Microenvironments Explain the Mismatch between Photochemical Absorptivity and Reactivity

Photochemistry is at the forefront of many modern technologies, from additive manufacturing to phototherapeutics, to sun protection and organic synthesis. For centuries, it was believed that an absorbance spectrum – showing the likelihood of a photon to be absorbed by a chromophore at a given wavelength – is an accurate predictor of how well a photochemical process will proceed when irradiated with different colours of light. Over the last decade this paradigm has been repeatedly challenged for many photochemical systems, as a distinct mismatch between the absorption spectrum and the wavelength resolved photochemical reactivity has been observed. Herein, we unravel the underlying mechanisms behind the mismatched reactivity and absorbance. Initially, we probe the impact that an equilibrium established between reversible photochemical processes has on the mismatch. Subsequently, we establish a critical link between photophysics and photochemistry with a theory based on the selective excitation of specific microenvironments leading to molecular transitions that allow for favourable wavelength-dependent reactivity. Time-resolved and steady-state fluorescence spectroscopy measurements confirm the presence of this selectivity, with both displaying significant red-edge effects that are observed in fluorescence spectroscopy literature, further supporting our theory. By synthetically tethering chromophores together, we further evidence the importance of microenvironments and their wavelength-dependent excited-state lifetimes, presenting the missing link that explains the mismatch for many photochemical systems. The implications of the theory presented herein stretch from additive manufacturing to photodynamic therapy and beyond, meaning that researchers can leverage photochemical mismatches of their design by simply changing the properties of the environment surrounding the chromophore.