Interference between Franck–Condon and Herzberg–Teller Terms in the Condensed-Phase Molecular Spectra of Metal-Based Tetrapyrrole Derivatives

Abstract

The commonly used Franck–Condon (FC) approximation is inadequate for explaining the electronic spectra of compounds that possess vibrations with substantial Herzberg–Teller (HT) couplings. Metal-based tetrapyrrole derivatives, which are ubiquitous natural pigments, often exhibit prominent HT activity. In this paper, we compare the condensed phase spectra of zinc–tetraphenylporphyrin (ZnTPP) and zinc–phthalocyanine (ZnPc), which exhibit vastly different spectral features in spite of sharing a common tetrapyrrole backbone. The absorption and emission spectra of ZnTPP are characterized by a lack of mirror symmetry and nontrivial temperature dependence. In contrast, mirror symmetry is restored, and the nontrivial temperature-dependent features disappear in ZnPc. We attribute these differences to FC–HT interference, which is less pronounced in ZnPc because of a larger FC component in the dipole moment that leads to FC-dominated transitions. A single minimalistic FC–HT vibronic model reproduces all the experimental spectral features of these molecules. These observations suggest that FC–HT interference is highly susceptible to chemical modification.

Decorrelated singlet and triplet exciton delocalization in acetylene-bridged Zn-porphyrin dimers

“…Despite this expectation, the Q state appears in linear absorption spectra with relatively large intensity due to the non-Condon vibronic coupling mechanism mediated by Herzberg–Teller coupling to the B state. 50,52,53…”

“…In seminal work, Gouterman proposed that the excited state processes including optical absorption of porphyrin molecules could be described by a four-orbital model of two nearly degenerate occupied orbitals, a 1u and a 2u , and a doubly degenerate pair of unoccupied orbitals, e (x) g and e (y) g , which are related by a 90°rotation about an axis through the molecule's center. 49,50 We show the spatial distributions of electron density in the occupied a 1u and a 2u orbitals in the panels of Fig. 1.…”

“…Despite this expectation, the Q state appears in linear absorption spectra with relatively large intensity due to the non-Condon vibronic coupling mechanism mediated by Herzberg-Teller coupling to the B state. 50,52,53 In addition to explaining the basic features of the porphyrin linear absorption spectra, the four-orbital model predicts that different ring substituents perturb the energetic ordering of the a 1u and a 2u orbitals by changing Coulomb integrals. For example, ring substituents bound at particular positions around the macrocycle that donate electron density to the porphyrin ring cause higher Coulomb repulsion effects in the orbitals located at those atomic sites.…”

Phosphorescent Properties of Heteroleptic Ir(III) Complexes: Uncovering Their Emissive Species

“…For these calculations, we tested the performance of different exchange correlation (xc) functionals for the optimization and Hessian calculations of the involved triplet excited state. Based on the analysis of the results of the phosphorescence rate and spectra calculations under different approximations, we provide an appropriate computational recipe to perform these simulations in a computationally accurate and efficient way. − …”

Assessing the Determinants of Cavity Polariton Relaxation Using Angle-Resolved Photoluminescence Excitation Spectroscopy

“…Following ultrafast nonradiative relaxation of the B exciton prepared by photoexcitation of the Soret resonance, electronic population resides in the Q exciton of Zn-ligated porphyrins for nearly 10 ns, which leads to measurable fluorescence spectra whose features are controlled by non-Condon vibronic coupling. ,, Upon substituting Cu 2+ for Zn 2+ , one drives ultrafast formation of a molecular trip-doublet, which borrows oscillator strength from the B exciton via exchange interactions to produce weak fluorescence in recovering of sing-doublet ground state . The presence of these spectrally distinct light emission features in both Cu- and Zn-ligated porphyrins suggests that one can use their appearance following photoexcitation of cavity polaritons formed from the chromophores’ B excitons to assess coherent light absorption and nonradiative decay processes, which could allow researchers to better estimate the relaxation rates of hybrid light–matter states.…”