Intra- and Intermolecular Photoinduced Energy and Electron Transfer between Oligothienylenevinylenes and N-Methylfulleropyrrolidine

Abstract

The photophysical properties of a homologous series of oligothienylenevinylenes (nTVs) and the inter- and intramolecular photoinduced energy- and electron-transfer processes between an nTV as a donor and N-methylfulleropyrrolidine (MP−C60) as an acceptor are described as a function of oligomer length (n = 2, 3, 4, 6, 8, and 12) in apolar and polar solvents. Whereas the shorter oligomers fluoresce and have singlet excited-state [nTV(S1)] lifetimes of 280 ps (2TV) and 1360 ps (3TV), the S1 lifetimes of the longer oligomers (n > 3) are extremely short because of a fast thermal decay, resulting in negligible quantum yields for fluorescence and intersystem crossing to the triplet state [nTV(T1)]. Using photoinduced absorption (PIA) spectroscopy, we demonstrate, however, that the nTV(T1) state can be populated indirectly via intermolecular triplet energy transfer using photoexcited MP−C60 as a triplet sensitizer for n > 2. This enabled the spectral determination of the nTV (T n ← T1) transition as a function of chain length (n = 3−12) in toluene (ε = 2.38). In a more polar solvent, o-dichlorobenzene (ODCB, ε = 9.93), the MP−C60(T1) state acts both as an oxidizing agent toward the nTVs, resulting in the formation of a metastable radical ion pair (nTV+• + MP−C60 -•) for n > 2, and as a triplet sensitizer, to produce the nTV(T1) state. In addition to the intermolecular transfer reactions, we investigated the corresponding intramolecular photoinduced energy- and electron-transfer reactions in systems in which MP−C60 and nTV (n = 2−4) are covalently linked. The results indicate that, after photoexcitation of the nTV moiety, an ultrafast singlet energy transfer to MP−C60 occurs, followed by an intramolecular electron transfer. The intramolecularly charge-separated state is the lowest-energy excited state in polar media, e.g., ODCB. In apolar media, e.g., toluene, the formation of the intramolecularly charge-separated state occurs only for n > 2 and concurrent with fluorescence and intersystem crossing to the MP−C60(T1) state. The discrimination between energy and electron transfer is rationalized using a continuum model.

Identification of a triplet pair intermediate in singlet exciton fission in solution

“…S18-S21), resulting in a long-lived signature that closely matches the features observed under direct excitation (Fig. 4C, Bottom) (26,27). The remarkably sharp triplet absorption features we observe for the triplet exciton have been observed in a small set of other systems (18,28), where the rigidity of the molecule gives narrow absorption bandwidths, and are a distinct advantage of solution studies of acenes compared with the solid state.…”

Near-IR Broadband-Absorbing trans-Bisphosphine Pt(II) Bisacetylide Complexes: Preparation and Study of the Photophysics

“…This quenching effect of the fluorescence of the peripheral Bodipy ligand is attributed to energy transfer, since we proved with the Gibbs free energy changes (ΔG CS ) that intramolecular electron transfer is prohibited in Pt−1. 74 Same quenching effect was observed in toluene, as well as in dichloromethane (DCM), MeCN, and MeOH; thus, this quenching effect is unlikely due to polarity-enhanced ICT effect ( Figure S24, see Supporting Information). The EnT rate constants were estimated at k EnT = 9.0 × 10 9 s −1 using eq 5.…”

Generation and Recombination Kinetics of Optical Excitations in Poly(3-dodecylthienylenevinylene) with Controlled Regioregularity

“…34 We thus conclude that the dark exciton in PTV-CR is in fact the 2A g state, which has been extensively discussed in the literature in relation with the photophysics of π-conjugated polymers and oligomers. 16,33,34 Particularly, electroabsorption of PTV synthesized by thermal conversion of precursor polymer gave supportive evidence of the 2A g exciton being below 1B u . 35 As a matter of fact, it is no surprise that the lowest-lying exciton in PTV is the dark 2A g .…”

“…11À14 Electron transfer between these systems and C 60 fullerene molecules has been suggested, although a study in the ultrafast regime suggests this process may be quite inefficient. 15,16 It is currently unclear whether the low efficiencies observed in PTVbased solar cells to date are due to poor charge transfer or other unknown reasons, and more detailed optical and structural investigations need to be conducted to realize the potential of this polymer. 14,17,18 Regioregularity (RR) has been shown to improve the crystallinity of solution cast films of poly(alkylthiophenes) with respect to their regiorandom (RRa) counterparts.…”