Accounts of Chemical Research最新文献

Photochemical methods have become indispensable in modern organic synthesis by enabling unique reactivities under mild conditions through electron transfer, energy transfer, and other radical-based pathways. In contrast to thermally driven reactions, however, photochemical processes are fundamentally governed by the delivery and utilization of photons. Wavelength, light intensity, photon flux, optical path length, and reactor geometry collectively determine how efficiently photons are absorbed and translated into chemical reactivity. Importantly, increasing light intensity does not necessarily improve performance: excessive photon flux can promote side reactions, catalyst deactivation, or product degradation. Effective photochemistry therefore requires deliberate matching of light-source emission to photocatalyst absorption and careful control of photon dose rather than indiscriminate intensification.

The complexity of photon management increases further in multiphasic systems containing gases or solids. Gas–liquid interfaces introduce refraction and reflection due to refractive index differences, leading to photon losses in regimes dominated by large bubbles, while finely dispersed bubbles can instead redirect light and enhance local absorption. Solid photocatalysts introduce additional challenges by scattering light anisotropically while simultaneously participating in the reaction. Scattering redistributes photons within─and sometimes out of─the reaction medium, complicating mechanistic interpretation and making mixing and hydrodynamics critical design parameters.

Scaling photochemical transformations from laboratory to production scale demands the parallel scaling of photon supply. Increasing optical power introduces challenges related to heat dissipation, nonuniform irradiation, and reactor design. Treating photons as reagents, quantified in equivalents relative to the substrate, provides a unifying framework for identifying photon-limited regimes and distinguishing them from limitations imposed by intrinsic kinetics or mass transfer. Systematic variation of wavelength and intensity not only enables robust scale-up but also yields mechanistic insight by revealing rate-limiting steps in multicomponent catalytic cycles.

In this Account, we describe how photon control, characterization, light interactions, and photoreactor engineering together define the efficiency, reproducibility, and scalability of photochemical processes. In addition, we discuss fundamental photonic principles for photochemistry and highlight strategies that enable predictable, selective, and industrially relevant photochemistry across reaction conditions and scales.

The electron deficiency of boron promotes the formation of multicenter σ and π bonds that endow its clusters and solids with exceptional structural diversity. While bulk boron favors cage-like frameworks, clusters often adopt planar or quasi-planar motifs composed of triangles that evolve into tubular and cage-like architectures as their size increases. Many of these clusters are stabilized by delocalized σ and π bonds that are associated with fluxional behavior and multiple aromaticity.

Metal doping enriches this chemistry. Transition metals use their d or f orbitals to couple with the boron framework, generating metal-centered rings, metallo-boron nanotubes, and metalloborophenes. In contrast, alkali and alkaline-earth metals have long been viewed as simple counterions, yet recent findings reveal that they can orchestrate deep structural reorganizations by combining charge transfer with efficient orbital overlap. Lithium, for example, leads to a quasi-planar → tubular → cage evolution in B₁₂ clusters via strong electrostatic attraction to the boron framework, whereas beryllium engages in pronounced covalent Be–B interactions that yield rare architectures such as the Archimedean Be₄B₁₂⁺ cage, the B–Be sandwich B₇Be₆B₇, and four-ring tubular forms like Be₂B₂₄⁺.

In heavier alkaline-earth systems, the participation of (n–1)d orbitals (Ca, Sr, Ba) introduces transition-metal-like covalent interactions, producing highly symmetric rings and tubular clusters. This Account summarizes how electrostatic and covalent interactions jointly control geometry and bonding in boron–metal systems, defining the rich landscape of boron chemistry.