We demonstrate that the 10-phenyl-10 H-phenothiazine radical cation (PTZ) has a manifold of excited doublet states accessible using visible and near-infrared light that can serve as super-photooxidants with excited-state potentials is excess of +2.1 V vs SCE to power energy demanding oxidation reactions. Photoexcitation of PTZ in CHCN with a 517 nm laser pulse populates a D electronically excited doublet state that decays first to the unrelaxed lowest electronic excited state, D' (τ < 0.3 ps), followed by relaxation to D (τ = 10.9 ± 0.4 ps), which finally decays to D (τ = 32.3 ± 0.8 ps). D' can also be populated directly using a lower energy 900 nm laser pulse, which results in a longer D'→D relaxation time (τ = 19 ± 2 ps). To probe the oxidative power of PTZ photoexcited doublet states, PTZ was covalently linked to each of three hole acceptors, perylene (Per), 9,10-diphenylanthracene (DPA), and 10-phenyl-9-anthracenecarbonitrile (ACN), which have oxidation potentials of 1.04, 1.27, and 1.6 V vs SCE, respectively. In all three cases, photoexcitation wavelength dependent ultrafast hole transfer occurs from D, D', or D of PTZ to Per, DPA, and ACN. The ability to take advantage of the additional oxidative power provided by the upper excited doublet states of PTZ will enable applications using this chromophore as a super-oxidant for energy-demanding reactions.

Singlet fission (SF) converts a singlet exciton into two triplet excitons in two or more electronically coupled organic chromophores, which may then be used to increase solar cell efficiency. Many known SF chromophores are unsuitable for device applications due to chemical instability or low triplet state energies. The results described here show that efficient SF occurs in derivatives of 9,10-bis(phenylethynyl)anthracene (BPEA), which is a highly robust and tunable chromophore. Fluoro and methoxy substituents at the 4- and 4′-positions of the BPEA phenyl groups control the intermolecular packing in the crystal structure, which alters the interchromophore electronic coupling, while also changing the SF energetics. The lowest excited singlet state (S1) energy of 4,4′-difluoro-BPEA is higher than that of BPEA so that the increased thermodynamic favorability of SF results in a (16 ± 2 ps)−1 SF rate and a 180% ± 16% triplet yield, which is about an order of magnitude faster than BPEA with a comparable triplet yield. By contrast, 4-fluoro-4′-methoxy-BPEA and 4,4′-dimethoxy-BPEA have slower SF rates, (90 ± 20 ps)−1 and (120 ± 10 ps)−1, and lower triplet yields, (110 ± 4)% and (168 ± 7)%, respectively, than 4,4′-difluoro-BPEA. These differences are attributed to changes in the crystal structure controlling interchromophore electronic coupling as well as SF energetics in these polycrystalline solids.

Strongly oxidizing photosensitizers (superoxidants) based on organic radical cations are capable of driving energy-demanding reactions using low-energy photons. Here, we show that the peri-xanthenoxanthene radical cation (PXX+•) has an electronic excited state (D1) with a τ = 124 ps lifetime in CH3CN at 295 K. Photoexcitation of PXX+• covalently attached to electron deficient 9,10-bis­(trifluoromethyl)­anthracene (TMFA) using an 885 nm laser pulse drives oxidation of TFMA with unity quantum yield. Extending the PXX+•-TFMA dyad to a molecular triad having a 9,10-diphenylanthracene terminal hole acceptor, PXX+•-TFMA-DPA, and selectively exciting PXX+• results in formation of PXX-TFMA-DPA+• with a 46% quantum yield and a τ = 11.5 ± 0.6 ns lifetime. This work demonstrates that the PXX+• D1 electronic excited state can serve as a promising superoxidant for challenging oxidation reactions relevant to solar-energy applications.

This paper describes spectroscopic evidence for the photoinduced transfer of a hole from the biexcitonic state of a CdS quantum dot (QD) to a phenothiazine (PTZ) molecular acceptor, covalently linked to the QD through phenyldithiocarbamate (PTC), with power-dependent yields of 8–21%. Visible and near-infrared transient absorption spectroscopy (TA) data suggest that the mechanisms of hole extraction include direct hole transfer from the QD’s valence band to PTZ in 2.4 ± 0.2 ps, or trapping of holes at the QD surface in ∼1 ps, followed by sequential hole transfer to PTZ. Both of these mechanisms potentially out-compete Auger recombination of biexcitonic states, which occurs within these QDs in 20 ± 1 ps. These results suggest that the PTC linkage will be useful for extracting multiple holes from a QD photosensitizer or solo photocatalyst to drive multistep oxidation reactions.