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

Abstract

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.

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

“…Similar triplet state ESA bands were observed by Sartor et al for modified versions of NPP with phenyl or biphenyl core substituents, albeit shifted to longer wavelength by the additional core conjugation. 13,16 At shorter time delays, the TEA spectra show solvent-dependent evolutions, with loss of absorption at wavelengths above 550 nm and growth then decay of an intermediate band peaking near 520 nm (with a secondary peak at wavelengths around 390 nm). This double-peaked feature is more prominent for spectra measured in DCM or DMF than in toluene where it is barely discernible as a broad feature above 500 nm.…”

“…[5][6][7][8][9][10] For these OPC classes, structural design principles have been developed based on both the observation of as-grown polymer properties [11][12][13] and transient absorption spectroscopy studies of excited-state lifetimes and SET rates. [14][15][16][17] Transient absorption spectroscopy has also been applied to other examples of photoredox reactions and over timescales spanning sub-picosecond to millisecond to explore multi-step reaction mechanisms. 15,[18][19][20][21] The complex photochemistry of these OPCs depends sensitively on the molecular architecture and the choice of solvent.…”

Interrogation of O-ATRP Activation Conducted by Singlet and Triplet Excited States of Phenoxazine Photocatalysts

“…Certainly, high-yield long-lived triplet excited states are commonly desired in solution-phase photoredox catalysis 41 and can be engineered in organic systems without heavy atoms by exploiting molecular substructures with orthogonal π orbitals to increase spin-orbit coupling and promote intersystem crossing. [42][43][44][45][46] However, the predecessor singlet excited states often have lifetimes on the order of ones to tens of nanoseconds, which can be long enough to engage in bimolecular photochemistry as long as there is sufficient concentration of the reactive partner.…”

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

“…Sartor et al have recently contributed detailed time-dependent studies of the photochemistry of structurally similar phenoxazine PCs. 24,43 In the current work, we report a more comprehensive study of an O-ATRP cycle using the organic photocatalyst 5,10di(naphthalen-1-yl)-5,10-dihydrophenazine (PCBN). This PC was chosen from an array of possible structural modications of the dihydrophenazine core to explore why it exhibits superior polymerization control to other dihydrophenazine-based PCs.…”