photon energy of 2.9 eV. 34,35 This means that in the time-resolved PL experiment in the partially conjugated polymers the initially excited state must be delocalized over four or more repeat units. The distribution of conjugation lengths statistically is expected to peak at short length in partially conjugated polymers.…”

“…27 The position of the PL maximum in the partially conjugated polymers at 3 ps is at 2.25 eV, indicating that emission comes predominantly from the conjugated segments of Ͼ10 repeat units long, because shorter PPV oligomers show emission peaks at significantly higher photon energies. 34 PL depolarization in the partially conjugated polymers within 10 ps represents at least one excitation hop to a neighboring conjugated segment; thus, adding the lengths of the initially excited and emitting segments in the partially conjugated polymers, we estimate that the primary exciton in the fully conjugated MEH-PPV is delocalized over a minimum of 14 repeat units ͑ϳ110 Å͒. We note that our estimate is a lower limit, because partially conjugated polymers are expected to show kinks at the conjugation breaks and a single excitation hop between segments is likely to give a greater depolarization than exciton relaxation in the smoothly curved segment of the fully conjugated polymer.…”

Charge recombination in a poly(para-phenylene vinylene)-fullerene derivative composite film studied by transient, nonresonant, hole-burning spectroscopy

“…From studies on oligomer model compounds it is known that the ionization energy of PV oligomers decreases with increasing conjugation length. 18 Also the energy of the first singlet-excited state decreases with increasing oligomer length, 19 leading to a redshift of the lowest energy absorption band with increasing conjugation length. Explanation in terms of the dynamic Burstein-Moss shift 20 seems unlikely as no changes in hole position could be measured as a function of excitation density.…”

Propriedades ópticas de filmes finos de MEH-PPV preparados por “spin-coating” em diferentes velocidades de rotação.

“…Uma relação entre o fator Huang-rhys S e o grau de conjugação n é dado por: (HAGLEr et al, 1991;PEETErS et al, 2000) ( )…”