Prediction of Internal Reorganization Energy in Photoinduced Electron Transfer Processes of Molecular Dyads

Photophysics and charge transfer in oligo(thiophene) based conjugated diblock oligomers

This paper reports an investigation of the electronic structure and photophysical properties of two "diblock" π-conjugated oligomers (T4-TBT and T8-TBT) that feature electron rich tetra(thiophene) (T4) or octa(thiophene) (T8) segments linked to an electron poor 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (TBT) moiety. Electrochemistry and UV-visible absorption spectroscopy reveals that the diblock oligomers display redox and absorption features that can be attributed to the Tn and TBT units. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations support the experimental electrochemistry and optical spectroscopy results, suggesting that the frontier orbitals on the diblock oligomers retain characteristics of the individual π-conjugated segments. However, low energy optical transitions are anticipated to arise from Tn to TBT charge transfer. Fluorescence spectroscopy on the diblock oligomers reveals that the oligomers feature strongly solvent dependent fluorescence. In non-polar solvents (hexane, toluene), the emission is structured with a moderate Stokes shift; however, in more polar solvents the emission becomes broader, and red-shifts significantly. Transient absorption spectroscopy on timescales from femtoseconds (fs) to microseconds (μs) reveals that in non-polar solvents excitation produces a singlet excited state (LE) that decays uniformly to the ground state in parallel with intersystem crossing to a triplet state. By contrast, in more polar solvents, excitation produces a very short-lived excited state (1-3 ps) which evolves rapidly into a second excited state that is attributed to the charge transfer (CT) state. The fast dynamics are associated with crossing from the LE state, which is populated initially by photoexcitation, into the CT state, which then decays to the ground state. The photophysical properties and dynamics of the LE and CT excited states are very similar for T4-TBT and T8-TBT, suggesting that the length of the oligo(thiophene) segment does not have a strong influence on the energy, structure or dynamics of the LE and CT excited states.

Estimation of Excited-State Geometries of Benzene and Fluorobenzene through Vibronic Analyses of Absorption Spectra

The parameters used in theoretical modeling of vibrational patterns within Franck-Condon (FC) approximation can be adjusted to match the vibrationally well-resolved experimental absorption spectrum of molecules. These simulation parameters can then be used to reveal the structural changes occurring between the initial and final states assuming the harmonic oscillator approximation holds for both states. Such a theoretical approach has been applied to benzene and fluorobenzene to disclose the first excited-state geometries of both compounds. The carbon-carbon bond length of benzene in the ¹B_2u state has been calculated as 1.430 Å, which is in very good agreement with the experimental bond length of 1.432 Å. The FC spectral fit method has been exploited to reveal the ¹B₂ state of fluorobenzene as well. Commonly employed density functional theory (DFT) and time-dependent DFT methods have been used to calculate the ground- and excited-state geometries of both compounds, respectively. The comparison of geometrical parameters and vibrational frequencies at the relevant states shows that frequently used hybrid functionals perform quite well in the ground state, whereas their performances drop considerably while predicting the excited-state properties. Among the hybrid functionals studied, TD-B3LYP with 6-31+G(d) basis set can be chosen to calculate the excited-state properties of molecules, albeit with much less anticipation of accuracy from the performance that B3LYP usually shows at the ground state.

Reducing the internal reorganization energy via symmetry controlled π-electron delocalization

The magnitude of the reorganization energy is closely related to the nonradiative relaxation rate, which affects the photoemission quantum efficiency, particularly for the emission with a lower energy gap toward the near IR (NIR) region. In this study, we explore the relationship between the reorganization energy and the molecular geometry, and hence the transition density by computational methods using two popular models of NIR luminescent materials: (1) linearly conjugated cyanine dyes and (2) electron donor-acceptor (D-A) composites with various degrees of charge transfer (CT) character. We find that in some cases, reorganization energies can be significantly reduced to 50% despite slight structural modifications. Detailed analyses indicate that the reflection symmetry plays an important role in linear cyanine systems. As for electron donor-acceptor systems, both the donor strength and the substitution position affect the relative magnitude of reorganization energies. If CT is dominant and creates large spatial separation between HOMO and LUMO density distributions, the reorganization energy is effectively increased due to the large electron density variation between S0 and S1 states. Mixing a certain degree of local excitation (LE) with CT in the S1 state reduces the reorganization energy. The principles proposed in this study are also translated into various pathways of canonically equivalent π-conjugation resonances to represent intramolecular π-delocalization, the concept of which may be applicable, in a facile manner, to improve the emission efficiency especially in the NIR region.

Multiscale Study of the Charge Transport Properties of Silicone Rubber Oligomers

Polymer dielectrics are widely used as insulation in electronic and electrical equipment. The charge transport process in polymer dielectrics under a high electric field, which is considered to result in electrical aging and even breakdown of equipment insulation, has attracted considerable attention. Based on a localized charge transfer model, the charge transport mechanism for typical silicone rubber (SR) insulating materials was studied by multiscale methods. A model of silicone rubber oligomers under standard conditions was generated by classical molecular dynamics. The frontier molecular orbitals and projected density of states for the SR oligomer were obtained via quantum chemistry methods. The electronic coupling, reorganization energy, and free energy difference for both electron and hole transfer processes between adjacent SR molecules in the molecular dynamics model were calculated. Both hole transfer and electron transfer in SR conform to an intermolecular hopping mechanism due to their high intramolecular reorganization energy and low intermolecular electronic coupling. The results of normal mode analysis for reorganization energy indicate that the high-temperature approximation holds for charge transport in SR around or above room temperature. The charge transfer trajectory and charge mobility in SR were simulated based on kinetic Monte Carlo simulations. The hole and electron mobilities at room temperature were calculated to be 7.24 × 10^-11 and 2.76 × 10^-9 cm²/Vs, respectively, which agrees with the experimental data. Both electron transport and hole transport in SR show thermal activation characteristics, with corresponding activation energies of 358 and 314 meV, respectively. This work suggests a physical model to describe the charge transport process in SR polymer dielectrics.

How to Predict Excited State Geometry by Using Empirical Parameters Obtained from Franck-Condon Analysis of Optical Spectrum

Excited state geometries of molecules can be calculated with highly reliable wavefunction schemes. Most of such schemes, however, are applicable to small molecules and can hardly be viewed as error-free for excited state geometries. In this study, a theoretical approach is presented in which the excited state geometries of molecules can be predicted by using vibrationally resolved experimental absorption spectrum in combination with the theoretical modelling of vibrational pattern based on Franck-Condon approximation. Huang-Rhys factors have been empirically determined and used as input for revealing the structural changes occurring between the ground and the excited state geometries upon photoexcitation. Naphthalene molecule has been chosen as a test case to show the robustness of the proposed theoretical approach. Predicted 1B_2u excited state geometry of the naphthalene has similar but slightly different bond length alternation pattern when compared with the geometries calculated with CIS, B3LYP, and CC2 methods. Excited state geometries of perylene and pyrene molecules are also determined with the presented theoretical approach. This powerful method can be applied to other molecules and specifically to relatively large molecules rather easily as long as vibrationally resolved experimental spectra are available to use.

Photoinduced Electron Transfer Reactions of Water Soluble Porphyrins in Zeolite Environment

Excited state interactions of zeolite adsorbed porphyrins have been investigated by steady state luminescence quenching technique with certain antioxidants such as reduced glutathione, ascorbic acid and L-cysteine. The zeolite supported porphyrins, meso-tetra (N-methyl-4-pyridyl) porphyrin (H₂TMPyP⁴⁺) and zinc tetra(N-methyl-4-pyridyl) porphyrin (ZnTMPyP⁴⁺) were prepared and characterized by various techniques such as Diffuse Reflectance Spectra (DRS), Scanning Electron Microscope (SEM), powder X-Ray Diffraction (XRD) and BET surface area. The interaction of zeolites with porphyrins are shown to increase the lifetime of the singlet excited state of porphyrins and decays are biphasic in nature. The splitting of the emission band of porphyrins occurs in 1:1 glycerol: water solution due to the changes in the dielectric of the solvation sphere associated with porphyrin. The Stern-Volmer plots of I₀/I vs quencher concentration [Q] were linear in the whole range of [Q] used. These studies revealed the effective quenching for zinc porphyrin compared to free base porphyrin. The effect of quenchers and zeolite acidity has also been studied and the quenching rate constant (k_q) is found in the order of 10⁹ M^-1 s^-1. The quenching reaction obeys Rehm-Weller Equation and is shown to be due to thermodynamically favoured electron transfer from quenchers to the excited singlet state of porphyrins (reductive quenching).