We present a theoretical investigation of the effects of substitution of the triphenylamine (TPA) block on the overall properties of materials based on small push–pull molecules designed as donors ...for organic photovoltaics (OPV). In particular, we exploit modern computational techniques such as density functional theory (DFT), time-dependent DFT, molecular dynamics, and the Marcus theory to analyze the charge and exciton transport properties in crystalline and amorphous phases of four compounds in which one phenyl ring of the TPA block of 2-(5-{4-methyl(phenyl)aminophenyl}thiophen-2-yl)methylenemalononitrile is replaced with a methyl, an α-naphthyl, and a β-naphthyl. Our calculations unveil the molecular rationale behind the different transport properties observed in the experiments. We show that, although the effects of the substituents on the electronic and optical properties are negligible, they have an impact on the molecular packing of the crystalline structure, thus explaining the different macroscopic transport properties (observed and calculated). In particular, the substitution of a phenyl with a methyl favors face-to-face π–π packing in the crystal structure and allows a good π-orbital overlap and high hopping rates. On the other hand, the introduction of an α-naphthyl group generates a steric hindrance that negatively affects the transport properties. Moreover, the investigated substitutions do not significantly influence the degree of local order in the amorphous bulks displaying complete disorder and low hole mobilities. These results, in agreement with the experimental findings, suggest that our computational approach is able to account for the macroscopic effect of subtle transformations of a molecular structure on transport properties and thus can be further employed to obtain valuable insights into the molecular design of optimized active materials for OPV.
A case study of 1,8-dihydroxy-2-napthaldehyde (DHNA)exhibiting an excited-state intramolecular double proton transfer resulting in photophysical properties sensitive to the surrounding ...environmenthas been used to assess the performance of electrostatic embedding approaches designed to accurately recover the effects of a bulk crystalline environment on calculated photophysical properties. The first approach, based on time-dependent density functional theory (TD-DFT) applied in a QM/QM′ scheme, makes use of a background point charge distribution which can accurately reproduce the exact ground-state Ewald potential of the bulk crystal. The second approach seeks to “optimize” these charges in a self-consistent manner in order to reproduce the electrostatic field produced by the environment at the excited state. Using these two approaches, both absorption and emission properties of molecular crystals, such as the position and the relative shift in the emission bands in the solid state with respect to solution, can be accurately reproduced. More generally, the results obtained show how these computationally affordable approaches can be used to predict the excited-state behavior of molecules in condensed phases, thus allowing their employment to predict or design new molecular materials with enhanced photophysical properties.
The structures of low-lying singlet excited states of nine π-conjugated heteroaromatic compounds have been investigated by the symmetry-adapted cluster-configuration interaction (SAC-CI) method and ...the time-dependent density functional theory (TDDFT) using the PBE0 functional (TD-PBE0). In particular, the geometry relaxation in some ππ* and nπ* excited states of furan, pyrrole, pyridine, p-benzoquinone, uracil, adenine, 9,10-anthraquinone, coumarin, and 1,8-naphthalimide as well as the corresponding vertical transitions, including Rydberg excited states, have been analyzed in detail. The basis set and functional dependence of the results was also examined. The SAC-CI and TD-PBE0 calculations showed reasonable agreement in both transition energies and excited-state equilibrium structures for these heteroaromatic compounds.
We present an analysis on the behavior of the TD-DFT approach in the determination of excited-state structures with particular attention to single and double bonds. The analysis is based on a direct ...comparison with the highly correlated CASPT2 ab initio approach. Six DFT exchange-correlation functionals differing in the Hartree−Fock exchange percentage and the type of correlation functional are considered and applied to the study of seven prototype organic molecules characterized by two families of excitations (acrolein, acetone, diazomethane, and propanoic acid anion for n−π* and cis-1,3-butadiene, trans-1,3-butadiene, and pyrrole for π−π*), and three protonated Schiff bases, used as model chromophores for 11-cis retinal. Our analysis allows pinpointing specific correlations between accuracy of the various functionals and category of excitation and/or type of chemical bond involved in the corresponding geometry relaxation. We confirm the role of the long-range correction of the potential to obtain a balanced description of excitation energies and excited-state structures, but we also point out that, for a small system, B3LYP and PBE0 also give results close to CASPT2.
We individuate a photoinduced electron transfer (PeT) as a quenching mechanism affecting rhodamine B photophysics in solvent. The PeT involves an electron transfer from the carboxylate group to the ...xanthene ring of rhodamine B. This is finely modulated by the subtle balance of coulombic and non-classical interactions between the carboxyphenyl and xanthene rings, also mediated by the solvent. We propose the use of an electronic density based index, the so called DCT index, as a new tool to assess and quantify the nature of the excited states involved in non-radiative decays near the region of their intersection. In the present case, this analysis allows us to gain insight on the interconversion process from the bright state to the dark state responsible for the quenching of rhodamine B fluorescence. Our findings encourage the use of density based indices to study the processes affecting excited state reactions that are characterized by a drastic change in the excitation nature, in order to rationalize the photophysical behavior of complex molecular systems.
The mechanism of base to base intermolecular proton shuttling occurring in the excited state proton transfer reaction between 7-hydroxy-4-(trifluoromethyl)coumarin (CouOH) and concentrated ...1-methylimidazole base (1-MeId) in toluene solution is disclosed here by means of a computational approach based on Density Functional Theory (DFT) and Time Dependent DFT (TD-DFT). These methods allow us to characterize both the ground and excited state potential energy surfaces along the proton shuttling coordinate, and to assess the nature of the emitting species in the presence of an excess of 1-MeId. As a result, the tautomerism of CouOH is found to be photo-activated and, from a mechanistic point of view, the calculations clearly show that the overall driving force of the entire shuttling is the coumarin photoacidity, which is responsible for both the first proton transfer event and the strengthening of the following chain mechanism of base to base proton hopping.
Hazards posed by chemical incompatibility, especially in a large-scale industrial environment, warrant a deeper understanding of the mechanisms of the reactions involved in these phenomena. In this ...study, reactions between ammonium nitrate and two sodium salts, namely, sodium nitrate and sodium nitrite, have been studied by ab initio calculations (with density functional theory, DFT) and experimental calorimetric methods (with differential scanning calorimetry, DSC, and heat flux calorimetry, HFC). The agreement between theoretical and experimental results allows an understanding of the thermal decomposition behaviors of the two sodium salts when exposed to ammonium nitrate. Moreover, this study highlighted the critical role of the water that appears to promote the incompatibility between ammonium nitrate and sodium nitrite.
In this work, we investigated the properties of the triplet excited states of heterocyclic compounds including their geometries, electronic properties, and phosphorescence energies by using both the ...direct symmetry-adapted cluster-configuration interaction (SAC-CI) method and the TD-DFT approach with the PBE0 exchange-correlation functional (TD-PBE0). The target states are the ππ* and nπ* triplet states of furan, pyrrole, pyridine, p-benzoquinone, uracil, adenine, 9,10-anthraquinone, coumarin, and 1,8-naphthalimide as well as the Rydberg states. The present benchmark demonstrates that these two methods provide reasonably accurate geometries for the excited states of these heterocyclic compounds. The calculated Stokes shifts, which reflect geometry changes, were consistent for both these methods. The trends of agreement with experimental or reference values obtained for a panel of exchange-correlation functionals used to compute the absolute emission energies from the triplet states, differ from those found for the singlet excited states. Some of the low-lying triplet excited states were examined in detail for the first time, including vibrational analysis.
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