Coordination Chemistry Reviews 254 (2010) 2560–2573 Contents lists available at ScienceDirect Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr Review Photon upconversion based on sensitized triplet–triplet annihilation Tanya N. Singh-Rachford, Felix N. Castellano ∗ Department of Chemistry, Center for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403, United States Contents 1. Introduction ..........................................................................................................................................2560 1.1. Original experimental observations of TTA in solution ......................................................................................2560 1.2. Requirements for the sensitizer and acceptor/annihilator molecules .......................................................................2561 2. Photon upconversion in solution ....................................................................................................................2561 2.1. Development of metal–organic upconverting compositions ................................................................................2561 2.2. Upconversion quantum yields ............................................................................................................... 2565 2.3. Triplet–triplet annihilation rate constants in solution.......................................................................................2566 3. Alternative acceptor/annihilators....................................................................................................................2567 4. Quartic incident light power dependence realized in photon upconversion .......................................................................2568 5. Photon upconversion in rubbery polymeric materials ..............................................................................................2568 6. Concluding remarks ..................................................................................................................................2571 Acknowledgments ...................................................................................................................................2572 References ........................................................................................................................................... 2572 article info abstract Article history: Photon upconversion, the process wherein light of long wavelength is frequency converted to photons Received 6 October 2009 of higher energy, is readily achieved at low incident power through sensitized triplet–triplet anni- Accepted 22 January 2010 hilation (TTA) in various chromophore combinations spanning the UV to the near-IR. This emerging Available online 1 February 2010 wavelength-shifting technology truly represents a viable route towards converting low energy terres- trial solar photons into light adequate to drive electron transfer in operational photovoltaics. Generalized Keywords: molecular design constraints, all operational examples reported to date, and measurement techniques Photon upconversion applied to these low power nonlinear processes are reviewed in this contribution. In many instances, Nonlinear photochemistry Triplet–triplet annihilation direct visualization of this phenomenon is presented in solution and within various polymeric host Triplet sensitization materials. Transient spectroscopy © 2010 Elsevier B.V. All rights reserved. Multiphoton excitation Low Tg polymers Solid-state upconversion 1. Introduction chromophores produced photoluminescence blue-shifted with respect to the exciting light and the emission intensity displayed 1.1. Original experimental observations of TTA in solution nonlinear incident light fluence dependence. The results of these original findings were interpreted as a two-quantum process Photon upconversion accomplished through sensitized TTA was involving triplet excited states leading to anti-Stokes delayed flu- first introduced by Parker and Hatchard over 40 years ago wherein orescence emanating from the acceptor’s singlet excited state. upconverted fluorescence was observed from donor/acceptor solu- Investigators remained fixated on utilizing various combinations of tion mixtures containing phenanthrene/naphthalene or proflavin aromatic hydrocarbons to generate upconverted photons through hydrochloride/anthracene [1]. Selective excitation of the donor this sensitized TTA mechanism for many of the years that followed [2–5]. Some of the initial research in this area incorporating metal- containing chromophoric species included the observation of S2 delayed fluorescence in metalloporphyrins [6] and TTA-producing ∗ Corresponding author. delayed fluorescence in K4[Pt2(P2O5H2)4], “platinum pop” [7]. Out- E-mail address: [email protected] (F.N. Castellano). side of these latter spurious reports, further developments in 0010-8545/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ccr.2010.01.003 T.N. Singh-Rachford, F.N. Castellano / Coordination Chemistry Reviews 254 (2010) 2560–2573 2561 TTA-sensitized photon upconversion would not emerge until the 21st century. Most recently, this field has experienced a significant reju- venation spawned by the introduction of various metallated triplet sensitizers used in conjunction with appropriate accep- tors/annihilators spanning the visible-to-near-IR region of the spectrum. These combinations of chromophores have successfully generated upconverted photons that can now be routinely dis- cerned by the naked eye. The visualization of these newly conceived sensitized TTA-producing compositions in solution and a variety Scheme 1. Generalized energy level diagram of the upconversion processes of polymeric media at low excitation power are reviewed herein. between the triplet states of the sensitizer molecule and the triplet acceptor molecule leading to the singlet delayed fluorescence. Colored solid lines represent Molecular design constraints and measurement techniques that are radiative processes. GS is ground state, ES is excited state, ISC is intersystem crossing, applied to these low power nonlinear processes are also presented. TTET is triplet–triplet energy transfer, and TTA is triplet–triplet annihilation. 1.2. Requirements for the sensitizer and acceptor/annihilator anti-Stokes to the excitation light possessing a spectral profile molecules identical to the acceptor molecule. The intensity of this singlet fluo- rescence displays a quadratic (x2) incident light power dependence The sensitized triplet–triplet annihilation mechanism involves as TTA requires reaction between two sensitized triplet acceptor the transfer of energy between a sensitizer (donor) molecule and an molecules. acceptor/annihilator. Hence, several factors are considered when determining the proper combination of chromophores in a viable 2. Photon upconversion in solution upconverting scheme. Sensitizers are selected based on their abil- ity to absorb light in the visible-to-near-IR region of the spectrum 2.1. Development of metal–organic upconverting compositions allowing for low energy excitation and must possess a relatively long triplet excited state lifetime, typically on the order of several The first example of photon upconversion that emerged from microseconds and beyond. This latter requirement is necessary to our research group involved observations resulting from long facilitate conditions enabling efficient diffusional-based quench- 2+ wavelength excitation of [Ru(dmb)2(bpy-An)] (dmb is 4,4 - ing. In our research group, metallated sensitizers including those dimethyl-2,2-bipyridine and bpy-An is 4-methyl-4(9-anthryl)- that exhibit metal-to-ligand charge transfer (MLCT) excited states 2,2 -bipyridine) in CH3CN [8]. Here the triplet MLCT excited state as well as heavy metal-containing porphyrins and phthalocya- is higher in energy than the triplet state of anthracene, while the ␲ nines are generally studied since the presence of the -conjugated singlet excited state of anthracene is higher in energy than that aromatic rings in the latter shift their respective absorption and of the 1MLCT and is lower than two times the triplet state of emission maxima towards the red and near-IR region of the anthracene [9–13], thereby satisfying the energy requirements for spectrum. The heavy metal present in the porphyrins and phthalo- sensitized TTA. Upon selective excitation of the MLCT transition at cyanines (Pd or Pt) strongly enhances the spin-orbit coupling which 450 nm, intramolecular energy transfer produces long-lived triplet yields singlet-triplet intersystem crossing (ISC) efficiencies near anthracene and when two energized chromophore-quencher unity. In order to observe bimolecular quenching of the triplet molecules diffuse to form an encounter complex leading to TTA, one excited state of the sensitizer the triplet acceptor energy must ground state and one excited singlet state chromophore is gener- be lower than the triplet energy of the sensitizer. The greater the ated. The latter decays radiatively to the ground state as evidenced energy difference between the triplet sensitizer and triplet accep- by delayed fluorescence emission from anthracene [8]. It should tor, the greater the driving force for this reaction and generally be noted that although intramolecular singlet energy transfer pro- speaking, the more favorable the triplet energy transfer process. It vides an efficient nonradiative deactivation pathway [11,14–16], is also