Put Your Backbone into It: Excited-State Structural Relaxation of PffBT4T-2DT Conducting Polymer in Solution

Conformational and energetic disorder in organic semiconductors reduces charge and exciton transport because of the structural defects, thus reducing the efficiency in devices such as organic photovoltaics and organic light-emitting diodes. The main structural heterogeneity is because of the twisting of the polymer backbone that occurs even in polymers that are mostly crystalline. Here, we explore the relationship between polymer backbone twisting and exciton delocalization by means of transient absorption spectroscopy and density functional theory calculations. We study the PffBT4T-2DT polymer which has exhibited even higher device efficiency with nonfullerene acceptors than the current record breaking PCE11 polymer. We determine the driving force for planarization of a polymer chain caused by excitation. The methodology is generally applicable and demonstrates a higher penalty for nonplanar structures in the excited state than in the ground state. This study highlights the morphological and electronic changes in conjugated polymers that are brought about by excitation

Put Your Backbone into It: Excited-State Structural Relaxation of PffBT4T-2DT Conducting Polymer in Solution

Conformational and energetic disorder in organic semiconductors reduces charge and exciton transport because of the structural defects, thus reducing the efficiency in devices such as organic photovoltaics and organic light-emitting diodes. The main structural heterogeneity is because of the twisting of the polymer backbone that occurs even in polymers that are mostly crystalline. Here, we explore the relationship between polymer backbone twisting and exciton delocalization by means of transient absorption spectroscopy and density functional theory calculations. We study the PffBT4T-2DT polymer which has exhibited even higher device efficiency with nonfullerene acceptors than the current record breaking PCE11 polymer. We determine the driving force for planarization of a polymer chain caused by excitation. The methodology is generally applicable and demonstrates a higher penalty for nonplanar structures in the excited state than in the ground state. This study highlights the morphological and electronic changes in conjugated polymers that are brought about by excitation

Excitation-Wavelength Dependent Photocycle Initiation Dynamics Resolve Heterogeneity in the Photoactive Yellow Protein from Halorhodospira halophila

Photoactive yellow proteins (PYPs) make up a diverse class of blue-light-absorbing bacterial photoreceptors. Electronic excitation of the p-coumaric acid chromophore covalently bound within PYP results in triphasic quenching kinetics; however, the molecular basis of this behavior remains unresolved. Here we explore this question by examining the excitation-wavelength dependence of the photodynamics of the PYP from Halorhodospira halophila via a combined experimental and computational approach. The fluorescence quantum yield, steady-state fluorescence emission maximum, and cryotrapping spectra are demonstrated to depend on excitation wavelength. We also compare the femtosecond photodynamics in PYP at two excitation wavelengths (435 and 475 nm) with a dual-excitation-wavelength-interleaved pump–probe technique. Multicompartment global analysis of these data demonstrates that the excited-state photochemistry of PYP depends subtly, but convincingly, on excitation wavelength with similar kinetics with distinctly different spectral features, including a shifted ground-state beach and altered stimulated emission oscillator strengths and peak positions. Three models involving multiple excited states, vibrationally enhanced barrier crossing, and inhomogeneity are proposed to interpret the observed excitation-wavelength dependence of the data. Conformational heterogeneity was identified as the most probable model, which was supported with molecular mechanics simulations that identified two levels of inhomogeneity involving the orientation of the R52 residue and different hydrogen bonding networks with the p-coumaric acid chromophore. Quantum calculations were used to confirm that these inhomogeneities track to altered spectral properties consistent with the experimental results.peerReviewe

Red States versus Blue States in Colloidal Silicon Nanocrystals: Exciton Sequestration into Low-Density Traps

The ultrafast exciton photodynamics of red-emitting and blue-emitting colloidal Si nanocrystals are contrasted under low (1.5 mJ/cm²) and high (9.1 mJ/cm²) excitation powers with broadband transient absorption spectroscopy. While the low-power initiated transient signals differ strongly for the two samples, the high-power signals exhibit similar nonmonotonic kinetics, resulting in a new population formed on a 10 to 30-ps time scale with a sample independent spectrum and decay kinetics. This phenomenon is ascribed to the saturation of low-density red-emitting and blue-emitting traps via a state-filling mechanism to populate new meta-stable states at higher excitation powers. The states responsible for blue emission and high-power populations are ascribed to traps from low-density nitrogen and oxygen impurities, respectively, and share similar charge-transfer character with the silicon nanocrystal core

Excitation-Wavelength-Dependent Photocycle Initiation Dynamics Resolve Heterogeneity in the Photoactive Yellow Protein from <i>Halorhodospira halophila</i>

Photoactive yellow proteins (PYPs) make up a diverse class of blue-light-absorbing bacterial photoreceptors. Electronic excitation of the <i>p</i>-coumaric acid chromophore covalently bound within PYP results in triphasic quenching kinetics; however, the molecular basis of this behavior remains unresolved. Here we explore this question by examining the excitation-wavelength dependence of the photodynamics of the PYP from <i>Halorhodospira halophila</i> via a combined experimental and computational approach. The fluorescence quantum yield, steady-state fluorescence emission maximum, and cryotrapping spectra are demonstrated to depend on excitation wavelength. We also compare the femtosecond photodynamics in PYP at two excitation wavelengths (435 and 475 nm) with a dual-excitation-wavelength-interleaved pump–probe technique. Multicompartment global analysis of these data demonstrates that the excited-state photochemistry of PYP depends subtly, but convincingly, on excitation wavelength with similar kinetics with distinctly different spectral features, including a shifted ground-state beach and altered stimulated emission oscillator strengths and peak positions. Three models involving multiple excited states, vibrationally enhanced barrier crossing, and inhomogeneity are proposed to interpret the observed excitation-wavelength dependence of the data. Conformational heterogeneity was identified as the most probable model, which was supported with molecular mechanics simulations that identified two levels of inhomogeneity involving the orientation of the R52 residue and different hydrogen bonding networks with the <i>p</i>-coumaric acid chromophore. Quantum calculations were used to confirm that these inhomogeneities track to altered spectral properties consistent with the experimental results