Investigations of Oscillator Strength Focusing and Beyond
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The effectiveness of solar energy capture and conversion materials derives from their ability to absorb light and to transform the excitation energy into energy stored in free carriers or chemical bonds. The Thomas-Reiche-Kuhn sum rule mandates that the integrated (electronic) oscillator strength of an absorber equals the total number of electrons in the structure. Typical molecular chromophores place only about 1% of their oscillator strength in the UV/Vis window, so individual chromophores operate at about 1% of their theoretical limit. We explore the distribution of oscillator strength as a function of excitation energy to understand this circumstance. To this aim, we use familiar independent-electron model Hamiltonians as well as first-principles electronic structure methods. While model Hamiltonians capture the qualitative electronic spectra associated with π-electron chromophores, these Hamiltonians mistakenly focus the oscillator strength in the fewest low-energy transitions. Advanced electronic structure methods, in contrast, spread the oscillator strength over a very wide excitation energy range, including transitions to Rydberg and continuum states, consistent with experiment. Our analysis rationalizes the low oscillator strength in the UV/Vis spectral region in molecules, a step toward the goal of oscillator strength manipulation and focusing.
Two types of oscillator strength focusing strategies are proposed. The first one is to borrow conclusions made in optics to build one-dimensional potential models through quantum-optical analogy. We prove that by using this analogy, oscillator strength associated with the HOMO→LUMO transition in the model can be maximized. However, using model potentials as a guide for the design of linear absorbers by simply matching the HOMO energy of each molecular building block to the potential in the model is not sufficient to fully reproduce the model’s results in real molecules qualitatively. This might result from the overestimation of the potential models.
The second strategy for oscillator strength focusing is to perturb the electronic structure of molecules. Mechanical stress (stretching or compression) applied along the molecular axis of polyenes is simulated by adding spring bond constraints in ab initial calculations, with the hypothesis that σ bonds would be greatly disturbed and the σ–σ* energy gap would be reduced. By stretching polyene molecules by 20%, the oscillator strength associated with the σ → σ* transition is significantly enhanced and becomes comparable to the π → π* value, while the excitation energy drops by up to 4 eV. We also show that electrostatic fields can be used to alter the electronic structure of polyenes so as to enhance their absorption in the UV/Vis region. Our time-dependent density functional theory calculations on single molecules (either in gas phase or absorbed on gold) and on molecular stacks indicate that the oscillator strength integrated over the visible spectral range up to the near UV can be increased by an order of magnitude depending on the strength of the applied field. This enhancement and its oscillatory response to the field intensity are rationalized using a shielded superlattice potential model and molecular orbital analysis. Our study prompts future experimental investigations of the use of electric fields to modulate the light absorption properties of materials based on linear conjugated molecules.
Red fluorescent proteins are widely used for deep-issue imaging and in super-resolution techniques due to their low photo-sensitivity and minimized light-scattering by endogenous biomolecules. mCherry is one of the most photostable red fluorescent proteins, but not the brightest. Efforts have been made but failed to enhance the fluorescent intensity of mCherry in vitro by intuitive mutagenesis aiming to create local electric field at the chromophore. In this work, we carry out a systematic exploration of the optimal field direction and intensity that maximize the electric field effect. We find that electric field applied in the +x or –x direction along the chromophore axis induces most notable changes. The enhancement of the maximum absorption is up to 45% in the presence of +x electric field with a field strength of 1.5 V/nm. Residue sites for mutagenesis are identified targeting the formation of the optimal local electric field, which is a promising strategy to improve the brightness of red fluorescent proteins.
Lastly, we elucidate basic side chain effect on the conductivity of self-assembled cyclic peptide nanotubes, an attractive bioinspired material for proton conducting devices. Experimental measurements find lysine-containing peptide nanotubes to be much better conductor than arginine- and histidine-containing counterparts. Molecular dynamics are carried out on hexamer model systems (with and without structural constraints). By analyzing four possible proton transfer configurations in the snapshots, effective proton transfer rates and mean-squared proton couplings are calculated for the three systems. Our results show that the flexibility of the lysine side chain enables a large contribution of cross-layer proton transfer to the overall proton transfer rate, yet this configuration requires certain degree of freedom in structure distortions. A relatively large mean-square proton coupling for lysine-containing peptides is another key determinant of its high conductivity. Our study sheds lights on rational designs of highly conductive synthetic peptide nanostructures, and promotes further investigations on improved accuracy in the modeling of cyclic peptide nanotube systems.
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