The influence of nanoparticles on the excitation energies of the photochromic dihydroazulene/vinylheptafulvene system.

This paper studies how nanoparticles affect photochromic systems, focusing on the influence of gold nanoparticles on the optical properties of the dihydroazulene/vinylheptafulvene (DHA/VHF) system. This investigation is done using a combined quantum mechanical/molecular mechanical approach treating the photochromic system quantum mechanically. The gold nanoparticle is described as gold atoms with atomic polarizabilities using molecular mechanics, thus leaving out excitations of the gold nanoparticle. The photochromic molecule is described by density functional theory using the long-range corrected functional CAM-B3LYP and the correlation consistent basis set aug-cc-pVDZ. The results show that the optical properties of the photochromic system are affected by the presence of the nanoparticle. The vinylheptafulvene molecule is especially influenced, when increasing the interaction between the molecule and the gold nanoparticle. We observe that the optical properties of the photochromic system are influenced strongly by conformational changes, relative orientation compared to the gold nanoparticle, and the molecule-cluster distance. We observe that the different molecules are affected differently by the nanoparticle. Furthermore, we suggest that experiments should be carried out to investigate how the molecules coordinate to the nanoparticle.

The influence of nanoparticles on the polarizabilities and hyperpolarizabilities of photochromic molecules.

We consider how nanoparticles affect molecular photoswitches and our focus is on how the polarizabilities and hyperpolarizabilities of the dihydroazulene/vinylheptafulvene system changes, when the compounds interact with gold nanoparticles. We have utilized a combined quantum mechanical/molecular mechanical approach, where the photochromic molecule is described by time-dependent density functional theory using the long-range-corrected CAM-B3LYP functional and the correlation consistent aug-cc-pVDZ basis-set. The nanoparticles are described by gold atoms having an atomic polarizability. The calculations showed that the polarizabilities of the photochromic molecules are affected by the gold nanoparticles, but the hyperpolarizabilities change significantly as the interactions between the nanoparticles and the photochromic molecules are increased.

Density Functional Theory Study of the Solvent Effects on Systematically Substituted Dihydroazulene/Vinylheptafulvene Systems: Improving the Capability of Molecular Energy Storage.

Former work has improved the energy storage capacity of the dihydroazulene/vinylheptafulvene photo/thermoswitch by substitution with NH2 and NO2 in vacuum. This work extends the former by investigating the solvent effects systematically using cyclohexane, toluene, dichloromethane, ethanol, and acetonitrile and comparing them with the inclusion of vacuum calculations. The investigation includes more than 8000 calculations using density functional theory for comparison of energy storage capacities, activation energies for the thermal conversion of vinylheptafulvene to dihydroazulene, and UV-Vis absorption spectra. We thereby establish design and solvent guidelines in order to obtain an optimal performance of the dihydroazulene/vinylheptafulvene system for use in a solar energy harvesting and storing device.

Theoretical Investigation of Substituent Effects on the Dihydroazulene/Vinylheptafulvene Photoswitch: Increasing the Energy Storage Capacity.

We have investigated the effects of substituents on the properties of the dihydroazulene/vinylheptafulvene photoswitch. The focus is on the changes of the thermochemical properties by placing electron withdrawing and donating groups on the monocyano and dicyano structures of the parent dihydroazulene and vinylheptafulvene compounds. We wish to increase the energy storage capacity, that is, the energy difference between the dihydroazulene and vinylheptafulvene isomers, of the photoswitch by computational molecular design and have performed over 9000 electronic structure calculations using density functional theory. Based on these calculations, we obtain design rules for how to increase the energy storage capacity of the photoswitch. Furthermore, we have investigated how the activation energy for the thermally induced vinylheptafulvene to dihydroazulene conversion depends on the substitution pattern, and based on these results, we have outlined molecular design considerations for obtaining new desired target structures exhibiting long energy storage times. Selected candidate systems have also been investigated in terms of optical properties to elucidate how sensitive the absorption maxima are to the functionalizations.

Characterisation of dihydroazulene and vinylheptafulvene derivatives using Raman spectroscopy: The CN-stretching region.

The effect of adding electron donating and withdrawing groups on the dihydroazulene (DHA)/vinylheptafulvene (VHF) photochromic system has been investigated using Raman spectroscopy in CS2 solutions. The photoswitching between DHA and VHF is often characterised with UV-Vis spectroscopy. However, Raman spectroscopy can also be used for this purpose and give structural insight, as the light induced ring-opening from DHA to VHF causes changes in the CN-stretching frequencies. The CN-stretching frequencies in DHA and VHF are isolated and optimal for the identification of DHA and VHF. The DHA system is also investigated in the solid state.

Computational methodology study of the optical and thermochemical properties of a molecular photoswitch.

We assess how the utilization of different DFT functionals for obtaining the equilibrium geometries and vibrational frequencies affect the description of the thermochemistry and subsequent calculation of the optical properties of a dihydroazulene-vinylheptafulvene photoswitch. The assessment covers nine popular DFT functionals (BLYP, B3LYP, CAM-B3LYP, M06-L, M06, M06-2X, PBE, PBE0, and ωB97X-D) in conjugation with five different Pople style basis sets (6-31+G(d), 6-31++G(d,p), 6-311+G(d), 6-311++G(d,p), and 6-311++G(3df,3pd)). It is identified that only CAM-B3LYP, M06-2X, and PBE0 are able to quantitatively describe the correct trends in the thermochemical properties. The subsequent calculation of the optical properties using the CAM-B3LYP functional shows that there is little difference in whether the CAM-B3LYP, M06-2X, or PBE0 functionals have been used to calculate the equilibrium geometries. Utilizing the identified functionals, we investigate how the number of electron withdrawing cyano substituents influence the thermochemistry and optical properties of the molecular photoswitch.