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To gain a deep understanding on structure-property relationship of all-small-molecule nonfullerene organic solar cells, donor molecules SM1 and SM2 with and without cyano group substituents, respectively, were selected, for which SM1-based device has obvious higher power conversion efficiency (10.11%) than SM2-based device (5.32%) in the same condition. Here a systemic study combining quantum chemistry with molecular dynamics was conducted to explore what improve the performance of SM1-based organic solar cells from the theoretical perspective. The results indicate that the cyano substituents in SM1 not only could induce a redshift absorption spectrum through decreasing the energy gap but also lead to better intramolecular charge transfer, which is beneficial to charge transfer at donor/acceptor interface. More interestingly, the SM1/acceptor interface presents an additional charge transfer state (CT state) below the first optically Frenkel exciton state (FE state) except that its FE state has stronger absorption compared with SM2/acceptor interface, which plays an important role in improving its charge transfer rate (kinter-CT) through increasing electronic coupling of charge transfer (VDA-CT). Thus it is speculated that molecular design considering additional CT states below FE state is a promising strategy in organic solar cells like SM1-based device with good performance.

As compared with the promising near-infrared (NIR) absorbing trans-squaraines (SQ), cis-squaraines (HSQ) possess special absorption spectra showing better performance and are being attracted as promising NIR chromophores in dye-sensitized solar cells (DSSCs). For further extending the absorption toward the NIR region, a rigidified dithiophene spacer 4H-Silolo[3,2-b:4,5-b’]dithiophen/e,4,4,-dimethyl (DTS) was introduced into the skeleton of HSQ (HSQ-DTS). A comprehensive analysis on the light harvesting ability, intramolecular charge separation, interfacial electron injection and dye regeneration were conducted by means of density functional theory (DFT) and time-dependent DFT calculations on the dye@(TiO2)38 system. Our results demonstrated that as compared with HSQ, HSQ-DTS could exhibit panchromatic absorption with more favorable charge separation ability, more injected electrons with inhibited charge recombination process due to the larger distance between the cation hole and the semiconductor surface. The superior performance of dye HSQ-DTS satisfied our original design intent. Therefore, based on these inspiring results, another six different rigidified dithiophene spacers (1-6) were also investigated to screen out more efficient cis-squaraine sensitizers. We find 3 with 4H-Phospholo[3,2-b:4,5-b’]dithiophene,4-ethynyl-,4-oxide possesses the best performance with the maximum short-circuit current ( J s c m a x ) achieved 31.60 mAcm−2, which is expected to support future experimental and theoretical studies.

For a photochromic spiro-diarylethene derivative (Spiro-DAE), its photocyclization reaction and multicolour fluorescence modulation performances were investigated based on a series of density functionals calculations in this work. Some elementary properties such as geometric and electronic structures at ground and the first excited state (S1), absorption and emission spectra in respective open-ring and closed-ring structures were compared, which indicate the cyclization reaction may occur at S1. It is further verified by the lower activation barrier (3.4 kcal/mol) of the transition state at S1 potential energy surface. The photochromic reaction rates for both cyclization and cycloreversion processes were quantitatively estimated, which suggests the much smaller radiative and internal conversion rates contribute to the higher photocyclization rate and thus efficiency. Furthermore, we suspect that the dihydroacridine fragment may make important contribution to the cyclization reaction occurring at S1 considering its contribution to electronic structure.

An interesting luminescent mechanochromic 9-anthryl gold(I) isocyanide complex has been reported. Its polymorphs perform different luminescent properties and their luminescence spectra are both bathochromically shifted upon grinding. This curious phenomenon arouses our interest to disclose how intermolecular weak interactions tune the luminescent properties from a theoretical perspective. The QM/MM approach was employed through all the calculations to consider the sensitive dependence of geometric and electronic structures on intermolecular interactions. The results show that the monomer is enough to describe the fluorescence of α phase crystal and its almost perpendicular dihedral angle between anthracene unit and phenyl ring leads to the forbidden emission from the first excited state which breaks Kasha’s rule. While the larger spin orbital coupling between excited singlet and triplet states of dimer makes key contribution to the phosphorescence emission of β phase crystal which is related with its near planar structure and stronger intermolecular interactions. Moreover, multiple intermolecular interactions including strong aurophilic interaction were found to be responsible for the red-shifted luminescence of the γ phase crystal. Therefore, the distinct molecular structures and intermolecular interactions codetermine the different photophysical processes in these crystal phases.