Perspective: How can ultrafast laser spectroscopy inform the design of new organic photoredox catalysts for chemical and materials synthesis?

Advancing Organic Photoredox Catalysis: Mechanistic Insight through Time-Resolved Spectroscopy

The rapid development of light-activated organic photoredox catalysts has led to the proliferation of powerful synthetic chemical strategies with industrial and pharmaceutical applications. Despite the advancement in synthetic approaches, a detailed understanding of the mechanisms governing these reactions has lagged. Time-resolved optical spectroscopy provides a method to track organic photoredox catalysis processes and reveal the energy pathways that drive reaction mechanisms. These measurements are sensitive to key processes in organic photoredox catalysis such as charge or energy transfer, lifetimes of singlet or triplet states, and solvation dynamics. The sensitivity and specificity of ultrafast spectroscopic measurements can provide a new perspective on the mechanisms of these reactions, including electron-transfer events, the role of solvent, and the short lifetimes of radical intermediates.

Characterization of the Reversible Intersystem Crossing Dynamics of Organic Photocatalysts Using Transient Absorption Spectroscopy and Time-Resolved Fluorescence Spectroscopy

Thermally activated delayed fluorescence (TADF) emitters are molecules of interest as homogeneous organic photocatalysts (OPCs) for photoredox chemistry. Here, three classes of OPC candidates are studied in dichloromethane (DCM) or N,N-dimethylformamide (DMF) solutions, using transient absorption spectroscopy and time-resolved fluorescence spectroscopy. These OPCs are benzophenones with either carbazole (2Cz-BP and 2tCz-BP) or phenoxazine/phenothiazine (2PXZ-BP and 2PTZ-BP) appended groups and the dicyanobenzene derivative 4DP-IPN. Dual lifetimes of the S1 state populations are observed, consistent with reverse intersystem crossing (RISC) and TADF emission. Example fluorescence lifetimes in DCM are (5.18 ± 0.01) ns and (6.22 ± 1.27) μs for 2Cz-BP, (1.38 ± 0.01) ns and (0.32 ± 0.01) μs for 2PXZ-BP, and (2.97 ± 0.01) ns and (62.0 ± 5.8) μs for 4DP-IPN. From ground state bleach recoveries and time-correlated single photon counting measurements, triplet quantum yields in DCM are estimated to be 0.62 ± 0.16, 0.04 ± 0.01, and 0.83 ± 0.02 for 2Cz-BP, 2PXZ-BP, and 4DP-IPN, respectively. 4DP-IPN displays similar photophysical behavior to the previously studied OPC 4Cz-IPN. Independent of the choice of solvent, 4DP-IPN, 2Cz-BP, and 2tCz-BP are shown to be TADF emitters, whereas emission by 2PXZ-BP and 2PTZ-BP depends on the molecular environment, with TADF emission enhanced in aggregates compared to monomers. Behavior of this type is representative of aggregation-induced emission luminogens (AIEgens).

Photoredox-HAT Catalysis for Primary Amine α-C-H Alkylation: Mechanistic Insight with Transient Absorption Spectroscopy

The synergistic use of (organo)photoredox catalysts with hydrogen-atom transfer (HAT) cocatalysts has emerged as a powerful strategy for innate C(sp³)-H bond functionalization, particularly for C-H bonds α- to nitrogen. Azide ion (N₃^-) was recently identified as an effective HAT catalyst for the challenging α-C-H alkylation of unprotected, primary alkylamines, in combination with dicyanoarene photocatalysts such as 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN). Here, time-resolved transient absorption spectroscopy over sub-picosecond to microsecond timescales provides kinetic and mechanistic details of the photoredox catalytic cycle in acetonitrile solution. Direct observation of the electron transfer from N₃^- to photoexcited 4CzIPN reveals the participation of the S₁ excited electronic state of the organic photocatalyst as an electron acceptor, but the N₃^• radical product of this reaction is not observed. Instead, both time-resolved infrared and UV-visible spectroscopic measurements implicate rapid association of N₃^• with N₃^- (a favorable process in acetonitrile) to form the N₆^•- radical anion. Electronic structure calculations indicate that N₃^• is the active participant in the HAT reaction, suggesting a role for N₆^•- as a reservoir that regulates the concentration of N₃^•.

Potential-Dependent Temporal Dynamics of CO Surface Concentration in Electrocatalytic CO2 Reduction

Beyond the identity and structure of an intermediate, changes in its concentration on and near the electrode surface with time are a critical component to understand and improve selectivity and reactivity in electrochemical transformations. We apply pulsed-potential electrochemical Raman scattering microscopy to measure the potential-dependent temporal evolution of CO formed during electrocatalytic CO₂ reduction in acetonitrile on Ag electrodes. At driving potentials positive of the onset potential as determined by cyclic voltammetry, CO accumulates on the electrode surface at time scales longer than 1 s. Near the ensemble onset potential, CO resides on the electrode surface for approximately 100 ms. At potentials known to evolve CO from the electrode surface, CO remains adsorbed on the electrode for less than 10 ms. The time scales accessible in our strategy are nearly 3 orders of magnitude faster than transient Raman or infrared measurements, allowing direct measurement of the temporal evolution of intermediates.

Rational Design of Photocatalysts for Controlled Polymerization: Effect of Structures on Photocatalytic Activities

Over the past decade, the use of photocatalysts (PCs) in controlled polymerization has brought new opportunities in sophisticated macromolecular synthesis. However, the selection of PCs in these systems has been typically based on laborious trial-and-error strategies. To tackle this limitation, computer-guided rational design of PCs based on knowledge of structure-property-performance relationships has emerged. These rational strategies provide rapid and economic methodologies for tuning the performance and functionality of a polymerization system, thus providing further opportunities for polymer science. This review provides an overview of PCs employed in photocontrolled polymerization systems and summarizes their progression from early systems to the current state-of-the-art. Background theories on electronic transitions are also introduced to establish the structure-property-performance relationships from a perspective of quantum chemistry. Typical examples for each type of structure-property relationships are then presented to enlighten future design of PCs for photocontrolled polymerization.

Ultrafast transient absorption spectroelectrochemistry: femtosecond to nanosecond excited-state relaxation dynamics of the individual components of an anthraquinone redox couple

Many photoactivated processes involve a change in oxidation state during the reaction pathway and formation of highly reactive photoactivated species. Isolating these reactive species and studying their early-stage femtosecond to nanosecond (fs-ns) photodynamics can be challenging. Here we introduce a combined ultrafast transient absorption-spectroelectrochemistry (TA-SEC) approach using freestanding boron doped diamond (BDD) mesh electrodes, which also extends the time domain of conventional spectrochemical measurements. The BDD electrodes offer a wide solvent window, low background currents, and a tuneable mesh size which minimises light scattering from the electrode itself. Importantly, reactive intermediates are generated electrochemically, via oxidation/reduction of the starting stable species, enabling their dynamic interrogation using ultrafast TA-SEC, through which the early stages of the photoinduced relaxation mechanisms are elucidated. As a model system, we investigate the ultrafast spectroscopy of both anthraquinone-2-sulfonate (AQS) and its less stable counterpart, anthrahydroquinone-2-sulfonate (AH2QS). This is achieved by generating AH2QS in situ from AQS via electrochemical means, whilst simultaneously probing the associated early-stage photoinduced dynamical processes. Using this approach we unravel the relaxation mechanisms occurring in the first 2.5 ns, following absorption of ultraviolet radiation; for AQS as an extension to previous studies, and for the first time for AH2QS. AQS relaxation occurs via formation of triplet states, with some of these states interacting with the buffered solution to form a transient species within approximately 600 ps. In contrast, all AH2QS undergoes excited-state single proton transfer with the buffered solution, resulting in formation of ground state AHQS- within approximately 150 ps.

The excited-state dynamics of the radical anions of cyanoanthracenes

The radical anion of 9,10-dicyanoanthracene (DCA) has been suggested to be a promising chromophore for photoredox chemistry, due to its nanosecond excited-state lifetime determined from indirect measurements. Here, we investigate the excited-state dynamics of the radical anion of three cyanoanthracenes, including DCA˙-, produced by photoinduced electron transfer in liquid using both pump-probe and pump-pump probe transient electronic absorption spectroscopy. All three excited radical ions are characterised by a 3-5 ps lifetime, due to efficient non-radiative deactivation to the ground state. The decay pathway most probably involves D1/D0 conical intersection(s), whose presence is favoured by the enhanced flexibility of the radical anions relative to their neutral counterparts. The origin of the discrepancy with the nanosecond lifetime of DCA˙-* reported previously is discussed. These very short lifetimes limit, but do not preclude, photochemical applications of the cyanoanthracene anions.

Direct Observation of Reactive Intermediates by Time-Resolved Spectroscopy Unravels the Mechanism of a Radical-Induced 1,2-Metalate Rearrangement

Radical-induced 1,2-metalate rearrangements of boronate complexes are an emerging and promising class of reactions that allow multiple new bonds to be formed in a single, tunable reaction step. These reactions involve the addition of an alkyl radical, typically generated from an alkyl iodide under photochemical activation, to a boronate complex to produce an α-boryl radical intermediate. From this α-boryl radical, there are two plausible reaction pathways that can trigger the product forming 1,2-metalate rearrangement: iodine atom transfer (IAT) or single electron transfer (SET). Previous steady-state techniques have struggled to differentiate these pathways. Here we apply state-of-the-art time-resolved infrared absorption spectroscopy to resolve all the steps in the reaction cycle by mapping production and consumption of the reactive intermediates over picosecond to millisecond time scales. We apply this technique to a recently reported reaction involving the addition of an electron-deficient alkyl radical to the strained σ-bond of a bicyclo[1.1.0]butyl boronate complex to form a cyclobutyl boronic ester. We show that the previously proposed SET mechanism does not adequately account for the observed spectral and kinetic data. Instead, we demonstrate that IAT is the preferred pathway for this reaction and is likely to be operative for other reactions of this type.

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

Organic photocatalysts (PCs) are gaining popularity in applications of photoredox catalysis, but few studies have explored their modus operandi. We report a detailed mechanistic investigation of the electron transfer activation step of organocatalyzed atom transfer radical polymerization (O-ATRP) involving electronically excited organic PCs and a radical initiator, methyl 2-bromopropionate (MBP). This study compares nine N-aryl modified PCs possessing dihydrophenazine, phenoxazine, or phenothiazine core chromophores. Transient electronic and vibrational absorption spectroscopies over subpicosecond to nanosecond and microsecond time intervals, respectively, track spectroscopic signatures of both the reactants and products of photoinduced electron transfer in N,N-dimethylformamide, dichloromethane, and toluene solutions. The rate coefficients for electron transfer exhibit a range of values up to ∼10¹⁰ M^-1 s^-1 influenced systematically by the PC structures. These rate coefficients are an order of magnitude smaller for catalysts with charge transfer character in their first excited singlet (S₁) or triplet (T₁) states than for photocatalysts with locally excited character. The latter species show nearly diffusion-limited rate coefficients for the electron transfer to MBP. The derived kinetic parameters are used to model the contributions to electron transfer from the S₁ state of each PC for different concentrations of MBP. Comparisons of singlet and triplet reactivity for one of the phenoxazine PCs reveal that the rate coefficient k_ET(T₁) = (2.7 ± 0.3) × 10⁷ M^-1 s^-1 for electron transfer from the T₁ state is 2 orders of magnitude lower than that from the S₁ state, k_ET(S₁) = (2.6 ± 0.4) × 10⁹ M^-1 s^-1. The trends in bimolecular electron transfer rate coefficients are accounted for using a modified Marcus theory for dissociative electron transfer.

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

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 spectroscopy. 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,N-dimethylformamide (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 a considerable dependence on the photocatalyst structure and the solvent. The 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. A comparison of our results with previously reported O-ATRP performances of the various photoredox catalysts shows that high triplet-state quantum yields are not a prerequisite for controlling polymer dispersity. For example, the photocatalyst 5,10-bis(4-cyanophenyl)-5,10-dihydrophenazine, 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 behavior of organic photoredox catalysts in O-ATRP reactions.

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

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-d8, complete ground-state recovery is (31 ± 3) % by internal conversion (IC) from the photo-excited state (or from S1 after IC but before complete vibrational relaxation), (13 ± 2) % via direct decay from vibrationally relaxed S1 (most likely radiative decay, with an estimated radiative lifetime of 13 ns) and (56 ± 3) % via the T1 state (with intersystem crossing (ISC) rate coefficient k ISC = (3.3 ± 0.2) × 108 s−1). 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 S1 and T1 states.

Mapping the multi-step mechanism of a photoredox catalyzed atom-transfer radical polymerization reaction by direct observation of the reactive intermediates

The rapid development of new applications of photoredox catalysis has so far outpaced the mechanistic studies important for rational design of new classes of catalysts. Here, we report the use of ultrafast transient absorption spectroscopic methods to reveal both mechanistic and kinetic details of multiple sequential steps involved in an organocatalyzed atom transfer radical polymerization reaction. The polymerization system studied involves a N,N-diaryl dihydrophenazine photocatalyst, a radical initiator (methyl 2-bromopropionate) and a monomer (isoprene). Time-resolved spectroscopic measurements spanning sub-picosecond to microseconds (i.e., almost 8 orders of magnitude of time) track the formation and loss of key reactive intermediates. These measurements identify both the excited state of the photocatalyst responsible for electron transfer and the radical intermediates participating in propagation reactions, as well as quantifying their lifetimes. The outcomes connect the properties of N,N-diaryl dihydrophenazine organic photocatalysts with the rates of sequential steps in the catalytic cycle.

Picosecond to millisecond tracking of a photocatalytic decarboxylation reaction provides direct mechanistic insights

The photochemical decarboxylation of carboxylic acids is a versatile route to free radical intermediates for chemical synthesis. However, the sequential nature of this multi-step reaction renders the mechanism challenging to probe. Here, we employ a 100 kHz mid-infrared probe in a transient absorption spectroscopy experiment to track the decarboxylation of cyclohexanecarboxylic acid in acetonitrile-d₃ over picosecond to millisecond timescales using a photooxidant pair (phenanthrene and 1,4-dicyanobenzene). Selective excitation of phenanthrene at 256 nm enables a diffusion-limited photoinduced electron transfer to 1,4-dicyanobenzene. A measured time offset in the rise of the CO₂ byproduct reports on the lifetime (520 ± 120 ns) of a reactive carboxyl radical in solution, and spectroscopic observation of the carboxyl radical confirm its formation as a reaction intermediate. Precise clocking of the lifetimes of radicals generated in situ by an activated C-C bond fission will pave the way for improving the photocatalytic selectivity and turnover.

Picosecond to Nanosecond Manipulation of Excited-State Lifetimes in Complexes with an FeII to TiIV Metal-to-Metal Charge Transfer: The Role of Ferrocene Centered Excited States

Time-resolved transient absorption spectroscopy and computational analysis of D-π-A complexes comprising Fe^II donors and Ti^IV acceptors with the general formula ^RCp₂Ti(C₂Fc)₂ (where ^RCp = Cp*, Cp, and ^MeOOCCp) and ^TMSCp₂Ti(C₂Fc)(C₂R) (where R = Ph or CF₃) are reported. The transient absorption spectra are consistent with an Fe^III/Ti^III metal-to-metal charge-transfer (MMCT) excited state for all complexes. Thus, excited-state decay is assigned to back-electron transfer (BET), the lifetime of which ranges from 18.8 to 41 ps. Though spectroscopic analysis suggests BET should fall into the Marcus inverted regime, the observed kinetics are not consistent with this assertion. TDDFT calculations reveal that the singlet metal-to-metal charge-transfer (¹MMCT) excited state for the Fe^II/Ti^IV complexes is not purely MMCT in nature but is contaminated with the higher-energy ¹Fc (d-d) state. For the diferrocenyl complexes, ^RCp₂Ti(C₂Fc)₂, the ratio of MMCT to Fc centered character ranges from 57:43 for the Cp* complex to 85:15 for the ^MeOOCCp complex. For the diferrocenyl and monoferrocenyl complexes investigated herein, the excited-state lifetimes decrease with increased ¹Fc character. The effect of Cu^I coordination was also analyzed by time-resolved transient absorption spectroscopy and reveals the elongation of the excited-state lifetime by 3 orders of magnitude to 63 ns. The transient spectra and TDDFT analysis suggest that the long-lived excited state in Cp₂Ti(C₂Fc)₂·CuX (where X is Cl or Br) is a triplet iron species with an electron arrangement of Ti^IV-³Fe^II-Cu^I.

Effects of Naphthyl Connectivity on the Photophysics of Compact Organic Charge-Transfer Photoredox Catalysts

Modular chromophoric systems with minimal electronic coupling between donor and acceptor moieties are well suited for establishing predictive relationships between molecular structure and excited-state properties. Here, we investigate the impact of naphthyl-based connectivity on the photophysics of phenoxazine-derived orthogonal donor-acceptor complexes. While compounds in this class are themselves interesting as potent organic photocatalysts useful for visible-light-driven organocatalyzed atom-transfer radical polymerization and small-molecule synthesis, many other systems (e.g., phenazine, phenothiazine, and acridinium) exploit charge-transfer excited states involving a naphthyl substituent. Therefore, aided by the facile tunability of the phenoxazine architecture, we aim to provide mechanistic insight into the effects of naphthyl connectivity that can help inform the understanding of other systems. We do so by employing time-resolved and steady-state spectroscopies, cyclic voltammetry, and temperature-dependent studies on two chemical series of phenoxazine compounds. In the first series ( N-aryl 3,7-dibiphenyl phenoxazine), we find high sensitivity of photophysical behavior to naphthyl connectivity at its 1 versus 2 positions, including a drop in the intersystem-crossing yield (ΦISC) from 0.91 ( N-1-naphthyl) to 0.54 ( N-2-naphthyl), which we attribute to the establishment of an excited-state equilibrium in the singlet manifold. Drawing on the synthetic tunability afforded by phenoxazine, a modified series ( N-aryl 3,7-diphenyl phenoxazine) is chosen to circumvent this equilibrium, thereby isolating the impact of naphthyl connectivity on charge-transfer energy and triplet formation. We conclude that donor-acceptor distance is a key design parameter that influences a host of excited-state and dynamical properties and can have an outsized impact on photochemical function.