Exciton Dynamics of Photoexcited Pendant Porphyrin Polymers in Solution and in Thin Films

Abstract

Several new polymers with rotatable zinc porphyrin pendants have been synthesized and their optical spectroscopic and photophysical properties, including upconversion efficiencies, determined in both fluid solution and thin films. Comparisons made with the β-substituted zinc tetraphenylporphyrin monomers and ZnTPP itself reveal that the yield of triplets resulting from either Q-band or Soret-band excitation of the polymers is surprisingly small. A detailed kinetic analysis of the fluorescence decays and transient triplet absorptions of the substituted monomers and their corresponding polymers reveals that this phenomenon is due to two parallel internal singlet quenching processes assigned to transient intrachain excimer formation. Consequently, the yields of upconverted S 2 fluorescence resulting from Q-band excitation in the degassed polymers are significantly diminished in both fluid solution and thin films. Implications of these results for the design of polymer upconverting systems are discussed.

Spectroscopic and Dynamic Properties of Electronically Excited Pendant Porphyrin Polymers with Backbones of Differing Flexibility

“…Observing at 442 nm (with a 10 nm bandwidth) appears to capture components of both the unperturbed S 2 emission with a maximum near 425 nm and a red-shifted band of emission that is similar in location and shape to that previously observed in the flexible pendant porphyrin polymer with the saturated hydrocarbon backbone. 20 We tentatively assign this latter weak component to porphyrin S 2 •••S 0 excimer emission, which is expected to be red-shifted relative to the unperturbed S 2 fluorescence and has been seen previously in the delayed S 2 emission of ZnTPP, resulting from triplet− triplet annihilation in degassed fluid solution. 23 In the latter report, the intensity of the delayed upconverted S 2 fluorescence, resulting from the intermolecular 2 T 1 → S 2 + S 0 process, is seen to be reduced by self-quenching of the S 2 state by its companion S 0 product molecule.…”

“…The fluorescence spectra of the porphyrin-substituted norbornene monomer, its polymer, and ZnTPP as a reference, 1 each excited at 405 and 561 nm in toluene at room temperature, are shown in Figure 2. Figure S2 in the Supporting Information provides a comparison of these fluorescence spectra with those previously reported 20 for the pendant porphyrin polymer (p-EL pol) with a flexible backbone. A further comparison of these two polymers (N pol and p-EL pol) is provided in Figure 3 where the differences in their fluorescence spectra obtained when exciting at 410 nm (at the wavelength of the hypsochromic-shifted Soret band of the norbornene polymer) and at 425 nm (near the maximum of the unshifted Soret band of the norbornene polymer) are recorded.…”

“…The nanosecond S 1 fluorescence decay profiles of the norbornene monomer and polymer are unremarkable and are similar to those previously seen in the flexible pendant porphyrin polymers. 20 Despite its meso-trialkyl porphyrin substitution, the monomer has an S 1 lifetime of 2.15 ns, similar to that of the ZnTPP standard. The polymer exhibits biexponential fluorescence decay with a major component of 0.64 ns and a minor component whose lifetime is similar to that of the monomer.…”

“…The structures of the zinc porphyrin-substituted N-arylpyrrolidine-fused norbornene monomer and its Grubbs-catalyzed ROMP polymer synthesized for this study are shown in Scheme 1. The structure of a representative pendant porphyrin polymer with a saturated hydrocarbon backbone, previously reported, 20 is also provided for comparison purposes. Details of the syntheses are provided in the Supporting Information.…”

“…We have found evidence of rapid intrachain triplet biexcitonic annihilation in a diphenylanthracene pendant polymer excited by sensitization methods 21 and have tried to find evidence of similar processes in a pendant zinc porphyrin polymer by observing upconverted delayed S 2 fluorescence following excitation in the porphyrin Q bands. 20 However, in the latter case, upconversion proved to be inefficient because the polymer employed in these experiments had a saturated carbon backbone that permitted internal rotation of the chromophores, resulting in kinetically competitive excimeric quenching of the excited species. The recent work of Lin et al 9 who have synthesized and characterized a number of pendant porphyrin polymers with rigid backbones and a range of E/Z isomeric pendant conformations has sustained our interest in these materials.…”