Suppressing non-radiative losses in organic semiconductors caused by high-frequency molecular vibrations

Abstract For molecular semiconductors, exciton-vibration coupling, particularly to high-frequency carbon-carbon stretch modes (1000 – 1600 cm -1 ), accelerates non-radiative loss, and this limits the performance of light emitting diode (LED), fluorescent biomarkers and photovoltaic devices. Systems with low bandgaps are most affected by this, as captured by the Energy-Gap law 1 , which predicts that the non-radiative transition rates increase nearly exponentially with the inverse of the number of vibrational quanta needed to span the semiconductor bandgap 2,3 . Since organic systems are primarily based on carbon-carbon bonds, vibrational coupling to high-frequency modes has been thought to be unavoidable. Here, using broadband impulsive vibrational spectroscopy and first-principles modelling, we demonstrate that when the exciton wavefunction has substantial charge-transfer character with spatially disjoint electron and hole density, high-frequency vibrational modes can be efficiently decoupled from the exciton. For instance, high-efficiency deep-red/NIR LED emitters, based on doublet (TTM-3PCz, TTM-TPA) and thermally-activated delayed fluorescence (APDC-DTPA), do not show any substantial coupling to the high-frequency modes >1000 cm -1 . We propose two reasons for this, firstly, localization of high-frequency modes to either donor or acceptor moiety means they do not significantly perturb the exciton energy or its spatial distribution. Secondly, such systems allow for individual selection of hole and electron levels which has enabled empirical optimization of hole donor moieties such as triphenylamine (TPA) and N-phenylcarbazole (PCz). These moieties have nitrogen p z non-bonding character for the hole accepting level which do not efficiently couple to the high frequency vibrational modes that modulate π-bond order. Our results provide a new mechanistic picture of how vibrational modes couple to excitons and exposes hitherto unknown design rules for the suppression of non-radiative decay channels in organic molecules.