A holistic approach for anthracene photochemistry kinetics

Laser-induced reversible photocycloadditions with anthracene monomer (A) are of key interest in generating light responsive materials. Upon dimerization (forward reaction) these species form dimers A₂ that can be cleaved to regenerate the monomer A (reverse reaction). Thus, if such moieties are incorporated in a polymer network, light-shaping of the material properties becomes possible. The photocycloaddition of A is mostly displayed as a simple wavelength (λ) gated on/off switch with long UV light inducing bond formation and short UV light bond cleavage. However, the real situation is far more complex as the forward and reverse reactions are concomitantly initiated in the shorter λ regime and their competition is strongly influenced by the reaction conditions. Herein, we report a λ dependent kinetic study to determine the forward and reverse photochemical reactivity, which can serve as a blueprint to design large-scale polymeric network materials. We introduce fundamental photoreaction rate laws for cases of exclusive dimerization, exclusive dimer cleavage, and competitive absorption and reactions. The λ-dependent kinetic parameters are determined based on a protocol covering dedicated short timescale experiments under specific conditions, either starting with a dominant presence of A or A₂. Model validation is performed using long time scale irradiation experimental data. In particular, the λ region between 260 and 330 nm is explored to determine the optimum λ for A₂ cleavage and to showcase the concentration variations of A and A₂ at various λ, including both situations of reactant depletion and equilibrium settlement.