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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.

SUBMITTER: Roy PP

PROVIDER: S-EPMC9393888 | biostudies-literature | 2022 Aug

REPOSITORIES: biostudies-literature

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

Roy Partha Pratim PP Kundu Sohang S Makri Nancy N Fleming Graham R GR

The journal of physical chemistry letters 20220805 32

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 com ...[more]

PMID: 35929598

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Project description:Interatomic potentials for the B2Σ1/2+ states of CsAr, CsXe, and RbXe have been determined through comparisons of experimental B ← X absorption spectra for alkali vapor-rare gas mixtures with calculations of the Franck-Condon factors (FCFs) associated with free-free transitions of thermal atomic pairs. Simulations of optical transitions of alkali-rare gas atomic pairs between the thermal and vibrational continua of the X2Σ1/2+ and B2Σ1/2+ states of the molecule, responsible for the blue satellites of the Cs and Rb D2 resonance lines in a rare gas background, require the incorporation of ground-state J values above ∼400 into the FCF calculations and proper normalization of the free-particle wave functions. Absorption spectra computed on the basis of several X and B state interatomic potentials available in the literature were found to be sensitive to the height of the B2Σ1/2+ state barrier, as well as the X2Σ1/2+ state repulsive wall contour and the location of the van der Waals minimum. Other spectral simulations entailed iterative modifications to a selected B2Σ1/2+ interatomic potential, again coupled with comparison to experimental B ← X spectra. Comparisons of calculated spectra with experiment yield a CsXe B2Σ1/2+ potential, for example, exhibiting a barrier height of 76 cm-1 at 5.2 Å and yet is nearly flat at smaller values of internuclear separation (R). The latter contrasts with previous theoretical calculations of VB(R) in the vicinity of the barrier maximum. For the CsAr molecule, the B2Σ1/2+ barrier height was found to be 221 cm-1, which is within 3% of the value determined from pseudopotential calculations incorporating the spin-orbit effect. Reproducing Cs-rare gas experimental absorption spectra also requires the existence of a broad, shallow potential well lying beyond the B2Σ1/2+ barrier that, for CsAr, has a dissociation energy (De ∼ 24 cm-1) a factor of 3 larger than values predicted by theory. Similar results are obtained for the RbXe and CsXe complexes.

Dataset Information

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

Publications

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

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