Extending the molecular conjugation of phosphorescence units to accurately modulate ultralong organic room temperature phosphorescence

Abstract

We successfully modulate the spectral shift of ultralong room temperature phosphorescence by extending the molecular conjugation of phosphorescence units and colorful afterglow can be achieved from the phosphorescence units. Four phosphorescence units (BCz-2, BCz-1, PyCz and BrPyCz) were comprehensively compared in this work. Doping these phosphorescence units into PMMA, we found that their phosphorescence bands were red shifted sequentially from 490 nm and 550 nm to 627 nm and 629 nm. It is true that the ultralong phosphorescence of phosphorescence units in toluene solution at 77 K and in a polymer film is assigned to their monomer phosphorescence (intrinsic T₁ decay). Compared with BCz-2 and BCz-1, PyCz and BrPyCz give out red afterglow because pyrene has the largest molecular conjugation. As PyCz@PMMA emits weak red afterglow at room temperature, we propose two methods to enhance room temperature red afterglow of PyCz, including using of rigid polymer PVA and incorporation of a bromine atom. Intense red afterglow can be observed for PyCz@PVA at room temperature because the rigid environment of PVA greatly suppresses the nonradiative relaxation pathways of triplet excitons. Interestingly, bromination on the benzene ring of PyCz (namely BrPyCz) significantly promotes intersystem crossing (ISC) efficiency and red afterglow of BrPyCz@PMMA is further boosted at room temperature but hardly alters the T₁ energy level so their ultralong phosphorescence band positions are kept unchanged. TD-DFT results verify that extension of molecular conjugation reduces energy levels of T₁ (2.61 eV, 2.32 eV, 2.03 eV and 2.03 eV) and directly results in a red shift of monomer phosphorescence of BCz-2, BCz-1 and (Br)PyCz. To our best knowledge, this study may be a significant work in precisely tuning the organic phosphorescence color of phosphorescence units and may give some suggestions for development of organic phosphorescence.