Density functional theory investigation of photoelectric conversion in graphene quantum dot/Ir(III) complex nanocomposites: the influence of π-conjugation in cyclometalating ligands

Using density functional theory (DFT), this study investigates the photoelectric performance of nanocomposites formed by coupling graphene quantum dots (GQDs) with Ir(III) complexes. The goal is to evaluate the influence of different π-conjugation levels in cyclometalating ligands and determine the most efficient ligand for energy conversion in the nanocomposite. The analysis covers seven distinct Ir(III) complexes, each featuring a common bpy ligand but differing diimine ligands. These complexes are linked to GQDs through amide connections. The study comprehensively examines electronic structure, absorption spectra, charge transfer, and chemical reactivity. Our results show that increased ligand π-conjugation causes a redshift in the absorption spectrum due to smaller highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps, ultimately enhancing light harvesting. This effect becomes more pronounced when GQDs are incorporated. For less-conjugated ligands, attaching GQDs enhances metal-to-ligand charge transfer, facilitating electron injection into TiO2. Moreover, higher conjugation and GQD coupling reduce chemical hardness while increasing chemical potential and electrophilicity, thus improving electron acceptance. Furthermore, strategic structural variations modify free energy changes for electron injection and ground-state regeneration. Notable is the inclusion of perylene and pyrene moieties in the ligand, which accelerates injection and extends recombination lifetimes, while GQD incorporation accelerates injection across all ligands. Additionally, photocurrent generation primarily influences energy conversion efficiency. Finally, adding GQDs enhances the first-order hyperpolarizability, further boosting light harvesting. This study demonstrates the potential of tuning ligand π-conjugation and GQD interfaces to optimize optoelectronic properties and charge transfer dynamics, thereby enhancing solar energy conversion in GQD/Ir(III) complex systems.

Refine the evaluation of photophysical properties of organometallic chromophores under confined molecular crystal conditions

Many impressive results have been achieved in the researches and developments of luminescent chromophore materials by combining experimental synthesis and characterization with the cooperative theoretical calculation. However, the existing theoretical studies are usually based on the intrinsic properties of isolated molecules and extend their properties to the whole molecular material directly, which will lead to the persistence of errors and affect the computational design of molecular materials with different morphology. Therefore, the study of multimolecular systems needs to further consider the environmental effects on molecules. This work is based on the calculation of a series of crystalline Ir(III) complexes under background charge conditions to reveal how the surrounding charge affects the photophysical properties of a series of transition metal Ir(III) complex materials. Through this method, the study of crystalline complexes is found to be more authentically reproduced the charge transfer state, energy level, and reorganization energy, etc., and shows the changes of luminescence characteristics and efficiency. The change of the electronic structure of the target molecule would be characterized more comprehensively, thus obtaining more accurate results for the excited states properties of molecular materials.

Substituent effects on the structure and properties of (para-C5H4X)Ir(PH3)3 complexes in the ground state (S0) and first singlet excited state (S1): DFT and TD-DFT investigations

The ground and lowest singlet excited state geometries of selected ( para-C5H4X)Ir(PH3)3 iridabenzene complexes ( para-substituent = NH2, OMe, Me, H, F, Cl, CCl3, CF3, NO2) are optimized using the MPW1PW91 procedure employing the LanL2DZ(Ir) and 6-311G(d, p) (C, H, N, O, P, F, Cl, P) basis sets. The excited state is generated using the time-dependent density function method. The effects of electron-donating groups and electron-withdrawing groups on the energy, atomization energy, rotational constants, and frontier orbital energies in the first singlet excited state (S1) of iridabenzene are investigated and compared to those of the ground state (S0). The Ir–C and Ir–P bonds in the studied molecules are analyzed by electron localization function and localized-orbital locator methods. The correlations between the Ir-C and Ir–P bond distances, electron localization function, and localized-orbital locator values Hammett constants (σp) and dual parameters (σI and σR) are given for the two studied states. The para-delocalization index is used for investigation of the aromaticity of the studied complexes.

Regulating vibrational modes to improve quantum efficiency: insights from theoretical calculations on iridium(iii) complexes bearing tridentate NCN and NNC chelates

We reported a theoretical study on [(NCN)Ir(iii)(NNC)]+ tridentate Ir(iii) complexes for organic light-emitting diode (OLED) applications. With appropriate chemical modifications onto the NCN ligand of [(NCN)Ir(iii)(NNC)]+, several rotational and stretching vibration modes were weakened or even eliminated, resulting in a weaker vibrational coupling between the emissive state and the ground state, therefore slowing down the non-radiative decay process, ultimately improving the quantum efficiency. We hope that these theoretical studies and the semi-quantitative prediction on phosphorescence efficiency could provide inspiration for the design of highly efficient phosphorescent materials.

The effect of the embedded o-carborane ligand on the photophysical properties of a cyclometalated Pt(ii) complex: a theoretical investigation

The embedded o-carborane ligands will have an impact on the photophysical properties and quantum efficiency of these highly efficient blue phosphorescent materials.

Synthesis and Characterization of a Series of Bis-homoleptic Cycloruthenates with Terdentate Ligands as a Family of Panchromatic Dyes

A series of six homoleptic bis-cyclometalated ruthenium complexes, Ru(N^N^C)₂, is reported where N^N^C is a 6-(2,4-difluoro-3-R³-phenyl)-4-R²-4'-R¹-2,2'-bipyridine with R³ = -H or -CF₃ and R² and R¹ = -COOEt or -CF₃. An effective synthesis of the ligands and the complexes is described. The UV-visible absorption studies demonstrate that these complexes are panchromatic dyes absorbing up to 900 nm. Importantly, the onset of absorption depends only on the substitution on the metalated phenyl, whereas the intensity of absorption throughout the spectra is a function of substituents on both the phenyl and the bipyridine moieties. The same trend is observed in electrochemistry as the redox gap depends only on the substitution on the metalated phenyl, whereas the oxidation and reduction potentials are a function of substituents on both the phenyl and the bipyridine moieties. Preliminary tests as sensitizer for dye-sensitized solar cells demonstrate that the number of anchoring groups on the dye has a major influence on the device efficiency.

Theoretical investigation on the effect of ancillary ligand modification for highly efficient phosphorescent platinum(ii) complex design

Reasonable modification of ancillary ligands for Pt(ii) complexes can effectively improve the quantum efficiency and strengthen the rigidity of luminescent materials in organic light-emitting diodes.

Theoretical study and design of highly efficient platinum(ii) complexes bearing tetradentate ligands for OLED

Theoretical study for improving the efficiency of a novel tetradentate platinum(ii) complexes in OLED by molecular modification.

Tuning the color of thermally activated delayed fluorescent properties for spiro-acridine derivatives by structural modification of the acceptor fragment: a DFT study

The broad range color tuning of spiro-acridine TADF materials has been demonstrated and elucidated via structural modification of the acceptor fragment.

Theoretical research on the effect of regulated π-conjugation on the photophysical properties of Ir(iii) complexes

Larger π-skeleton-conjugation in host ligands of Ir(iii) carbene complexes can result in remarkably blue-shifted emission and can effectively improve the quantum efficiency.

Theoretical Studies on Phosphorescent Materials: The Conjugation-Extended PtII Complexes

A theoretical study on the PtII complex A based on a dimesitylboron (BMes2)-functionalized [Pt(C^N)(acac)] (C^N = 2-phenyl-pyridyl, acac = acetylaceton) complex, as well as three conjugation-extended analogues of the methylimidazole (C*) ligand BMes2-[Pt(C^C*)(acac)] complexes B–D is performed. Their theoretical geometries, electronic structures, emission properties, and the radiative decay rate constants (kr) were also investigated. The energy differences between the two highest occupied orbitals with dominant Pt d-orbital components (Δddocc) of D both at the ground and excited states are the smallest of all. Compared with B, the charge transfer in D possesses a marked trend towards the extended conjugated group, while C changed inconspicuously. The lowest-lying absorptions and the phosphorescence of them can be described as a mixed metal-to-ligand charge transfer (MLCT)/intra-ligand π→π* charge transfer (ILCT) and 3MLCT/3ILCT, respectively. The variation of charge transfer properties induced by extended conjugation and the radiative decay rate constants (kr) calculated revealed that D is a more efficient blue phosphorescence material with a 497 nm emission transition.

Investigation and enhancement of the stability and performance of water reduction systems based on cyclometalated iridium(III) complexes

Water reduction systems that use a bis-cyclometalated Ir(III) photosensitiser (PS) along with homogeneous Pd complexes as a source of in-situ-formed colloidal Pd as the water reducing complex (WRC) and triethylamine (TEA) as the sacrificial electron donor were tested and characterised with respect to their photocatalytic H(2) production performance. It was confirmed that substitution of the 2-(pyridin-2-yl)benzen-1-ide (pyb) ligand in the well-known system [Ir(pyb)(2)(bpy)](+) (bpy=2,2'-bipyridine) by the fluorinated cyclometalating ligand 5-fluoro-2-(5-methylpyridin-2-yl)benzen-1-ide (Fmpyb) tremendously enhanced the H(2) production rate. Moreover, variation of the bidentate N^N ligand bpy by alkyl substitution in the 4,4'-position resulted in an increase in the H(2) production yield by a factor of three. The incident-photon-to-hydrogen-efficiency could be enhanced from 2.6 to 12.3%. Furthermore, a new dinuclear Co complex was used as a reduction catalyst and showed up to 760 turnovers after 20 h. A detailed study of the concentration impact of all components in the photoredox system was performed. DFT calculations were used to aid the explanation of the findings.