Negative Photochromism of Biaryl-Bridged Imidazole Dimers: Designing Biradical-Mediated Reaction Networks

Conspectus Negative photochromism reverses the usual behavior of T-type photochromic molecules: a thermally stable colored state is converted by light into a less absorbing metastable state and subsequently recovers thermally. Because the photoproduct attenuates excitation light less strongly, negative photochromism is attractive for optically dense materials and systems requiring an intrinsic thermal reset. The challenge is to combine long-wavelength excitation, efficient photoconversion, controllable state populations and thermal lifetimes, and adaptation to different operating environments. Our studies of biaryl-bridged imidazole dimers originated from efforts to accelerate radical recombination in hexaarylbiimidazole (HABI) photochromes by covalently bridging the two imidazole units and suppressing diffusional separation of the photogenerated radicals. Introduction of a 1,1′-binaphthyl bridge unexpectedly produced negative photochromism. In the resulting 1,1′-binaphthyl-bridged imidazole dimer (BN-ImD), the thermodynamically stable colored state (5MR) contains a diazafulvene chromophore absent from conventional HABI photochromes. Visible-light excitation cleaves a C–N bond to generate a short-lived biradical intermediate (BR), which recombines either to regenerate 5MR or to form the metastable colorless state (6MR). Thermal cleavage of the interimidazole C–C bond of 6MR regenerates BR, completing a thermally resettable 5MR–BR–6MR network. This behavior revealed that molecular bridging can control not only radical proximity but also the covalent states and reaction pathways accessible through BR. This reaction-network perspective provides several design coordinates. Extension of the π-conjugated bridge and donor substitution enable long-wavelength excitation, whereas differential tuning of 5MR and 6MR absorptions can produce large spectral separation and wavelength-selective bidirectional switching. Molecular asymmetry can bias BR recombination among competing covalent products, and excitation intensity can open nonlinear pathways to alternative states. Conformational restriction alters activation barriers and thermal recovery, while transient spectroscopy and global kinetic analysis separate BR formation, product branching, and thermal regeneration as distinct contributions. Structural modification can also open access to 7MR and 8MR states and generate kinetic hierarchies spanning rapid radical recombination, seconds-scale switching, and hours-to-days thermal equilibration. The 5MR/6MR contrast can also be translated into function, including turn-on fluorescence, four-photon-like fluorescence responses under two-photon excitation, operation in polar and aqueous media, and light-driven reorganization of lyotropic and chiral nematic liquid crystals. This Account organizes these developments around four interdependent design principles─spectral control, kinetic control, branching selectivity, and environmental integration. The central message is that negative photochromism in this family is best designed by controlling the reaction profile as a whole: which states are accessible, how BR is partitioned among them, how rapidly the connecting pathways operate, and how the network responds to excitation wavelength, photon flux, and environment.