Tanya N. Singh-Rachford

Boron Dipyrromethene Chromophores: Next Generation Triplet Acceptors/Annihilators for Low Power Upconversion Schemes

In the present study, the red-light absorbing platinum(II) tetraphenyltetrabenzoporphyrin (PtTPBP) was used as a triplet sensitizer in conjunction with two distinct iodophenyl-bearing BODIPY derivatives independently serving as triplet acceptors/annihilators poised for photon upconversion based on triplet-triplet annihilation. In deaerated benzene solutions, extremely stable and high quantum efficiency green (Phi(UC) = 0.0313 +/- 0.0005) and yellow (Phi(UC) = 0.0753 +/- 0.0036) upconverted emissions were observed from selective red excitation of the PtTPBP sensitizer at 635 +/- 5 nm. The current systems represent the first examples of photon upconversion where aromatic hydrocarbons do not serve the role of triplet acceptor/annihilator. Notably, the nature of the current chromophore compositions permitted highly reproducible upconversion quantum efficiency determinations while permitting the evaluation of the triplet-triplet annihilation quantum yields in both instances.

Influence of Temperature on Low-Power Upconversion in Rubbery Polymer Blends

The upconverting properties of a dye cocktail composed of palladium(II) octaethylporphyrin (PdOEP, triplet sensitizer) and 9,10-diphenylanthracene (DPA, triplet acceptor/annihilator) were investigated as a function of temperature in several low glass transition temperature (T(g)) polymer hosts including an ethyleneoxide-epichlorohydrin copolymer (EO-EPI) and the polyurethanes Texin 270, Texin 285, and Tecoflex EG-80A. Selective excitation of PdOEP at 544 nm in the presence of DPA in these materials resulted in anti-Stokes blue emission from DPA, a consequence of sensitized triplet-triplet annihilation (TTA) photochemistry, confirmed by the quadratic dependence of the upconverted fluorescence intensity with respect to incident light power. The upconversion process was completely suppressed by cooling a PdOEP/DPA blend film to below the T(g) of the respective polymer. However, the blue emission was clearly visible by the naked eye upon heating these films to room temperature (290 K). Subsequently, the upconverted emission intensity increased with increasing temperature and was found to be completely reversible upon several heating and cooling cycles provided the temperature remained below 400 K. Heating samples above this temperature resulted in unrecoverable failure of the material to produce upconverted photons. The phosphorescence intensity decay of PdOEP in the polymer host, Tecoflex EG-80A, adequately fits to a sum of two exponential functions as well as the Kohlrausch-Williams-Watts (KWW) stretched exponential model. Increasing the temperature of the film increases the complexity and heterogeneity of the system as evidenced by the lower beta values obtained from the KWW model as the temperature increases.

Pd(II) Phthalocyanine-Sensitized Triplet-Triplet Annihilation from Rubrene

Upconverted yellow singlet fluorescence from rubrene (5,6,11,12-tetraphenylnapthacene) was generated from selective excitation (lambdaex = 725 nm) of the red light absorbing triplet sensitizer palladium(II) octabutoxyphthalocyanine, PdPc(OBu)8, in vacuum degassed toluene solutions using a Nd:YAG/OPO laser system in concert with gated iCCD detection. The data are consistent with upconversion proceeding from triplet-triplet annihilation (TTA) of rubrene acceptor molecules. The TTA process was confirmed by the quadratic dependence of the upconverted delayed fluorescence intensity with respect to incident light, measured by integrating the corresponding kinetic traces as a function of the incident excitation power. In vacuum degassed toluene solutions, the red-to-yellow upconversion process is stable under continuous long wavelength irradiation and is readily visualized by the naked eye even at modest laser fluence (0.6 mJ/pulse). In aerated solutions, however, selective excitation of the phthalocyanine sensitizer leads to rapid decomposition of rubrene into its corresponding endoperoxide as evidenced by UV-vis (in toluene), 1H NMR (in d6-benzene), and MALDI-TOF mass spectrometry, consistent with the established reactivity of rubrene with singlet dioxygen. The upconversion process in this triplet sensitizer/acceptor-annihilator combination was preliminarily investigated in solid polymer films composed of a 50:50 mixture of an ethyleneoxide/epichlorohydrin copolymer, P(EO/EP). Films that were prepared under an argon atmosphere and maintained under this inert environment successfully achieve the anticipated quadratic incident power dependence, whereas air exposure causes the film to deviate somewhat from this dependence. To the best of our knowledge, the current study represents the first example of photon upconversion using a phthalocyanine triplet sensitizer, furthering the notion that anti-Stokes light-producing sensitized TTA appears to be a general phenomenon as long as proper energy criteria are met.

Low Power Visible-to-UV Upconversion

Low power visible-to-UV photon upconversion is demonstrated for the first time, achieved from two simple organic chromophores dissolved in benzene. Selective 442 nm excitation of the triplet sensitizer 2,3-butanedione (biacetyl) in the presence of the laser dye 2,5-diphenyloxazole (PPO) results in the observation of singlet fluorescence from the latter in the UV centered at 360 nm, anti-Stokes shifted by a record 0.64 eV with respect to the excitation. All of the experimental data are consistent with the upconverted singlet PPO fluorescence being produced as a result of biacetyl-sensitized triplet-triplet annihilation (TTA) of triplet excited PPO chromophores. Nanosecond laser flash photolysis performed under pseudo-first-order conditions revealed the bimolecular rate constant of triplet-triplet energy transfer between the biacetyl sensitizer and PPO acceptor, k(q) = 9.0 x 10(8) M(-1)s(-1). The TTA process was confirmed by the quadratic dependence of the upconverted integrated PPO emission intensity measured with respect to incident 442 nm light power density. The maximum quantum yield of the upconverted emission (0.0058 +/- 0.0002) was determined relative to 1,8-diphenyl-1,3,5,7-octatetraene, both measured with 0.389 W/cm(2) incident power density. The PPO triplet-triplet annihilation rate constant (k(TT)) was determined from transient absorption decays monitored at the peak of its characteristic triplet-to-triplet excited-state absorption (500 nm) as a function of incident pulsed laser fluence; this process attains the diffusion limit in benzene at room temperature, k(TT) = 1.1 +/- 0.1 x 10(10) M(-1) s(-1).

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

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 x 10(9) 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, k(TT) = 5.64 +/- 0.08 x 10(9) 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 k(TT) 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 T(g) polyurethane.

Nonlinear Photochemistry Squared: Quartic Light Power Dependence Realized in Photon Upconversion

Simultaneous two-photon excitation of a solution mixture of [Ru(dmb)3]2+ and 9,10-diphenylanthracene (DPA) using 860 nm light pulses from a Ti:Sapphire laser resulted in triplet energy transfer followed by triplet-triplet annihilation (TTA), ultimately leading to upconverted DPA fluorescence from the sensitized DPA triplets. The photochemistry sequence was confirmed by the unprecedented quartic (x4) incident light power dependence exhibited by this process, which incidentally generated a record anti-Stokes shift of 1.38 eV for sensitized TTA.