We explored the excited-state interactions of bimolecular, non-covalent systems consisting of cationic poly [(9,9-di(3,3'-N,N'-trimethyl-ammonium) propyl fluorenyl-2,7-diyl)-alt-co-(9,9-dioctyl-fluorenyl-2,7-diyl)] diiodide salt (PFN) and 1,4-dicyanobenzene (DCB) using steady-
A series of π-conjugated oligomer-acceptor dyads were synthesized that feature oligo(phenylene ethynylene) (OPE) conjugated backbones end-capped with a naphthalene diimide (NDI) acceptor. The OPE segments vary in length from 4 to 8 phenylene ethynene units (PEn-NDI, where n = 4, 6 and 8). Fluorescence and transient absorption spectroscopy reveals that intramolecular OPE → NDI charge transfer dominates the deactivation of excited states of the PEn-NDI oligomers. Both charge separation (CS) and charge recombination (CR) are strongly exothermic (ΔG ∼ -1.1 and ΔG ∼ -2.0 eV), and the driving forces do not vary much across the series because the oxidation and reduction potentials and singlet energies of the OPEs do not vary much with their length. Bimolecular photoinduced charge transfer between model OPEs that do not contain the NDI acceptors with methyl viologen was studied, and the results reveal that the absorption of the cation radical state (OPE) remains approximately constant (λ ∼ 575 nm) regardless of oligomer length. This finding suggests that the cation radical (polaron) of the OPE is relatively localized, effectively occupying a confined segment of n ≤ 4 repeat units in the longer oligomers. Photoinduced intramolecular electron transfer dynamics in the PEn-NDI series was investigated by UV-visible femtosecond transient absorption spectroscopy with visible and mid-infrared probes. Charge separation occurs on the 1-10 ps time scale with the rates decreasing slightly with increased oligomer length (β ∼ 0.15 Å). The rate for charge-recombination decreases in the sequence PE4-NDI > PE6-NDI ∼ PE8-NDI. The discontinuous distance dependence in the rate for charge recombination may be related to the spatial localization of the positive polaron state in the longer oligomers.
Controlling
the ultrafast dynamical process of photoinduced charge transfer at
donor–acceptor interfaces remains a major challenge for physical
chemistry and solar cell communities. The process is complicated by
the involvement of other complex dynamical processes, including hydrogen
bond formation, energy transfer, and solvation dynamics occurring
on similar time scales. In this study, we explore the remarkable impact
of hydrogen-bond formation on the interfacial charge transfer between
a negatively charged electron donating anionic porphyrin and a positively
charged electron accepting π-conjugated polymer, as a model
system in solvents with different polarities and capabilities for
hydrogen bonding using femtosecond transient absorption spectroscopy.
Unlike the conventional understanding of the key role of hydrogen
bonding in promoting the charge-transfer process, our steady-state
and time-resolved results reveal that the intervening hydrogen-bonding
environment and, consequently, the probable longer spacing between
the donor and acceptor molecules significantly hinders the charge-transfer
process between them. These results show that site-specific hydrogen
bonding and geometric considerations between donor and acceptor can
be exploited to control both the charge-transfer dynamics and its
efficiency not only at donor–acceptor interfaces but also in
complex biological systems.
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