Aggregation-Induced Enhancement of Molecular Phosphorescence Lifetime: A First-Principle Study

Abstract

Pure organic phosphorescent molecules are promising compounds for applications of phosphorescence, yet their utilization is restricted because of inefficient intersystem crossing (ISC) between singlet and triplet states. Molecular aggregation has been deemed a viable strategy to modulate molecular luminescence in solution, yet its impact on the phosphorescence is rarely investigated. In this work, we carried out firstprinciple studies to elucidate how aggregation of selected phosphorescent molecules will affect their phosphorescence behavior. Our calculations show that the overall ISC rate is appreciably enhanced, thanks to a decrease of energy gaps (ΔE) and an increase in the number of ISC channels between singlet and triplet states as the degree of aggregation develops. This facilitates singlet-to-triplet conversion. More importantly, the phosphorescence lifetime increases with the increase of the degree of molecular aggregation. The longlived phosphorescence associated with aggregation benefits from multiple factors, including small singlet−triplet gaps, enhanced overall ISC rates, and suppression of fluorescence. We believe this aggregation-induced ISC mechanism may be employed as an alternative approach for realizing persistent phosphorescence.

Phenothiazine–Quinoline Conjugates Realizing Intrinsic Thermally Activated Delayed Fluorescence and Room‐Temperature Phosphorescence: Understanding the Mechanism and Electroluminescence Devices

“…On the other hand, to harvest triplet excitons via TADF with room‐temperature phosphorescence (RTP) feature, a very good energy‐level tuning of these three excited states should occur at ambient conditions. [ 8 ] Designing such organic systems is a difficult task as purely organic materials are intrinsically incapable of efficient phosphorescence at ambient conditions due to fast nonradiative decay pathways ( k nr ) of the excited states, [ 9 ] although many design principles which rely on intra‐ and/or intermolecular interactions (lp••• π , π ••• π , hydrogen bonding), [ 10 ] aggregates, [ 11 ] host–guest chemistry, [ 12 ] deuteration, [ 13 ] and the presence of heavy elements [ 14 ] in the molecular backbone have been developed. Recently, few researchers demonstrated simultaneous TADF and RTP from single molecular systems using chemical modification, [ 15 ] mechanical force, [8b,16] conformational switching, [ 17 ] and change of host matrices.…”

Phenoxazine–Quinoline Conjugates: Impact of Halogenation on Charge Transfer Triplet Energy Harvesting via Aggregate Induced Phosphorescence

“…To avoid such obstacles, a large number of design principles based on harnessing the locally excited triplet ( 3 LE) energy, for example, intra- and/or intermolecular interactions (lone pair···π, π···π, hydrogen bonding), 15 − 17 aggregates, 18 − 21 host–guest, 22 , 23 deuteration, 24 excited states engineering, 25 internal heavy chalcogen atom effect, 26 and the presence of heavy elements in the molecular backbone, 10 have been adopted to reduce the nonradiative pathways and boost ISC rates at ambient conditions. Recently, a handful of reports have established that phosphorescence from the 3 CT state at ambient conditions can be achieved using charge transfer complexation, 1 , 27 − 29 excited-state hydrogen-bonding effect, 30 aggregate induced phosphorescence (AIP), 31 , 32 and orthogonal donor–acceptor (D–A) geometries. 1 , 33 On the other hand, thermally activated delayed fluorescent (TADF) material development relies on the orthogonal D–A structure with a close proximity of 3 LE, 1 CT, and 3 CT states which results in an increased reverse intersystem crossing (RISC) rate by reducing the energy gap between the 1 CT, 3 CT, and 3 LE states.…”

Asymmetric-donor (D2D2′)–acceptor (A) conjugates for simultaneously accessing intrinsic blue-RTP and blue-TADF

“…In addition, the molecular vibrations that readily increase the non-radiative pathways of the localized triplet excitons become another major bottleneck to harvest efficient phosphorescence yield. 36 To circumvent the above issues, crystal formation, 37,38 aggregation, 39,40 hostguest interactions 41 and deuteration 42 have been studied to reduce the nonradiative pathways under ambient conditions. To observe both blue-TADF (BTADF) and blue-RTP (BRTP) simultaneously with high PLQY from the SCOLMs under ambient conditions becomes a difficult task in the field of chemical physics.…”