Light blue rigid excited-state intramolecular proton transfer organic semiconductor laser chromophores

Abstract

Organic laser materials are compared based on their ability to exhibit amplified spontaneous emission (ASE) and the energy threshold (E^ASE_th) required to achieve it. Very broadly, ASE demonstrates whether energy gains exceed energy losses within a given system. Various material design strategies have been explored to maximise gains and minimise losses. Among these, excited state intramolecular proton transfer (ESIPT) materials enable the absorbing species to differ greatly from the emitting species through enol–keto tautomerisation, generating a substantial Stokes shift and eliminating self-absorption losses. However, the resulting large Stokes shift also makes the development of efficient blue-emitting ESIPT laser semiconductors challenging. This work leveraged a combinatorial design strategy in ESIPT while simultaneously endowing molecular rigidity through strategic placement of one or two bonds to increase the rigidity of the chromophore for constrained non-radiative decay pathways. As a result, uncharacteristically high intrinsic photoluminescence quantum yields of 33–55% are observed for these new ESIPT chromophores. They are also found to exhibit high solid-state PLQYs (34–49%) in the blue region. These new chromophores further facilitated low solid-state ASE thresholds (4.4–5.2 μJ cm⁻² at 6 wt% in a common host material, 4,4′-bis(N-carbazolyl)-1,1′-biphenyl). Thus, even without a resonating structure, all compounds outperformed the literature parent HPI-Ac single crystals with a laser threshold of 6.0 μJ cm⁻², showing the benefits of a rigidity approach to low solid-state ASE thresholds for blue ESIPT laser chromophores.