Complementing adiabatic and non-adiabatic methods to understand internal conversion dynamics in porphyrin derivatives

Comprehending internal conversion (IC) is essential for interpreting and forecasting photo-physical processes in organic molecules. Here we propose a novel first-principles approach to elucidate the role of molecular vibrations assisting IC in organic molecules, which complements static analysis of per-mode reorganization energies based on time-dependent density functional theory (TDDFT) calculations[1] and non-adiabatic molecular dynamics simulations[2]. This method is applied to the prototypical case of porphyrins to elucidate the dynamics of the vibrational modes driving the IC process within the Q band of a functionalized porphyrin (FP) molecule, in comparison with its bare, non-functionalized counterpart (BP).

We show that static normal mode analysis, paired with the computation of per-mode RE, can serve as a preliminary, approximate yet quick and effective step in investigating IC close to conical intersection. Moreover, we examine the differences in the evolution of excited states of the two molecules within the first 100 fs, and show how to relate single-point adiabatic quantum-chemical methods and dynamical non-adiabatic simulations to gain a deeper awareness of the effects of porphyrin functionalization on its IC dynamics. Finally, our theoretical results are compared to ultrafast multidimensional and transient spectroscopies[3] to provide a comprehensive explanation of the fast IC within ≈60 fs observed in FP as compared to the much longer decay rate reported for BP, especially focusing on the role of the functional groups. Our results successfully demonstrate the use of porphyrin-core functionalization to effectively modulate IC, offering new opportunities of application in the photocatalysis and optoelectronics fields.

References:

[1] P. Rukin, D. Prezzi, and C.A. Rozzi. Excited-state normal-mode analysis: The case of porphyrins, J. Chem. Phys. 159, 244103 (2023);

[2] P. Rukin, M. Fortino, D. Prezzi, and C.A. Rozzi. Complementing adiabatic and non-adiabatic methods to understand internal conversion dynamics in porphyrin derivatives. Manuscript submitted for publication.

[3] V. Petropoulos, P.S. Rukin, F. Quintela, M. Russo, L. Moretti, A. Moore, T. Moore, D. Gust, D. Prezzi, G.D. Scholes, E. Molinari, G. Cerullo, F. Troiani, C.A. Rozzi, and M. Maiuri. Vibronic Coupling Drives the Ultrafast Internal Conversion in a Functionalized Free-Base Porphyrin. J. Phys. Chem. Lett. 15, 16, 4461–4467 (2024).

Seminar realized in the framework of the funded projects:

- VIBETWO - Next Generation EU PRIN 2022, GA: 202284JP34 

- HARVEST - Italian Ministry of University and Research, within the program PRIN 2017, GA: 201795SBA3.