The electron–hole recombination kinetics of organic photovoltaics (OPVs) are known to be sensitive to the relative energies of triplet and charge‐transfer (CT) states. Yet, the role of exciton spin in systems having CT states above 1.7 eV—like those in near‐ultraviolet‐harvesting OPVs—has largely not been investigated. Here, aggregation‐induced room‐temperature intersystem crossing (ISC) to facilitate exciton harvesting in OPVs having CT states as high as 2.3 eV and open‐circuit voltages exceeding 1.6 V is reported. Triplet excimers from energy‐band splitting result in ultrafast CT and charge separation with nonradiative energy losses of <250 meV, suggesting that a 0.1 eV driving force is sufficient for charge separation, with entropic gain via CT state delocalization being the main driver for exciton dissociation and generation of free charges. This finding can inform engineering of next‐generation active materials and films for near‐ultraviolet OPVs with open‐circuit voltages exceeding 2 V. Contrary to general belief, this work reveals that exclusive and efficient ISC need not require heavy‐atom‐containing active materials. Molecular aggregation through thin‐film processing provides an alternative route to accessing 100% triplet states on photoexcitation.
Attaining long‐lived charge‐transfer (CT) states is of the utmost importance for energy science, photocatalysis, and materials engineering. When charge separation (CS) is slower than consequent charge recombination (CR), formation of a CT state is not apparent, yet the CT process provides parallel pathways for deactivation of electronically excited systems. The nuclear, or Franck‐Condon (FC), contributions to the CT kinetics, as implemented by various formalisms based on the Marcus transition‐state theory, provide an excellent platform for designing systems that produce long‐lived CT states. Such approaches, however, tend to underestimate the complexity of alternative parameters that govern CT kinetics. Here we show a comparative analysis of two systems that have quite similar FC CT characteristics but manifest distinctly different CT kinetics. A decrease in the donor‐acceptor electronic coupling during the charge‐separation step provides an alternative route for slowing down undesired charge recombination. These examples suggest that, while infrequently reported and discussed, cases where CR is faster than CS are not necessarily rare occurrences.
more » « less- Award ID(s):
- 1800602
- NSF-PAR ID:
- 10460627
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- International Journal of Chemical Kinetics
- Volume:
- 51
- Issue:
- 9
- ISSN:
- 0538-8066
- Page Range / eLocation ID:
- p. 657-668
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Electron donor–acceptor (DA) hybrids comprised of single‐wall carbon nanotubes (SWCNTs) are promising functional materials for light energy conversion. However, the DA hybrids built on SWCNTs have failed to reveal the much‐sought long‐lived charge separation (CS) due to the close proximity of the DA entities facilitating charge recombination. Here, we address this issue and report an elegant strategy to build multi‐modular DA hybrids capable of producing long‐lived CS states. For this, a nano tweezer featuring V‐shape configured BODIPY was synthesized to host SWCNTs of different diameters via π‐stacking. Supported by spectral, electrochemical, and computational studies, the established energy scheme revealed the possibility of sequential electron transfer. Systematic pump‐probe studies covering wide spatial and temporal scales provided evidence of CS from the initial1BODIPY* ultimately resulting in C60⋅−‐BODIPY‐SWCNT⋅
+ CS states of lifetimes in the 20‐microsecond range. -
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Abstract Electron donor–acceptor (DA) hybrids comprised of single‐wall carbon nanotubes (SWCNTs) are promising functional materials for light energy conversion. However, the DA hybrids built on SWCNTs have failed to reveal the much‐sought long‐lived charge separation (CS) due to the close proximity of the DA entities facilitating charge recombination. Here, we address this issue and report an elegant strategy to build multi‐modular DA hybrids capable of producing long‐lived CS states. For this, a nano tweezer featuring V‐shape configured BODIPY was synthesized to host SWCNTs of different diameters via π‐stacking. Supported by spectral, electrochemical, and computational studies, the established energy scheme revealed the possibility of sequential electron transfer. Systematic pump‐probe studies covering wide spatial and temporal scales provided evidence of CS from the initial1BODIPY* ultimately resulting in C60⋅−‐BODIPY‐SWCNT⋅
+ CS states of lifetimes in the 20‐microsecond range. -
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Abstract A far‐red absorbing sensitizer, BF2‐chelated azadipyrromethane (azaBODIPY) has been employed as an electron acceptor to synthesize a series of push‐pull systems linked with different nitrogenous electron donors, viz.,
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Abstract A panchromatic triad, consisting of benzothiazole (BTZ) and BF2‐chelated boron‐dipyrromethene (BODIPY) moieties covalently linked to a zinc porphyrin (ZnP) core, has been synthesized and systematically characterized by using1H NMR spectroscopy, ESI‐MS, UV‐visible, steady‐state fluorescence, electrochemical, and femtosecond transient absorption techniques. The absorption band of the triad, BTZ‐BODIPY‐ZnP, and dyads, BTZ‐BODIPY and BODIPY‐ZnP, along with the reference compounds BTZ‐OMe, BODIPY‐OMe, and ZnP‐OMe exhibited characteristic bands corresponding to individual chromophores. Electrochemical measurements on BTZ‐BODIPY‐ZnP exhibited redox behavior similar to that of the reference compounds. Upon selective excitation of BTZ (≈290 nm) in the BTZ‐BODIPY‐ZnP triad, the fluorescence of the BTZ moiety is quenched, due to photoinduced energy transfer (PEnT) from1BTZ*to the BODIPY moiety, followed by quenching of the BODIPY emission due to sequential PEnT from the1BODIPY* moiety to ZnP, resulting in the appearance of the ZnP emission, indicating the occurrence of a two‐step singlet–singlet energy transfer. Further, a supramolecular tetrad, BTZ‐BODIPY‐ZnP:ImC60, was formed by axially coordinating the triad with imidazole‐appended fulleropyrrolidine (ImC60), and parallel steady‐state measurements displayed the diminished emission of ZnP, which clearly indicated the occurrence of photoinduced electron transfer (PET) from1ZnP* to ImC60. Finally, femtosecond transient absorption spectral studies provided evidence for the sequential occurrence of PEnT and PET events, namely,1BTZ*‐BODIPY‐ZnP:ImC60→BTZ‐1BODIPY*‐ZnP:ImC60→BTZ‐BODIPY‐1ZnP*:ImC60→BTZ‐BODIPY‐ZnP.+:ImC60.−in the supramolecular tetrad. The evaluated rate of energy transfer,
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