Sensitized triplet–triplet annihilation based photon upconversion in full organic and hybrid multicomponent systems

Abstract

In the last 15 years, the attention dedicated to organic conjugated systems experienced outstanding growth because of the renewed interest in mechanisms involving triplet states such as singlet fission, thermally activated delayed fluorescence, and intersystem crossing enhanced phosphorescence. Photon upconversion via sensitized triplet–triplet annihilation ( sTTA) enables the conversion of low-energy photons into high-energy ones, and it has been proposed in multicomponent systems as an efficient managing strategy of non-coherent photons. This mechanism exploits the annihilation of two optically dark triplet states of emitter moieties to produce high-energy photons. The annihilating triplets are sensitized through Dexter energy transfer by a light-harvester, typically a conjugated molecule or a nanocrystal, so sTTA upconversion is usually performed in bi-component systems. The high yield observed at low excitation intensities stimulated thriving research in the field, leading to the development of a large family of fully organic and hybrid sTTA multicomponent upconverters. Here, we compare the evolution of these two families of systems with respect to the sTTA upconversion main figures of merit, highlighting the strengths and weaknesses of both approaches, according to the results reported in the literature. The data presented are also discussed in the perspective of future developments in the field, pointing out the challenges that are still to be faced for the technological use of the sTTA upconversion process.

Metal–Organic Bichromophore Lowers the Upconversion Excitation Power Threshold and Promotes UV Photoreactions

“…A quantitative analysis of the excitation power density threshold dependence on exact sensitizer/annihilator properties, as discussed previously, ,− is hampered by the overall complexity of our upconversion systems (Supporting Information Section 9), particularly with 1-MN and TP , for which bimolecular reverse TTET from the annihilators to the sensitizer complicates the situation (see below and Figure S27). However, in the case of the PPO annihilator, the physical origin of the lower I th values for Ir Naph in comparison to that for Ir ref can be rationalized by the considerably longer excited-state lifetime of the sensitizer (Figure d).…”

“…Aside from the excitation power threshold, the upconversion efficiency (ϕ sTTA‑UC ) is a key figure of merit. ,,, To determine ϕ sTTA‑UC for our systems, the ratios of upconversion luminescence intensity and the prompt sensitizer emission intensity of a reference solution containing no annihilator were measured as a function of the excitation power density (Figures b,c and S22–S25), to provide data sets such as those in Figure f (see Supporting Information Section 7 for details). For the Ir Naph / PPO couple, ϕ sTTA‑UC is roughly 70% higher than for the Ir ref / PPO combination (Table ), suggesting that the bichromophore concept and the triplet reservoir effect might be generally helpful to boost ϕ sTTA‑UC in addition to I th .…”

Triplet–Triplet Annihilation Upconversion in a Porphyrinic Molecular Container

“…TTA was first observed in solutions of anthracene more than 60 years ago. − Since then, the concept of using two functional componentsi.e., a sensitizer (Sn) and an annihilator (An)to convert light energies has been adopted ,− widely within the scientific community. The energy-transfer steps of TTA-UC often proceed as follows: first, a sensitizer in its ground state ( 1 Sn) is excited (Figure a) to a singlet excited state ( 1 Sn*) by absorbing low-energy incident photons before undergoing intersystem crossing (ISC), leading to a triplet exciton ( 3 Sn*).…”

High-Efficiency Near-Infrared-to-Visible Photon Upconversion in Poly(vinyl alcohol) Porous Film

“…Developing solid-state TTA-UC materials is a considerable hurdle in this field. Among the diverse approaches, such as the use of polymer hosts, − crystals, , sensitized metal oxides, − and thermal deposited semiconductor-based devices, − the approaches based on the use of polymeric materials could provide exceptional advantages from a manufacturing standpoint. , These challenges can be resolved through the realization of effective TET via close contact between chromophores in rigid matrices. , To create this dense environment, a blending of large quantities of chromophores into a polymer host was proposed and resulted in promising performance. − Nevertheless, these attractive approaches have been demonstrated in the conversion of photons within the visible (vis) range, whereas insufficient progress has been achieved in near-infrared (NIR)-to-vis UC in polymeric materials . One aspect hampering the utilization of this dense chromophore approach for NIR-to-vis conversion is the generation of undesirable chromophore aggregations that lower the UC performance because they form loss channels in the TTA-UC system during concentration quenching of emitters, self-TTA in sensitizers, and other deactivation processes of triplet states. ,− As a solution, from a synthetic chemistry approach, bulky substituents were introduced into the chromophores to reduce the aggregate-induced loss channels; however, this contended with the distance between the chromophores that needs to be close enough to enable efficient TET …”

“…27−29 Nevertheless, these attractive approaches have been demonstrated in the conversion of photons within the visible (vis) range, whereas insufficient progress has been achieved in near-infrared (NIR)-to-vis UC in polymeric materials. 30 One aspect hampering the utilization of this dense chromophore approach for NIR-to-vis conversion is the generation of undesirable chromophore aggregations that lower the UC performance because they form loss channels in the TTA-UC system during concentration quenching of emitters, self-TTA in sensitizers, and other deactivation processes of triplet states. 8,31−36 As a solution, from a synthetic chemistry approach, bulky substituents were introduced into the chromophores to reduce the aggregate-induced loss channels; however, this contended with the distance between the chromophores that needs to be close enough to enable efficient TET.…”