Thermally Activated Delayed Photoluminescence: Deterministic Control of Excited-State Decay

Abstract

Thermally activated photophysical processes are ubiquitous in numerous organic and metal–organic molecules, leading to chromophores with excited-state properties that can be considered an equilibrium mixture of the available low-lying states. Relative populations of the equilibrated states are governed by temperature. Such molecules have been devised as high quantum yield emitters in modern organic light-emitting diode technology and for deterministic excited-state lifetime control to enhance chemical reactivity in solar energy conversion and photocatalytic schemes. The recent discovery of thermally activated photophysics at CdSe nanocrystal–molecule interfaces enables a new paradigm wherein molecule–quantum dot constructs are used to systematically generate material with predetermined photophysical response and excited-state properties. Semiconductor nanomaterials feature size-tunable energy level engineering, which considerably expands the purview of thermally activated photophysics beyond what is possible using only molecules. This Perspective is intended to provide a nonexhaustive overview of the advances that led to the integration of semiconductor quantum dots in thermally activated delayed photoluminescence (TADPL) schemes and to identify important challenges moving into the future. The initial establishment of excited-state lifetime extension utilizing triplet–triplet excited-state equilibria is detailed. Next, advances involving the rational design of molecules composed of both metal-containing and organic-based chromophores that produce the desired TADPL are described. Finally, the recent introduction of semiconductor nanomaterials into hybrid TADPL constructs is discussed, paving the way toward the realization of fine-tuned deterministic control of excited-state decay. It is envisioned that libraries of synthetically facile composites will be broadly deployed as photosensitizers and light emitters for numerous synthetic and optoelectronic applications in the near future.

Sensitizing phosphorescent and radical emitters via triplet energy translation from CsPbBr3 nanocrystals

“…1 A possible reason is that the triplet energy of NCA is only ∼0.14 eV lower than the NC exciton energy, and therefore there exists a possibility of thermally activated reverse TET from NCA triplets to NCs, shortening the triplet lifetime. 23,45 Indeed, we can observe a persistent weak dip at ∼450 nm in the μs-TA spectra in Fig. 2b, which can be attributed to a quasi-equilibrium between the NC and NCA excited states established through the bi-directional TET.…”

Bridge control of photophysical properties in benzothiazole-phenoxazine emitters – from thermally activated delayed fluorescence to room temperature phosphorescence

“…4,5 The transition from the first singlet excited state to the ground state (S 1 -S 0 , i.e. fluorescence) represents an exchange between two electronic states with identical spin multiplicity and as such is a spin-allowed transition, 6 generally with a lifetime in the nanoseconds regime. On the other hand, the transition from the first triplet excited state to the ground state (T 1 -S 0 , i.e.…”

“…phosphorescence) occurs between two electronic states with different spin multiplicity and is therefore a spin-forbidden and considerably slower transition (typically in the order of microseconds). 6,7 While the second generation OLED devices are able to harvest triplet excitons by spin-orbit coupling (SOC) enhanced phosphorescence from heavy metal complexes, the third generation focusses on triplet up-conversion strategies to achieve near-unity internal quantum efficiency and singlet emission. There are three leading mechanisms for the indirect generation and emission of singlet excited states, i.e.…”

Crystalline Metal‐Organic Materials with Thermally Activated Delayed Fluorescence

“…Besides TADF, recently another TADPL emission mechanism has been exploited. [123,124] The similarities of TADF and TADPL are that 1) thermal trigger upconversion in excited states, 2) the relative populations of the equilibrated excited states are controlled by temperature. The difference is that TADF is involved S1 and T1 states, while TADPL is a more complicated progress which is involved triplet−triplet energy transfer (TTET) and its reverse progress (rTTET) in a pair of donor-acceptor chromophores.…”

“…[124] TADPL emission is observed in the system incorporating quantum dots and organic dyes, which are used to tune the lifetime of excited states. [123] The comparison between TADF and TADPL is expected to help readers to gain more insight into the TADF progress.…”