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Assignment of the Triplet-to-Triplet Absorption Spectrum of Pyrene
Jeffrey R Reimers1,2, Ayano Yamamoto3, Chihaya Adachi3
1International Centre for Quantum and Molecular Structures and Department of Physics, Shanghai University, Shanghai200444, China.
The Journal of Physical Chemistry. A
|August 13, 2026
Summary
This study validates pyrene
Area of Science:
- Photochemistry and Spectroscopy
- Theoretical Chemistry
Background:
- The triplet-to-triplet absorption spectrum of pyrene is crucial for technological applications.
- Previous studies primarily focused on intense green-to-UV transitions, neglecting weak IR and red absorption bands.
- The IR band is critical for practical applications, yet poorly understood.
Purpose of the Study:
- To vindicate the spectral assignment of Langelaar et al. (1970) for pyrene's triplet-to-triplet absorption.
- To identify and characterize previously unaccounted absorption regions in pyrene's spectrum.
- To investigate the role of vibronic coupling and nonadiabatic effects in pyrene's excited states.
Main Methods:
- Utilized advanced computational methods: CAM-B3LYP density-functional, coupled-cluster, and complete active space self-consistent field (CASSCF) calculations.
- Analyzed Franck-Condon spectral progressions and identified new absorption regions.
- Investigated vibronic coupling, Herzberg-Teller effects, and nonadiabatic couplings.
Main Results:
- Vindicated the assignment of four Franck-Condon spectral progressions from Langelaar et al.
- Identified three new absorption regions, including a novel dark state.
- Predicted strong vibronic coupling and Herzberg-Teller effects explaining unassigned absorptions.
- Predicted rapid energy relaxation to the T2 state (3B2g) via nonadiabatic couplings.
- Evidence for low-lying conical intersections involving triplet states (1)3B2u and (1)3Ag.
Conclusions:
- The study provides a comprehensive understanding of pyrene's triplet-to-triplet absorption spectrum, including weak regions.
- Computational results support previous assignments and reveal new spectral features and underlying electronic processes.
- Identified mechanisms for rapid energy relaxation within the triplet manifold, relevant for photophysical applications.
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