Achieving Colorful Ultralong Organic Room-Temperature Phosphorescence by Precise Modification of Nitrogen Atoms on Phosphorescence Units

Abstract

We successfully tune ultralong organic room-temperature phosphorescence (UORTP) by a simple strategy of precisely modifying nitrogen atoms on Phosphorescence Units, and colorful ultralong phosphorescence can be achieved. We for the first time investigate the structure–function relationship between phosphorescence properties and molecular structures of Phosphorescence Units. With BCz and BCz-1 as comparison, eight new Phosphorescence Units were synthesized by introducing one or two nitrogen atoms to the naphthalene moiety. For all the 10 Phosphorescence Units, their room-temperature ultralong phosphorescence in the PMMA film should be assigned to monomer phosphorescence from intrinsic T1 decay. For Phosphorescence Units series I (BCz, NBCz-1, NBCz-2, NBCz-3, NBCz-4, NBCz-5, and NBCz-6), introducing one nitrogen atom to the naphthalene moiety can significantly affect the phosphorescence properties of Phosphorescence Units, and the effect is quite complicated. For modification on the inner ring, the T1 energy level of NBCz-1 decreases, and the red shift of UORTP occurs while the T1 energy level of NBCz-2 increases and the blue shift of UORTP happens. For modification on the outer ring, no phosphorescence color change is observed for NBCz-3 and NBCz-4, but their phosphorescence lifetimes vary notably due to different intersystem crossing efficiencies; as the modification site approaches the central five-member ring, the T1 energy levels of NBCz-5 and NBCz-6 decrease, and their UORTP red shifts dramatically. For Phosphorescence Units series II (BCz, 2NBCz, BCz-1, and 2NBCz-1), introducing two nitrogen atoms to the outer six-member ring reduces energy level of T1 excitons and leads to incredible red shift of UORTP for BCz and 2NBCz while surprisingly energy levels of T1 excitons rise and UORTP blue shifts for BCz-1 and 2NBCz-1. Under the condition of proper modification sites, it is true that the more the additional nitrogen atoms, the more red-shifted the ultralong phosphorescence. This study may expand our knowledge of organic phosphorescence and lay the foundation for its future applications.

Photo- and Oxygen- Synergistically Controlled Selective Expression of Organic Phosphorescence Units

“…As for the mechanism of photoinduced UORTP color change, we deduced that NBCz units might undergo chemical reaction and molecular structure change under continuous UV irradiation. First, we revisited our previous work where eight new phosphorescence units were obtained by modification of nitrogen atoms. The UORTP color was unchanged for all other seven molecules after prolonged UV irradiation, suggesting that photoinduced UORTP color change is an exclusive property for NBCz.…”

“…After 5 s UV irradiation (Figure a), a yellowish-green afterglow came out, and the phosphorescence bands were located at 525 (τ = 387.85 ms), 571 (τ = 366.85 ms), and 621 nm (τ = 369.08 ms) (Figure b,e), which was totally different from those of PCBNBCz and FSO 2 NBCz. In our previous work, it is confirmed that these phosphorescence bands are assigned to the intrinsic monomer phosphorescence of N 2 BCz. Thus, the intrinsic monomer phosphorescence of the NBCz units was screened by N 2 BCz units.…”

“…The phosphorescence unit NBCz (7 H -indolo­[2,3- c ]­quinoline) was synthesized referring to our previous work. , NBCz was obtained from the intramolecular coupling reaction of N -(2-bromophenyl) quinoline-3-amine which was produced by a typical substitution reaction between 1-bromo-2-iodobenzene and 3-aminoquinoline. Two donor–acceptor (D–A) molecules, PCBNBCz and FSO 2 NBCz, were both gained via typical substitution reactions.…”

“…Photoresponsive UORTP is a very interesting topic as light has excellent temporal-spatial resolution. − Generally, the photoresponse of UORTP includes two typical responding modes: photoactivation and photoinduced color change. Till now, photoactivated UORTP in the doped polymer film due to photoinduced consumption of oxygen has been widely reported.…”

“…ESR and HRMS measurements confirmed that oxidation of NBCz occurs at the nitrogen atom of the quinoline ring via photogenerated superoxide radicals, which results in the UORTP color change. Revisiting our previous work, it is rational that the nitrogen atom of the quinoline ring of NBCz possesses reduction activity and tends to undergo oxidation under UV irradiation. TDDFT calculations proved that after oxidation of NBCz the T 1 energy level declines significantly, which is consistent with the redshift of UORTP.…”