Symmetry-breaking fluctuations split the porphyrin Q bands*

Porphyrins offer a malleable and cost-efficient platform to sculpt biomimetic technologies with tunable charge transfer, energy conversion, and photocatalytic properties. Yet, despite decades of research, the physical mechanisms underlying the Q-band splitting in their electronic spectra remain elusive. To address this long-standing question, we leverage universal statistical arguments to develop a novel theoretical and computational framework that enables us to simulate the optical spectroscopy of solvated chromophores in the presence of arbitrarily strong non-Condon fluctuations. Our theory bridges exact quantum-dynamical expressions with atomistic simulations, enabling us to capture for the first time the spectra of tetraphenylporphyrin and porphine in solution. Our work reveals that Q-band splitting arises from slow and fast fluctuations of distinct chemical moieties that turn symmetry-forbidden transitions bright. We exploit these insights to propose and confirm how chemical modifications tune the optical properties of related porphyrins, demonstrating the potential for developing design principles to tailor optoelectronic properties.