Broad‐Band Visible‐Light Excitable Room‐Temperature Phosphorescence Via Polymer Site‐Isolated Dye Aggregates

Abstract

Organic room‐temperature phosphorescent (RTP) materials are useful in optical imaging and sensing technologies. However, most organic phosphors, including red‐light emitting dyes, exhibit unusually large Stokes shifts, and require excitation with ultraviolet radiation, the high energy of which can induce substantial photo damage. Here a design concept of using dihydroxy‐functionalized naphthalenediimide (NDI, λabs < 400 nm) is demonstrated to initiate ring‐opening polymerizations of L‐lactide (PLA) and ε‐caprolactone (PCL), resulting in a linear polymer NDI‐PLA2/NDI‐PCL2. With the right combination of substituents, the design simultaneously realizes broadband visible light absorption (λabs = 450–650 nm) and near‐infrared RTP (λRTP = 700 nm) with millisecond‐long lifetimes. Polymer site isolations of the aggregation‐prone NDI phosphors may prevent aggregation‐caused quenching of RTP common in bulk, by forming various microscopic ground‐state aggregates, which exhibit triplet‐emitting states lower than that of the individual phosphor. The method can potentially be expanded onto other molecular RTP systems.

Origin of Red‐Shifted Phosphorescence from Triphenylamines: Triplet Excimer or Impurity?

“…[28b] When the BF 2 dbm is modified with a single atom, however, a more significant RTP shift in emission maximum (525 to 535 nm, 356 cm À 1 ) is obtained for BF 2 dbm(I) aggregates, [28a] presumably due to a mixed singlet transition dipole moment. Very recently, we also demonstrated that the formation of dibromo-substituted naphthalenediimide (NDI) aggregates leads to red-shifted RTP emission maximum vs. the monomer RTP (670 to 700 nm, 639 cm À 1 ) while the unsubstituted NDI aggregates have negligible spectral changes; [29] nonetheless, the shift is mostly due to re-distributed vibronic populations while the position of the 0-0 transition does not change much. Incidentally, to show that the mixing of singlet excited state into the triplet one could promote phosphorescence spectral shift for TPA aggregates (Figure 3), we compared the phosphorescence spectra of the TPA with three internal heavy atoms, tris(4bromophenyl)amine (TPA-3Br), at a concentration of 0.5 wt % and 12 wt % in PS at 80 K, respectively, and indeed observed a change in emission maximum from 433 to 455 nm, or 1117 cm À 1 in energy (Figure 3b).…”

Origin of Red‐Shifted Phosphorescence from Triphenylamines: Triplet Excimer or Impurity?

“…[28b] When the BF 2 dbm is modified with a single iodine atom, however, a more significant RTP shift in emission maximum (525 to 535 nm, 356 cm À 1 ) is obtained for BF 2 dbm(I) aggregates, [28a] presumably due to a mixed singlet transition dipole moment. Very recently, we also demonstrated that the formation of dibromo-substituted naphthalenediimide (NDI) aggregates leads to red-shifted RTP emission maximum vs. the monomer RTP (670 to 700 nm, 639 cm À 1 ) while the unsubstituted NDI aggregates have negligible spectral changes; [29] nonetheless, the shift is mostly due to re-distributed vibronic populations while the position of the 0-0 transition does not change much. Incidentally, to show that the mixing of singlet excited state into the triplet one could promote phosphorescence spectral shift for TPA aggregates (Figure 3), we compared the phosphorescence spectra of the TPA with three internal heavy atoms, tris(4bromophenyl)amine (TPA-3Br), at a concentration of 0.5 wt % and 12 wt % in PS at 80 K, respectively, and indeed observed a change in emission maximum from 433 to 455 nm, or 1117 cm À 1 in energy (Figure 3b).…”

Recent Advances of Pure Organic Room Temperature Phosphorescence Based on Functional Polymers

“…(d) Chemical structures of P1–P6. Reproduced with permission from ref . Copyright 2022 Wiley-VCH GmbH.…”

“…Zhang et al 37 synthesized a series polyester (P1−P6) through a solvent-free, ring-opening polymerization of D,L- lactide and ε-caprolactone using modified dihydroxy-functionalized naphthalenediimide (NDI) derivatives as initiators (Figure 4d). Because the NDI backbone can easily form aggregates, both monomer and aggregate RTP emissions were detected.…”