Abstract
Managing the excited-state decay by a supramolecular structure is a crucial issue for organic photovoltaics. We show that in thin films of metallo-supramolecular polymers made of bis(terpyridine-4′-yl)terthiophenes and coupling ions, the photoexcited states generated by ultrashort laser pulses at the wavelength of 440 nm decay by the bi-molecular annihilation predominantly controlled by the Förster transfer between singlet states. During this bi-molecular annihilation of singlet states, intermediate hot triplet pairs are formed, which subsequently dissociate into long-living diffusing triplet states. It explains a significant shortening of the triplet state rise time with increasing pump fluence. The diffusion coefficient of triplets showed power-law time dependence, with its exponent proportional to the pump fluence, decreasing thus the diffusivity of triplets.
Highlights
Metallo-supramolecular polymers (MSPs) are composed of defined simple or oligomeric organic molecules capped with chelating end-groups that spontaneously assemble into polymer chains by coordination to metal ions (Ciferri, 2002)
We show that in thin films of metallo-supramolecular polymers made of bis(terpyridine-4′-yl)terthiophenes and Zn2+ coupling ions, the photoexcited states generated by ultrashort laser pulses at the wavelength of 440 nm decay by the bi-molecular annihilation predominantly controlled by the Förster transfer between singlet states
Recent finding of Kinetics of the Photoexcited States an efficient singlet fission process in thin films of MSP based on α,ω−bis(tpy)terthiophene unimers assembled with Zn2+ ion couplers (Rais et al, 2017b) shows that some MSPs could be potentially exploited for better light harvesting in photovoltaics
Summary
Metallo-supramolecular polymers (MSPs) are composed of defined simple or oligomeric organic molecules capped with chelating end-groups (referred to as unimers) that spontaneously assemble into polymer chains by coordination to metal ions (dubbed ion couplers) (Ciferri, 2002). The dependences plotted in the log–log scale are less sensitive to the linearly or quadratically increasing value of F, we can still see general trends For the former case (Figure 8A), we see that the maximum of the peaks shifts systematically to the left with increasing pump fluence F if the triplet populations are obtained either from the spectral analysis ESA or by the subtraction of the ground-state bleach and singlet populations. We did not show the diffusion coefficient DT1d(t) in absolute values for several reasons: First, at early times, the evolution of triplets is strongly proportional to the decay rate of singlets This rate was obtained by fitting the experimental data, which brings some numerical uncertainty. In the system discussed in this article, the structure can be disturbed by both the electronic excitations and heat dissipation at stronger pump fluence which, subsequently, can affect triplet diffusivity
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