•Properties of dyes related to the Jsc and Voc of DSSCs were investigated based on DFT calculation.•The designed HIS-2a shows red-shifted absorption spectra and enhanced transition intensity in NIR region.•HIS-2a is a promising candidate to further increase efficiency.To overcome the shortcomings of traditional Ru dyes in low molar extinction coefficient (ϵ) of the metal-to-ligand charge transfer band in the near-infrared region, we designed and studied two Ru dyes, HIS-2a and HIS-2b derived from HIS-2. The intrinsic causes for varying short-circuit photocurrent density (Jsc) and open-circuit photovoltage (Voc) have been systematically investigated on the dyes and dye/(TiO2)38 systems including the conduction band energy shift, electronic structures, light harvesting efficiency, electron injection driving force, electronic coupling, dye regeneration driving force and so on. Those results suggest that, compared with HIS-2 and HIS-2b, HIS-2a has red-shifted absorption spectrum and increased transition intensity in NIR region, which would lead to a larger Jsc. At the same time, the conduction band energy of HIS-2a shifts little compared with other dyes, which will not have much effect on Voc. Hence, we could speculate that HIS-2a is a promising candidate with enhanced Jsc and a more efficient Ru(II) π-expanded terpyridyl dye. We also hope that this study will provide theoretical guidelines for the synthesis of new Ru dyes.To overcome the shortcomings of traditional Ru dyes in low molar extinction coefficient (ϵ) in the near-infrared region, two Ru dyes, HIS-2a and HIS-2b derived from HIS-2, were designed and studied in depth based on density functional theory calculations.

•The different performance of reported dyes was rationalized by DFT calculations.•Present a comprehensive strategy to design high-efficiency dye.•Designed dyes 6 and 7 are promising candidates to further improve efficiency.Factors associated with short circuit current density (Jsc) and open circuit photovoltage (Voc) of dye sensitized solar cells (DSSCs) have been analyzed through DFT and TDDFT calculations to explore the origin of the significant performance differences with only tiny structure difference (1.24% for 1 and 8.21% for 2) (Advanced Functional Materials 2012, 22, 1291–1302). Our results reveal that the insertion of phenyl ring in 2 enlarges the distance between the dye cation hole and the semiconductor surface and makes the benzothiadiazole (BTDA) unit, which has strong interaction with the electrolyte, far away from the semiconductor, resulting in a decreased charge recombination rate compared with that of 1. However, the insertion of phenyl ring also results in a distortion of the molecular structure, leading to a decreased light harvesting ability. Hence, two dyes (6 and 7) derived from 2 with better conjugation degree, farther position of BTDA unit and longer molecular length have been designed to keep the advantages and overcome the disadvantages of 2 simultaneously. The results demonstrate that we get the desired properties of dyes through reasonable molecular design, and these two dyes could be promising candidates in DSSC field and further improve the performance of the cell.

•One synthesized and two designed guanidinate-based iridium (III) complexes are investigated.•Phosphorescence quantum efficiency is evaluated with the aid of kr and knr values.•Different cyclometalated ligands can greatly affect the photophysical properties of complexes.•Changing the pyridine ring with diazole groups can significantly enhance the SOC effects.•The designed complex 3 is expected to be a promising candidate for highly efficient guanidinate-based phosphorescence emitter.A density functional theory/time-depended density functional theory was used to investigate the synthesized guanidinate-based iridium(III) complex [(ppy)2Ir{(NiPr)2C(NPh2)}] (1) and two designed derivatives (2 and 3) to determine the influences of different cyclometalated ligands on photophysical properties. Except the conventional discussions on geometric relaxations, absorption and emission properties, many relevant parameters, including spin-orbital coupling (SOC) matrix elements, zero-field-splitting parameters, radiative rate constants (kr) and so on were quantitatively evaluated. The results reveal that the replacement of the pyridine ring in the 2-phenylpyridine ligand with different diazole rings cannot only enlarge the frontier molecular orbital energy gaps, resulting in a blue-shift of the absorption spectra for 2 and 3, but also enhance the absorption intensity of 3 in the lower-energy region. Furthermore, it is intriguing to note that the photoluminescence quantum efficiency (ΦPL) of 3 is significantly higher than that of 1. This can be explained by its large SOC value  (n = 3–4) and large transition electric dipole moment (μS3), which could significantly contribute to a larger kr. Besides, compared with 1, the higher emitting energy (ET1) and smaller 2 value for 3 may lead to a smaller non-radiative decay rate. Additionally, the detailed results also indicate that compared to 1 with pyridine ring, 3 with imidazole ring performs a better hole injection ability. Therefore, the designed complex 3 can be expected as a promising candidate for highly efficient guanidinate-based phosphorescence emitter for OLEDs applications.Calculation results indicate that upon changing the main ligand of guanidinate-based complex 1 from 2-phenylpyridine to different phenyl-diazoles, significant SOC effects can be observed, leading to higher kr values compared to 1. The designed complex 3 would be considered as a promising candidate for highly efficient guanidinate-based phosphorescence emitter in OLEDs.