Where the vibronic coupling lies: promoting-mode frequencies and second-order rate theory for reverse intersystem crossing in multiresonance TADF emitters
Year: 2026
Type: preprint
Abstract: Rate models for reverse intersystem crossing in multiresonance delayed-fluorescence emitters usually place the vibrational spectrum on one promoting mode near 75 meV and treat the triplet–triplet vibronic coupling at second order. Neither approximation has been tested at scale. This work computes nonadiabatic couplings and signed spin–orbit vectors (PBE0/def2-SVP, Tamm–Dancoff) for all 121 emitters of the Hagai et al. compilation, and full vibrational spectra for 17 of them. The coupling-weighted promoting quantum has a median of 173 meV, in the aromatic ring-stretch region, with about 25 modes carrying 90% of the squared coupling. Collapsing the spectrum onto a 75 meV oscillator inflates the coupling by a factor of 1.5, and second-order theory then fails: over 120 emitters its rate departs from exact diagonalisation of the vibronic problem by a median of 0.49 orders of magnitude and lies further from experiment. At the computed 173 meV the median departure is 0.10 orders, although 21 emitters still differ by more than an order. On a multimode model that preserves the residual coupling matrix it falls to 0.02 orders for the 17 emitters with spectra, against 0.61 and 0.23 orders for the single-mode model on the same molecules. Both multimode treatments overestimate the measured rate for 15 of the 17 emitters, by a median of 1.2 orders, so the remaining error lies in inputs they share. For these emitters the breakdown of second-order theory comes from the single-mode reduction; the near-resonant regime remains to be tested with full spectra.
Source: ChemRxiv
Author: Ramarao Tata
Institution: Ohio University
Language: English
Subfield: Electrical and Electronic Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
Funder: National Science Foundation
OpenAlex ID: W7215121174