Symmetry breaking fluctuations split the porphyrin Q bands

TL;DR

This paper uncovers the physical mechanisms behind the splitting of porphyrin Q-bands, revealing how nuclear motion timescale separation influences their optical properties, and proposes design principles for tuning these properties.

Contribution

It introduces a novel theoretical approach combining quantum dynamics and atomistic simulations to accurately predict porphyrin spectra and explains the origin of Q-band splitting.

Findings

Q-band splitting results from nuclear motion timescale separation.

Chemical modifications can tune porphyrin optical properties.

The theory accurately predicts spectra of various porphyrins.

Abstract

Porphyrins offer a malleable and cost-efficient platform to sculpt bioinspired technologies with tunable charge transfer, energy conversion, and photocatalytic properties. Yet, despite decades of research, the physical mechanisms that determine their electronic spectra remain elusive. Even for metal-free porphyrins, no consensus exists on the origin of the splitting of their $Q$-bands, an energetic region critical in photosynthesis. We leverage our recent statistical treatment of molecular motions in the condensed phase to predict their linear absorption spectra. By bridging exact quantum-dynamical expressions with atomistic simulations, our theory is the first to capture the spectra of representative porphyrins in solution: porphine, tetraphenylporphyrin, and tetraphenylporpholactone. Our work reveals that $Q$-band splitting arises from extreme timescale separation of nuclear motions that turn symmetry-forbidden transitions bright. We exploit these insights to propose and confirm how chemical modifications tune the optical properties of porphyrins, demonstrating the potential for developing design principles to tailor their optoelectronic behavior.