Visible-Light-Initiated Free-Radical Polymerization by Homomolecular Triplet-Triplet Annihilation

Abstract

A new visible-light photoactivation mechanism based on sensitized triplet fusion between two excited ZnTPP chromophores achieves free-radical polymerization from acrylate monomers without additives. The combined results demonstrate that the upper S 2 excited state of ZnTPP is oxidatively quenched by the acrylates, resulting in the formation of an acrylate radical anion that engages in radical chain polymerization. Well-defined macro-and microscopic polymer gels were readily produced in a facile manner using green and yellow visible photons.

Ultra-small PbS nanocrystals as sensitizers for red-to-blue triplet-fusion upconversion

“…Control experiments where either the sensitizing PbS-CPA nanocrystals or DPA are removed show no gel formation (Fig. S5 †), consistent with previous reports that achieve photoinitiation via upconversion, 13,30,32 or via direct excitation with higher-energy photons. 57,58 Thus, we observe that the higherenergy DPA spin-singlet excited state generated via triplet fusion is able to activate the homolytic cleavage of the initiator, in line with other recent demonstrations of TUC-mediated photochemistry with other material systems.…”

“…57,58 Thus, we observe that the higherenergy DPA spin-singlet excited state generated via triplet fusion is able to activate the homolytic cleavage of the initiator, in line with other recent demonstrations of TUC-mediated photochemistry with other material systems. 13,14,30 Notably, the low concentrations (4 mM) of PbS used in this polymerization reaction were consistent with other TUC-mediated photochemistry systems, 13,14 as well as direct QD-catalyzed polymerizations. 57 Given the susceptibility of these ultra-small QDs to degrade in the presence of an active radical catalyst, future work should be focused towards other photochemical transformations, 13,31,59 stabilizing QD sensitizers to these reaction conditions, 57 and improvements to the overall efficiency of this photochemical reaction via optimization of sensitizer and annihilator concentrations , which may improve the maximum upconversion quantum efficiency (UCQE) for this system.…”

“…1,6,9,28,29 Importantly, it was recently demonstrated that triplet-fusion upconversion can enable photochemical transformations using incident long-wavelength light at low uences appropriate for tissue penetration. 13,14,[30][31][32] This is motivating an exploration of photochemical and biomedical applications, where large anti-Stokes shi upconversion could permit targeted drug-delivery without the non-specic photochemistry that accompanies the use of direct ultraviolet excitation. 31 Thus, red-to-blue upconversion-achieving large anti-Stokes shi while creating excited states with enough energy (z3.0 eV) to make and break chemical bonds-has emerged as an important step towards applications.…”

Diphenylanthracene Dimers for Triplet–Triplet Annihilation Photon Upconversion: Mechanistic Insights for Intramolecular Pathways and the Importance of Molecular Geometry

“…Thus, TTA-UC is of special interest not only for photovoltaics 7 − 10 but also in other solar energy conversion applications, such as photocatalysis 11 − 13 and photochemistry. 14 − 16 Two compounds are needed in this process: a sensitizer (S) and an annihilator (A). The sensitizer molecule absorbs one long wavelength photon and undergoes rapid intersystem crossing (ISC).…”

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

“…19 Synthetic advances enabling control of the photophysical properties of transition metal complexes and materials are revolutionizing triplet sensitization. [20][21][22][23][24][25][26][27][28][29] Numerous molecular triplet sensitizers featuring long-lived excited states contain rare-earth and precious second-or third-row transition metals, including Ru(II) [30][31][32][33] , Ir(III) [34][35][36][37] , Re(I) 38,39 , Pt(II) or Pd(II). 5,[40][41][42] Replacing precious metals with more earth-abundant transition metals reveals additional non-radiative decay pathways that markedly attenuate excited state lifetimes.…”