Charge Transfer as the Key Parameter Affecting Color Purity of TADF Emitters

TL;DR

This study investigates how charge transfer influences the emission bandwidth of TADF emitters, revealing that charge transfer character, not molecular rotation, primarily determines spectral sharpness, guiding future molecular design.

Contribution

It demonstrates that the intrinsic emission bandwidth of TADF molecules is mainly governed by charge transfer properties rather than conformational dynamics, offering new insights for designing narrow-spectrum emitters.

Findings

Charge transfer character primarily determines emission bandwidth.

Suppressing D-A dihedral rotation does not significantly narrow emission.

Intrinsic emission bandwidth is linked to charge transfer, not molecular motion.

Abstract

The key factors determining the emission bandwidth of thermally activated delayed fluorescence (TADF) are investi-gated by combining computational and experimental approaches. To achieve high internal quantum efficiencies (IQEs) in metal-free organic light emitting diode via TADF, the first triplet (T1) to first singlet (S1) reverse intersystem crossing (rISC) is promoted by configuring molecules in an electron donor-acceptor (D-A) alternation with a large di-hedral angle, which results in a small energy gap ({\Delta}EST) between S1 and T1 levels. This allows for effective non-radiative up-conversion of triplet excitons to singlet excitons that fluoresce. However, this traditional molecular de-sign of TADF results in broad emission spectral bands (full-width at half-maximum = 70-100 nm). Despite reports suggesting that suppressing the D-A dihedral rotation narrows the emission band, the origin of emission broadening remains elusive. Indeed, our results suggest that the intrinsic TADF emission bandwidth is primarily determined by the charge transfer character of the molecule, rather than its propensity for rotational motion, which offers a renewed perspective on the rational molecular design of organic emitters exhibiting sharp emission spectra.