Solvent-dependent photochemical dynamics of a phenoxazine-based photoredox catalyst

Abstract

<jats:title>Abstract</jats:title><jats:p>Organic substitutes for ruthenium and iridium complexes are increasingly finding applications in chemical syntheses involving photoredox catalysis. However, the performance of these organic compounds as electron-transfer photocatalysts depends on their accessible photochemical pathways and excited state lifetimes. Here, the UV-induced dynamics of N-phenyl phenoxazine, chosen as a prototypical N-aryl phenoxazine organic photoredox catalyst, are explored in three solvents, N,N-dimethyl formamide, dichloromethane and toluene, using ultrafast transient absorption spectroscopy. Quantum chemistry calculations reveal the locally excited or charge-transfer electronic character of the excited states, and are used to assign the transient electronic and vibrational bands observed. In toluene-d<jats:sub>8</jats:sub>, complete ground-state recovery is (31 ± 3) % by internal conversion (IC) from the photo-excited state (or from S<jats:sub>1</jats:sub> after IC but before complete vibrational relaxation), (13 ± 2) % via direct decay from vibrationally relaxed S<jats:sub>1</jats:sub> (most likely radiative decay, with an estimated radiative lifetime of 13 ns) and (56 ± 3) % via the T<jats:sub>1</jats:sub> state (with intersystem crossing (ISC) rate coefficient <jats:italic>k</jats:italic><jats:sub>ISC</jats:sub> = (3.3 ± 0.2) × 10<jats:sup>8</jats:sup> s<jats:sup>−1</jats:sup>). In dichloromethane, we find evidence for excited state N-phenyl phenoxazine reaction with the solvent. Excited state lifetimes, ISC rates, and ground-state recovery show only modest variation with changes to the solvent environment because of the locally excited character of the S<jats:sub>1</jats:sub> and T<jats:sub>1</jats:sub> states.</jats:p>

Singlet and Triplet Contributions to the Excited-State Activities of Dihydrophenazine, Phenoxazine, and Phenothiazine Organocatalysts Used in Atom Transfer Radical Polymerization

“…The TEAS spectra for PC-O3 are similar to those reported previously, with the prior PC-O3 study conducted in DMAc; 11,29 the bands of photoexcited PC-O3 observed with centres at 440 nm and 625 nm are assigned to ESA from the S1 CT-biphenyl state. Our observations of PC-O3 show rapid growth of these ESA features, with associated decay of a shoulder to the 625 nm band located around 540 nm (Figure S4 of Supporting Information).…”

“…The TEAS spectrum of PC-O1 in DMF clearly shows an intermediate triplet state developing on a nanosecond timescale, as the band assigned to S1 initially grows and then decays. 29 The early time growth of the S1 ESA feature (with time constant 14.8 ± 0.3 ps) is attributed to internal conversion after photoexcitation to a higher lying singlet state. Similar photodynamics are observed in the TEAS spectra of PC-O1 in DCM and in toluene.…”

“…(f) Minor contributions from a second time constant of 6.78 ± 0.26 ns in DMF and 6.10 ± 0.26 ns in DCM are also observed in the TCSPC experiments. (g) Time constants for PC-O1 were previously reported from our laboratory,29 and are added here for comparison. (h) Extends to 1.63 ± 0.03 µs with improved N2 purging.…”

Structure-Dependent Electron Transfer Rates for Dihydrophenazine, Phenoxazine, and Phenothiazine Photoredox Catalysts Employed in Atom Transfer Radical Polymerization

“…35,36 Second, transient vibrational and electronic absorption spectroscopy (TVAS and TEAS) measurements show that some of the LE-character catalysts can also undergo efficient ISC, although they are reported to be poor at controlling polymerization. 37 Although they display a range of photophysical attributes, catalysts with excited-state CT-character as a class seem to outperform their LE counterparts in controlling polymer dispersity. 23,38 To resolve why that is the case, both activation and deactivation ET steps must be examined.…”

Tuning the Excited-State Dynamics of Acetophenone Using Metal Ions in Solution

“…The excited-state dynamics of a photoactive organic molecule can be manipulated by various means. For example, addition of electron-withdrawing or -donating groups can reorder excited-state energy levels. ,, Recently, there has been growing awareness of the importance of the local chemical environment in regulating the overall effectiveness of photocatalysts. − Notably, Venkatraman and co-workers demonstrated that hydrogen bonding between benzophenone and the solvent environment can dramatically alter the photophysical properties of this prototypical photocatalyst . They were also able to demonstrate the preferential photoexcitation of benzophenone molecules transiently hydrogen-bonded to phenol co-solute molecules, thereby enhancing the rate of bimolecular reaction beyond the diffusional limit .…”