Efficient Triplet‐Triplet Annihilation Upconversion Sensitized by a Chromium(III) Complex via an Underexplored Energy Transfer Mechanism

Abstract

Sensitized triplet‐triplet annihilation upconversion (sTTA‐UC) mainly relies on precious metal complexes thanks to their high intersystem crossing (ISC) efficiencies, excited state energies, and lifetimes, while complexes of abundant first‐row transition metals are only rarely utilized and with often moderate UC quantum yields. [Cr(bpmp)2]3+ (bpmp=2,6‐bis(2‐pyridylmethyl)pyridine) containing earth‐abundant chromium possesses an absorption band suitable for green light excitation, a doublet excited state energy matching the triplet energy of 9,10‐diphenyl anthracene (DPA), a close to millisecond excited state lifetime, and high photostability. Combined ISC and doublet‐triplet energy transfer from excited [Cr(bpmp)2]3+ to DPA gives 3DPA with close‐to‐unity quantum yield. TTA of 3DPA furnishes green‐to‐blue UC with a quantum yield of 12.0 % (close to the theoretical maximum). Sterically less‐hindered anthracenes undergo a [4+4] cycloaddition with [Cr(bpmp)2]3+ and green light.

Towards Luminescent Vanadium(II) Complexes with Slow Magnetic Relaxation and Quantum Coherence

“… [8] A thoroughly studied ion with S = 3 / 2 is chromium(III). Chromium(III) complexes (quartet ground state, S = 3 / 2 ) with suitable ligand fields have been developed into highly versatile materials for photonic and photocatalytic applications, including sensing,[ 9 , 10 , 11 ] upconversion,[ 12 , 13 ] circularly polarized luminescence[ 14 , 15 ] and photo(redox) catalysis. [ 16 , 17 , 18 , 19 ] The magnetic ground state S = 3 / 2 of the highly luminescent molecular ruby [Cr(ddpd) 2 ] 3+ with the spin‐flip [20] emission band peaking at 778 nm [21] shows a long coherence time of 8.4(1) μs (ddpd= N , N ’‐dimethyl‐ N , N ’‐dipyridine‐2‐yl‐pyridine‐2,6‐diamine).…”

Oxidative Two-State Photoreactivity of a Manganese(IV) Complex using NIR Light

“…Thus, we assign this process to the population of the 2 LMCT/ 2 MC state via slow ISC with 1 = 780 fs. Although ISC is often ultrafast even for 3d transition metal complexes [44][45][46] , it can also be rather slow in the picosecond and even nanosecond range. [47][48][49][50][51][52][53] After the population of the 2 LMCT/ 2 MC state is completed, the shape of the spectrum remains basically unchanged, besides minor shifts and a small increase of the intensity of the newly formed band (2 = 9.7 ps), which we attribute to thermal relaxation and reorganization of the solvent cage (VR, vibrational relaxation) due to the substantial change of the dipole moment during the LMCT transition.…”

“…A similar retarding effect on the photoreduction, likely also affecting the cage escape rate, occurs when diluting the acetonitrile solution with redox-inert orthodichlorobenzene in 1:1 and 1:2 v/v ratios (Supplementary Figs. [45][46][47].…”

“…[37][38][39][40][41]23,24 Excited states with a multiplicity differing from the ground state multiplicity, for example triplet metal-to-ligand charge transfer states ( 3 MLCT), possess higher excited state lifetimes but are typically at lower energies than the corresponding singlet states ( 1 MLCT) leading to energy dissipation and consequently a loss of oxidizing/reducing power. Furthermore, an intersystem crossing (ISC) process 43 is required to reach the long-lived 3 MLCT states, which is often fast and efficient for transition metal complexes, even for first row metals, [44][45][46] yet in certain cases also slow and inefficient ISC processes have been reported. [47][48][49][50][51][52][53] In the present study we demonstrate that a mixed pyridine/guanidine complex with manganese(IV) as earth-abundant metal ion can be excited with low-energy NIR light to give two photoactive states, namely an unconventional, luminescent and long-lived mixed doublet ligand-to-metal charge transfer/metal-centered ( 2 LMCT/ 2 MC) excited state with some spin-flip admixture at lower energy capable of oxidizing moderately challenging substrates and a short-lived 4 LMCT excited state at high energy capable of oxidizing even extremely challenging substrates.…”