Molecular engineering of organic emitter molecules with inverted singlet–triplet energy gaps (INVEST) has emerged as a powerful approach for enhancing fluorescence efficiency through triplet harvesting. In these unique materials, the first excited singlet state (S1) lies below the lowest triplet state (T1), enabling efficient reverse intersystem crossing. Previous computational studies have focused on accurately calculating the inverted energy gap and establishing qualitative structure–property relationships. Here, we present quantitative relationships that link the molecular structure to the S1–T1 energy gap, ΔEST, by introducing a benchmark set of 15 heptazine-based INVEST molecules (HEPTA-INVEST15). We identify a strong linear correlation (R2 > 0.94) between ΔEST and both the degree of intramolecular charge transfer and the deviation from a single-excitation character, as quantified by %R1 values and transition density matrix norms. These trends persist across our expanded set of 44 mono-, di-, and tri-substituted heptazines (HEPTA-INVEST44), underscoring the generality of our findings. Notably, strongly electron-donating groups, such as −NH2, minimize the magnitude of inverted gaps in mono-substituted heptazines yet produce the most negative ΔEST in certain tri-substituted derivatives, a result arising from competing resonance effects and excited-state aromaticity. Although ΔEST shows no clear correlation with Hammett parameters, our results reveal that physically meaningful, computable descriptors offer a mechanistic foundation for the future data-driven design of INVEST emitters. These findings pave the way for machine-learning approaches that connect the molecular structure to ΔEST without requiring high-level excited-state calculations.

Awardees will work at national laboratories to solve America's most pressing energy challenges

Molecular engineering of organic emitter molecules with inverted singlet-triplet energy gaps (INVEST) has emerged as a powerful approach to achieve enhanced fluorescence efficiency through triplet har...

Quantum mechanical vibrational coherence transfer processes play important roles in energy relaxation, charge transfer, and reaction dynamics in chemical and biological systems but are difficult to directly measure using traditional condensed-phase nonlinear spectroscopies. Recently, we developed a new experimental capability to obtain two-dimensional infrared (2D IR) spectra of molecular systems in the gas phase that enables the direct measurement of coherence pathways. Herein, we report ultrafast 2D IR spectroscopy of the peptide glutathione (GSH) isolated and cryogenically cooled in the gas phase. Six vibrational modes were simultaneously excited within the amide I and II region. The spectral dynamics of both diagonal and off-diagonal cross peak features exhibit long-lived oscillatory behavior consistent with the presence of coherent vibrational dynamics. The oscillatory signatures deviate significantly from the expected quantum beating pathways predicted from standard nonlinear response theories. These deviations indicate the presence of additional nonlinear pathways, including coherence transfer processes. Quantum chemistry calculations indicate large anharmonic couplings between the excited vibrational modes in GSH and, critically, strong coupling between the excited modes and numerous low-frequency modes that act as a bath to mediate coherence transfer. The data provide important new benchmarks for modeling coherence transfer dynamics and system–bath interactions in open quantum systems free from solvent effects.

Quantum mechanical vibrational coherence transfer processes play important roles in energy relaxation, charge transfer, and reaction dynamics in chemical and biological systems, but are difficult to d...

Dimethyl disulfide (DMDS), one of the smallest organic molecules with an S-S bond, serves as a model system for understanding photofragmentation in polypeptides and proteins. Prior studies of DMDS pho...

Organic co-crystals have emerged as a promising class of semiconductors for next-generation optoelectronic devices due to their unique photophysical properties. This paper presents a joint experimenta...