Determinants of the efficiency of photon upconversion by triplet–triplet annihilation in the solid state: zinc porphyrin derivatives in PVA

Abstract

Spectroscopic, photophysical and computational studies designed to expose and explain the differences in the efficiencies of non-coherent photon upconversion (NCPU) by triplet-triplet annihilation (TTA) have been carried out for a new series of alkyl-substituted diphenyl and tetraphenyl zinc porphyrins, both in fluid solution and in solid films. Systematic variations in the alkyl-substitution of the phenyl groups in both the di- and tetraphenyl porphyrins introduces small, but well-understood changes in their spectroscopic and photophysical properties and in their TTA efficiencies. In degassed toluene solution TTA occurs for all derivatives and produces the fluorescent S product states in all cases. In PVA matrices, however, none of the di-phenylporphyrins exhibit measurable NCPU whereas all the tetraphenyl-substituted compounds remain upconversion-active. In PVA the NCPU efficiencies of the zinc tetraphenylporphyrins vary significantly with their steric characteristics; the most sterically crowded tetraphenyl derivative exhibits the greatest efficiency. DFT-D computations have been undertaken and help reveal the sources of these differences.

Low power threshold photochemical upconversion using a zirconium(iv) LMCT photosensitizer

“…It is well established that metalloporphyrin photosensitizers undergo homomolecular triplet-triplet annihilation that serves as a competing process with respect to the desired acceptor-based TTA. [67][68][69][70][71][72][73][74] Importantly, homomolecular TTA nor any other self-quenching process was observed in the Zr(IV) LMCT sensitizer, whose excited state lifetime remained constant across all sensitizer concentrations investigated. For the photochemical upconversion systems using CzPA series as acceptors/annihilators, the upconversion quantum efficiencies -although they do not rival that of the benchmark DPA -are still impressive when compared to the large body of existing anthracene-based annihilators.…”

“…The maximum hUC of 42.7% for Zr(IV)/DPA composition is likely reflecting the maximum normalized TTA efficiency (hTTA = 2 ´ FTTA) of 40% within experimental error of the hUC measurement. 70,71 The underlying mechanism of the TTA process is generally regarded as an encounter complex formation between two excited triplet acceptor/annihilator molecules. Triplet-triplet encounters yield the following spin statistics: 5/9 quintets, 3/9 triplets and 1/9 singlets.…”

Metal–Organic Framework Thin Film-Based Dye Sensitized Solar Cells with Enhanced Photocurrent

“…Herein, for the first time, we report an approach by assembling metal-free sensitizer, Bodipy derivative with poly(methyl methacrylate) (PMMA) [ 25 , 26 , 27 ] on –COOH exposed Zn-perylene SURMOF surface. Recently, the promising idea to use the polymers for the TTA-UC process has been reported [ 28 ]. The synthesis and characterizations of the Bodipy derivative ( Figures S1 and S2 ) can be found in Supporting Information .…”

Thermally Activated Delayed Fluorescence via Triplet Fusion

“…No upconverted emission was detected from control samples with sole PdTPTBP or DPA under the same condition. The upconversion emission of DPA may be derived from TTA of DPA or be the consequence of singlet energy transfer to DPA from higher excited singlet PdTPTBP formed through the triplet fusion of PdTPTBP itself. − The quenching efficiency of the phosphorescence of PdTPTBP increases from 7.8 to 73.1% upon increasing DPA from 2 to 50 mM with constant PdTPTBP, while the upconversion emission intensity increases and reaches a maximum with an upconversion quantum yield (Φ UC ) of 3.6% in the presence of 40 mM DPA but declines a little with more DPA (Figure a). The observation of an increasing trend of upconversion emission in the presence of more quenchers of triplet PdTPTBP suggests that the formation of a higher excited singlet sensitizer by TTA of triplet PdTPTBP and subsequent singlet energy transfer to DPA is not likely responsible for the upconverted emission.…”