1. TTA upconversion

TTA upconversion requires two main components which a both (metal)organic dyes (Fig. 1.1). Briefly, a sensitizer absorbs low energetic light and is excited to the first singlet state from which it undergoes the transition to the triplet excited state. As a consequence of energy transfer from the sensitizer to annihilator, triplet excited state of the latter is populated. Collision of two annihilators results in one of them being promoted to the singlet excited state which results in emission of fluorescence. Evidently, the sensitizer should ideally possess high molar absorption coefficients, high efficiency of inter-system crossing and have comparably long triplet state lifetimes. The annihilator should have the energy of the triplet state lower than that of the sensitizer, possess high fluorescence quantum yields and preferably not have much overlap between its emission spectrum and the absorption spectrum of the sensitizer to minimize light losses due to the inner-filter-effect. Since oxygen is a powerful quencher of the triplet states (both of the sensitizer and annihilator) it should be excluded from the system. However, this quenching can also be made use of to design optical oxygen sensor with anti-Stokes emission. [3]
Figure 1.1. Mechanism of the TTA-upconversion exemplified for a metalloporphyrin and a perylene used as sensitizer and annihilator, respectively. Reproduced from ref. [3].
We prepared new Pd(II) and Pt(II) complexes with Schiff bases and demonstrated that they are efficient sensitizers for TTA-systems. [4] The dyes possess very high molar absorption coefficients (> 100,000 M-1cm-1) combined with broad absorption bands, allowing for efficient collection of light in the orange-red part of the spectrum. Efficient upconversion is observed when these dyes are combined with perylenes as annihilators (Fig. 1.2). The upconverted fluorescence is clearly visible even under LED excitation.
Figure 1.2. Upconversion spectra upon excitation with red light of the toluene solutions containing a mixture of the Pd(II)-Schiff base complex and a perylene dye; a photographic image of the same solution (SG-5 as annihilator) upon excitation with 635 nm laser diode and 605 nm LED. Reproduced from ref. [4].
Recently we also reported new Pd(II) and Pt(II) complexes with highly electron-deficient alkylsulfon-substituted benzoporphyrins. Compared to the state-of-the-art benzoporphyrins these dyes benefit from much lower singlet-triplet energy gap which allows excitation at the longer wavelengths without the loss of the sensitizing efficiency. As can be seen (Fig. 1.3) some of the new dyes show efficient upconversion even upon excitation with deep red light. Since the new Schiff bases and benzo/naphthoporphyrins prepared in our group represent efficient sensitizers of TTA upconverison, their mixture was utilized to boost the efficiency of the upconversion by harvesting a much broader part of sunlight than is possible with individual indicators. [5] This work realized together with collaboration partners paves the way to more efficient photovoltaic devices.
Figure 1.3. (a) Emission spectra of 1×10-4 M Pt/Pd-O-S complexes in toluene in presence of 5×10-4 M Solvent Green 5 (SG5) as annihilator when excited with a 450 W xenon-lamp. (b) Photographic images of 5×10-5 M deoxygenated toluene solutions of various sensitizers and different annihilators (C = 2.5×10-4 M; Per = perylene, SG5 = solvent green 5 & LOr = Lumogen orange) upon excitation with several laser diodes (675 nm, 650 nm and 635 nm, top to bottom).
As was mentioned above, oxygen is a powerful triplet quencher and should be eliminated from the system to enable upconversion. However, this process can be also used to design highly sensitive oxygen sensors with anti-Stokes emission. We presented the first system for oxygen sensing which is neither based on multiphoton absorption nor on lanthanide upconversion. [3] Both sensitizer (Pt(II) porphyrin) and annihilator (perylene dye solvent green 5) has been dissolved in a solution of a high-boiling solvent which was placed in the pores of controlled pore glass beads. The latter were dispersed in a gas-permeable silicone rubber or Teflon AF matrix. Oxygen-dependent luminescence spectra were observed both for residual phosphorescence of the sensitizer and for the upconverted fluorescence of the annihilator. Notably, oxygen sensitivity of the latter is much higher (> 2 orders of magnitude). Also, the Stern-Volmer plots for the upconverted fluorescence are significantly different for the fluorescence intensity and fluorescence decay time. In fact, whereas the decay time plots are described by the linear fit, the intensity plots feature a quadratic dependency. Thus, oxygen sensors based on TTA provide a unique possibility of ultra-sensitive sensing of oxygen in different modalities, thus covering a broad dynamic range.

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References:

[3] Borisov, S. M.; Larndorfer, C.; Klimant, I. Triplet–Triplet Annihilation-Based Anti-Stokes Oxygen Sensing Materials with a Very Broad Dynamic Range. Advanced Functional Materials 2012, 22, 4360–4368. [4] Borisov, S. M.; Saf, R.; Fischer, R.; Klimant, I. Synthesis and Properties of New Phosphorescent Red Light-Excitable Platinum(II) and Palladium(II) Complexes with Schiff Bases for Oxygen Sensing and Triplet–Triplet Annihilation-Based Upconversion. Inorg. Chem. 2013, 52 (3), 1206–1216. [5] Monguzzi, A.; Borisov, S. M.; Pedrini, J.; Klimant, I.; Salvalaggio, M.; Biagini, P.; Melchiorre, F.; Lelii, C.; Meinardi, F. Efficient Broadband Triplet–Triplet Annihilation-Assisted Photon Upconversion at Subsolar Irradiance in Fully Organic Systems. Adv. Funct. Mater. 2015, 25 (35), 5617–5624.

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