Photoinduced Electron Transfer in Naphthalene Diimide End-Capped Thiophene Oligomers

Abstract

A series of linear thiophene oligomers containing 4, 6, 8, 10, and 12 thienylene units were synthesized and end-capped with naphthalene diimide (NDI) acceptors with the objective to study the effect of oligomer length on the dynamics of photoinduced electron transfer and charge recombination. The synthetic work afforded a series of nonacceptor-substituted thiophene oligomers, T, and corresponding NDI end-capped series, TNDI (where n is the number of thienylene repeat units). This paper reports a complete photophysical characterization study of the T and TNDI series by using steady-state absorption, fluorescence, singlet oxygen sensitized emission, two-photon absorption, and nanosecond-microsecond transient absorption spectroscopy. The thermodynamics of photoinduced electron transfer and charge recombination in the TNDI oligomers were determined by analysis of photophysical and electrochemical data. Excitation of the T oligomers gives rise to efficient fluorescence and intersystem crossing to a triplet excited state that is easily observed by nanosecond transient absorption spectroscopy. Bimolecular photoinduced electron transfer from the triplet states, T*, to N,N-dimethylviologen (MV) occurs, and by using microsecond transient absorption it is possible to assign the visible region absorption spectra for the one electron oxidized (polaron) states, T. The fluorescence of the TNDI oligomers is quenched nearly quantitatively, and no long-lived transients are observed by nanosecond transient absorption. These findings suggest that rapid photoinduced electron transfer and charge recombination occurs, NDI-(T)*-NDI → NDI-(T)-NDI → NDI-T-NDI. Preliminary femtosecond-picosecond transient absorption studies on TNDI reveal that both forward electron transfer and charge recombination occur with k > 10 s, consistent with both reactions being nearly activationless. Analysis with semiclassical electron transfer theory suggests that both reactions occur at near the optimum driving force where -ΔG ∼ λ.

Control of Photoinduced Charge Separation in Conjugated Polyelectrolyte Complexes through Microstructure-Dependent Exciton Delocalization

“…Aromatic diimides are attractive electron acceptors for assemblies given their thermal stability and structural planarity that promote intermolecular stacking. ,,− The viability of PTAK and ENDI as a photoinduced electron donor–acceptor pair is based on the relative highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) energies of poly­(3-hexylthiophene-2,5-diyl) (P3HT) and NDI with quaternary ammonium chains: −4.9/–2.7 eV for P3HT and −7.56/–4.41 eV for NDI derivatives. , Hence, it is possible to induce electron transfer (and not energy transfer) to the acceptor through selective photoexcitation of PTAK. Charge transfer in NDI-capped thiophene oligomers and NDI-thiophene copolymers is well studied. − In end-capped oligomers, strong exothermic forward electron transfer is gated by exciton migration to the donor–acceptor “interface” and/or via torsional twisting of a phenylene bridge . In contrast, ENDI and PTAK interact supramolecularly such that the mechanism for electron transfer is more likely to originate from coupling between localized polymer and charge-transfer exciton states, similar to that observed in donor–acceptor films. − …”

Performance of TDDFT Vertical Excitation Energies of Core‐Substituted Naphthalene Diimides

“…In this paper we present a systematic TDDFT study of core‐substituted naphthalene diimides (cNDIs) which are well known for their tunable absorption and emission properties, ranging over the entire visible to near‐infrared spectrum . The parent NDI molecule has been extensively used as an electron acceptor exhibiting a high π‐acidity, as well as for electron transport and photoinduced CT applications . An accurate quantum computational approach is indispensable to gain a comprehensive understanding of these excited‐state processes.…”

Photophysics of Oligothiophenes End‐Capped with Platinum(II) Auxochromes

“…We have used this method in previous studies of π‐conjugated oligomers to access the cation radicals and probe them spectroscopically [45–48] . Solutions containing 3 and 4 with MV 2+ (c=1 mM) were excited at 355 nm using a Nd : YAG laser.…”