Supermolecular-Chromophore-Sensitized Near-Infrared-to-Visible Photon Upconversion

Abstract

Selective near-IR (NIR) excitation (780 nm) of the conjugated supermolecule ruthenium(II) [15-(4'-ethynyl-(2,2';6',2''-terpyridinyl))-bis[(5,5',-10,20-di(2',6'-bis(3,3-dimethylbutoxy)phenyl)porphinato)zinc(II)]ethyne][4'-pyrrolidin-1-yl-2,2';6',2''-terpyridine] bis(hexafluorophosphate) (Pyr(1)RuPZn(2)) in solutions containing N,N-bis(ethylpropyl)perylene-3,4,9,10-tetracarboxylicdiimide (PDI) or tetracene gives rise to a substantial anti-Stokes energy gain (PDI, 0.70 eV; tetracene, 0.86 eV). Experimental data clearly demonstrate that this upconverted fluorescence signal is produced via Pyr(1)RuPZn(2)-sensitized triplet-triplet annihilation (TTA) photochemistry. The TTA process was confirmed by the quadratic dependence of the integrated (1)PDI* emission centered at 541 nm derived from 780 nm laser excitation. The T(1)→T(n) excited state absorption decay of Pyr(1)RuPZn(2), monitored at 900 nm as a function of PDI concentration, revealed Stern-Volmer and bimolecular quenching constants of 10 048 M(-1) and 5.9 × 10(8) M(-1) s(-1), respectively, for the PDI triplet sensitization process. The T(1)→T(n) PDI extinction coefficient at 560 nm (ε(T) = 6.6 × 10(4) M(-1) cm(-1)) was determined through the triplet energy transfer method utilizing anthracene as the donor chromophore. (3)PDI* transient triplet absorption dynamics observed as a function of 485 nm incident nanosecond pump laser fluence demonstrate a bimolecular (3)PDI*-(3)PDI* TTA rate constant (k(TT) = 1.0 ± 0.2 × 10(9) M(-1) s(-1)). The maximum quantum yield of the supermolecule-sensitized PDI upconverted emission (Φ(UC) = 0.0075 ± 0.0002) was determined relative to [Os(phen)(3)][PF(6)](2) at an incident laser power of 22 mW at 780 nm. This study successfully demonstrates NIR-to-visible photon upconversion and achieves a new record anti-Stokes shift of 0.86 eV for sensitized TTA, using the supermolecular Pyr(1)RuPZn(2)sensitizer. The stability of the Pyr(1)RuPZn(2)/PDI chromophore combination is readily apparent as continuous irradiation at 780 nm produces 541 nm centered fluorescence with no significant decrease in intensity measured over time domains exceeding several hours. The molecular components of these NIR-to-vis upconverting compositions illustrate that substantial anti-Stokes energy gains via a TTA process can be effortlessly realized.

Near‐Infrared→Visible Light Upconversion in a Molecular Trinuclear d–f–d Complex

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Molecular nonlinear-optical (NLO) phenomena are currently being exploited for the design of visible emitting bioprobes with unprecedented properties that result from 1) the transparency of living tissues toward low-energy near-infrared (NIR) incident radiation and 2) the improved focusing of the light from an excitation laser beam within nanometric volumes.[1] Nonresonant multiphoton absorption produces a single emission during the irradiation; this emission corresponds to a multiple of the energy of the incident beam, as, for example, in second-harmonic generation (Figure 1 a), while resonant multiphoton absorption populates a real excited state of the chromophore, which relaxes and fluoresces with the same characteristics as if sensitized by a linear excitation process (Figure 1 b).[2] Beyond the optimization of polarizable push-pull p-aromatic molecules for NLO optical response, [2] and the related development of upconverted fluorescence signals in polyaromatic platforms produced by metal-sensitized triplet-triplet annihilation photochemistry, [3] Le Bozec, Maury, Andraud, and their respective co-workers demonstrated that trivalent lanthanide ions (Ln III ) may efficiently contribute to the polarization of coordinated aromatic ligands for resonant multiphoton absorption.[4] Moreover, the wealth of accessible long-lived metal-centered luminescent emissive levels can be exploited for two-photon-excited time-gated fluorescence analyses of biological tissues. [5] Interestingly, the existence of several real excited states located between the ground state and the target excited state of lanthanide acceptors opens novel perspectives for the operation of alternative sensitizing mechanisms that take advantage of these electronic relays for successive linear excitations by one (Figure 1 c) or several (Figure 1 d) moieties, thus leading to an excited-state absorption (ESA) process or sequential energy transfer upconversion (ETU), both of which are followed by luminescence.

[6] Although they operate by different principles, that is, they involve real rather than virtual intermediate excited levels during the multiphoton absorption processes, two-photon upconversion fluorescence processes (Figure 1 c, d) can be considered as nonlinear because of the quadratic dependence of the efficiency (I) on the incident intensity of the excitation light (I / P 2 ).

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“…[2] Beyond the optimization of polarizable push-pull p-aromatic molecules for NLO optical response, [2] and the related development of upconverted fluorescence signals in polyaromatic platforms produced by metal-sensitized triplet-triplet annihilation photochemistry, [3] Le Bozec, Maury, Andraud, and their respective co-workers demonstrated that trivalent lanthanide ions (Ln III ) may efficiently contribute to the polarization of coordinated aromatic ligands for resonant multiphoton absorption.…”

“…[1] Nonresonant multiphoton absorption produces a single emission during the irradiation; this emission corresponds to a multiple of the energy of the incident beam, as, for example, in second-harmonic generation (Figure 1 a), while resonant multiphoton absorption populates a real excited state of the chromophore, which relaxes and fluoresces with the same characteristics as if sensitized by a linear excitation process (Figure 1 b). [2] Beyond the optimization of polarizable push-pull p-aromatic molecules for NLO optical response, [2] and the related development of upconverted fluorescence signals in polyaromatic platforms produced by metal-sensitized triplet-triplet annihilation photochemistry, [3] Le Bozec, Maury, Andraud, and their respective co-workers demonstrated that trivalent lanthanide ions (Ln III ) may efficiently contribute to the polarization of coordinated aromatic ligands for resonant multiphoton absorption. [4] Moreover, the wealth of accessible long-lived metal-centered luminescent emissive levels can be exploited for two-photon-excited time-gated fluorescence analyses of biological tissues.…”

“…[4,6] Upconversion has been regularly reported for trivalent lanthanide cations doped into low-phonon inorganic matrices, [6] particularly for applications in bioanalyses, telecommunications, and solar energy conversion, [7] but it is still unknown in molecular erbium complexes [8] because of the high effective vibrational energy " hw eff % 2000 cm À1 that is typical of the molecular vibrations of organic ligands or of closely interacting solvent molecules, which result in efficient nonradiative relaxation of 4f-4f transitions. [9] In their seminal contribution dedicated to the search for upconversion fluorescence in the molecular complexes Na 3 [Ln(pyridine-2,6-dicarboxylate) 3 ]·13 H 2 O (Ln = Nd, Er, Yb), Reinhard and Güdel indeed concluded that "there is no chance to induce and observe upconversion luminescence in these molecular compounds". [9] Such a statement usually stimulates synthetic chemists to design novel molecular systems that are able to potentially overcome the purported theoretical limitation.…”

“…An attractive sensitizer/acceptor pair for potential molecular upconversion should thus match the following three criteria: 1) each acceptor should be surrounded by at least two equidistant sensitizers, 2) the resonant S!A energy transfer processes should be optimized for an efficient population of the excited levels of the acceptor, while ensuring that the excited-state lifetimes of the remaining sensitizers are long enough for the occurrence of sequential energy transfer in an isolated molecule, [3] and 3) the acceptor must be protected from high-frequency vibrations in order to minimize the average energy of the interacting phonons.…”