Supra-Nanosecond Dynamics of a Red-to-Blue Photon Upconversion System

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Abstract
Blue-green upconverted emission from 2-chloro-bis-phenylethynylanthracene (2CBPEA) sensitized by the red-absorbing platinum(II)tetraphenyltetrabenzoporphyrin (PtTPBP) has been investigated in N,N-dimethylformamide (DMF). The upconverted singlet fluorescence of 2CBPEA resulting from its sensitized triplet−triplet annihilation (TTA) is observed following selective excitation of PtTPBP at 635 ± 5 nm. Stern−Volmer analysis of the photoluminescence quenching of PtTPBP by 2CBPEA yields a bimolecular quenching constant of 1.62 × 109 M−1 s−1, slightly below the diffusion limit in DMF at room temperature. The TTA process was confirmed by the quadratic dependence of the integrated upconverted singlet fluorescence emission profile of 2CBPEA measured as a function of 635 nm incident laser power. Time-resolved emission spectra following 630 nm nanosecond laser pulses illustrate the prompt nature of porphyrin phosphorescence quenching and the delayed nature of the upconverted singlet fluorescence from 2CBPEA. Transient absorption decays monitored at the peak of the characteristic 2CBPEA triplet−triplet excited-state absorption (490 nm) measured as a function of incident nanosecond 630 nm pump laser fluence recovered the rate constant for the sensitized TTA process, kTT = 5.64 ± 0.08 × 109 M−1 s−1. To calculate this rate constant, we determined the triplet−triplet extinction coefficient of 2CBPEA (12,500 M−1 cm−1 at 490 nm) utilizing triplet energy transfer from donors with known excited-state extinction coefficients, namely [Ru(bpy)3]2+ and 2-acetonaphthone and averaged these values. The current work, to the best of our knowledge, represents the first example of red-to-blue upconversion thus demonstrating another viable sensitized TTA process, as well as providing the first measurements of kTT in a photon upconverting scheme. As 2CBPEA is stable under ambient conditions, this chromophore represents an almost ideal candidate for light-producing sensitized TTA in aerated polymeric materials, and we demonstrate successful translation of the present donor−acceptor/annihilator system into a low Tg polyurethane.