The photochemical dynamics of three classes of organic photoredox catalysts employed in organocatalyzed atom-transfer radical polymerization (O-ATRP) are studied using time-resolved optical transient absorption and fluorescence spectroscopies. The nine catalysts selected for study are examples of N-aryl and core-substituted dihydrophenazine, phenoxazine and phenothiazine compounds with varying propensities for control of polymerization outcomes. Excited singlet state lifetimes extracted from the spectroscopic measurements are reported in N,Ndimethylformamide (DMF), dichloromethane (DCM) and toluene. Ultrafast (< 200 fs to 3 ps) electronic relaxation of the photocatalysts after photoexcitation at near-UV wavelengths (318-390 nm) populates the first singlet excited state (S1). The S1-state lifetimes range from 130 ps to 40 ns with considerable dependence on the photocatalyst structure and the solvent. Competition between ground-electronic state recovery and intersystem crossing controls triplet state populations and is a minor pathway in the dihydrophenazine derivatives, but is of greater importance for phenoxazine and phenothiazine catalysts. Comparison of our results with previously reported O-ATRP performances of the various photoredox catalysis shows that high triplet-state quantum yields are not a pre-requisite for controlling polymer dispersity. For example, the 5,10-di(4-cyanophenyl)-5,10-dihydrophenazine photocatalyst, shown previously to exert good polymerization control, possesses the shortest S1-state lifetime (135 ps in DMF and 180 ps in N,N-dimethylacetamide) among the nine examples reported here, and a negligible triplet state quantum yield. The results call for a re-evaluation of the excited state properties of most significance in governing the photocatalytic behaviour of organic photoredox catalysts in O-ATRP reactions.
2D electronic spectroscopy maps acquired in different configurations unveil intraband hot carrier cooling and interband multi-exciton recombination dynamics.
The ultrafast dynamics of a bimolecular excited state proton transfer (ESPT) reaction between the photoacid 7-hydroxy-4-(trifluoromethyl)-1-coumarin (CouOH) and 1-methylimidazole (MI) base in aprotic chloroform-d1 solution were investigated using ultrafast transient infrared (TRIR) and transient absorption (TA) spectroscopies. The excited state lifetime of the photoacid in solution is relatively short (52 ps) which at the millimolar photoacid and base concentrations used in our study precludes any diffusion-controlled bimolecular ESPT reactions. This allows the prompt ESPT reaction between hydrogen bonded CouOH and MI molecules to be studied in isolation, and the 'contact' ESPT dynamics to be unambiguously determined. Our time resolved studies reveal ultrafast ESPT from the CouOH moiety to hydrogen bonded MI molecules occurs within ~1 ps, tracked by unequivocal spectroscopic signatures of CouO-* photoproducts which are formed in tandem with HMI +. Some of the ESPT photoproducts subsequently p-stack to form exciplexes on a ~35 ps timescale, minimizing the attractive Coulombic forces between the oppositely charged aromatic molecules. For the concentrations of CouOH and MI used in our study (up to 8 mM), we saw no evidence for excited state tautomerization of coumarin anions.
Reaction
centers (RCs) are the pivotal component of natural photosystems,
converting solar energy into the potential difference between separated
electrons and holes that is used to power much of biology. RCs from
anoxygenic purple photosynthetic bacteria such as Rhodobacter
sphaeroides only weakly absorb much of the visible region
of the solar spectrum, which limits their overall light-harvesting
capacity. For in vitro applications such as biohybrid
photodevices, this deficiency can be addressed by effectively coupling
RCs with synthetic light-harvesting materials. Here, we studied the
time scale and efficiency of Förster resonance energy transfer
(FRET) in a nanoconjugate assembled from a synthetic quantum dot (QD)
antenna and a tailored RC engineered to be fluorescent. Time-correlated
single-photon counting spectroscopy of biohybrid conjugates enabled
the direct determination of FRET from QDs to attached RCs on a time
scale of 26.6 ± 0.1 ns and with a high efficiency of 0.75 ±
0.01.
Diketopyrrolopyrroles are a popular class of electron-withdrawing unit in optoelectronic materials. When combined with electron donating side-chain functional groups such as thiophenes, they form a very broad class of donor-acceptor...
Reaction centers (RCs) are the pivotal component of natural photosystems, converting solar energy into the potential difference between separated electrons and holes that is used to power much of biology. RCs from anoxygenic purple photosynthetic bacteria such as Rhodobacter sphaeroides only weakly absorb much of the visible region of the solar spectrum which limits their overall light-harvesting capacity. For in vitro applications such as bio-hybrid photodevices this deficiency can be addressed by effectively coupling RCs with synthetic light-harvesting materials. Here, we studied the time scale and efficiency of Förster resonance energy transfer (FRET) in a nanoconjugate assembled from a synthetic quantum dot (QD) antenna and a tailored RC engineered to be fluorescent. Time-correlated single photon counting spectroscopy of biohybrid conjugates enabled the direct determination of FRET from QDs to attached RCs on a time scale of 26.6 ± 0.1 ns and with a high efficiency of 0.75 ± 0.01.
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