Revisiting the mechanism of long-lived luminescence in host/guest organic crystals

Abstract

Room-temperature phosphoresence (RTP) in purely organic systems has attracted significant attention due to its promising applications in organic electronics, bioprobes or even security solutions. Despite the well-established observation of RTP, a comprehensive materials-perspective on the underlying photophysical mechanism is still missing. Here, we investigate an archetypical class of host/guest systems, carbazoles doped with benzoindoles, using periodic (time-dependent) density functional theory. We first confirm that the phosphoresence is due to the benzoindole guest. Second, we demonstrate that the triplet excited state located on the benzoindole guest is significantly lower in energy than charge-separated states between the host and the guest. This strongly suggests that an energy-transfer from the host to the guest, rather than charge-transfer, is involved in the RTP mechanism. Finally, we use the valence and conduction bandwidth obtained by periodic calculations to estimate the transfer integral of charge carriers. We underline the role in the RTP efficiency of the herringbone organization of the carbazole units. Having elucidated the key properties leading to RTP in this class of systems, we extended the analysis to a newly-designed OMe functionalized host. With this classical functionalization, used to increase the donor character of molecule, we clearly show both computationally and experimentally that the electronic and structural requirements of this host/guest system are preserved and indeed lead to a RTP observation. This corroborates the mechanistic insights and suggests that a design of organic host/guest RTP systems is within reach.