Effect of Binding Geometry on Charge Transfer in CdSe Nanocrystals Functionalized by N719 Dyes to Tune Energy Conversion Efficiency

Semiconductor quantum dots (QDs) functionalized by metal–organic dyes show great promise in photocatalytic and photovoltaic applications. However, the charge transfer direction and rateskey processes governing the efficiency of energy conversionare strongly affected by the QD–dye interactions, insights on which are challenging to obtain experimentally. We use density functional theory (DFT) and constrained DFT calculations to investigate a degree of sensitivity of the electronic level alignment and related QD–dye electronic couplings to binding conformations of N719 dye at the surface of the 1.5 nm CdSe QD. Our calculations reveal a lack of direct correlations between the strength of the QD–dye interaction in terms of their binding conformations and the donor–acceptor electronic couplings. While the QD–dye binding conformations are the most stable when the N719 dye is attached to the QD via two carboxylate groups, the strongest electronic coupling between the QD as an electron donor and the dye as an electron acceptor is observed in structures bonded via the isocyanate ligands. Such strong electronic couplings also are responsible for significant stabilization of the dye’s occupied orbitals deep inside in the valence band of the QD making the hole transfer from the photoexcited QD to the dye thermodynamically unfavorable in structures bound via isocyanates. Our results suggest that the most probable binding conformations are those occurring via two carboxylate linkers, which exhibit very weak electronic couplings contributing to the electron transfer from the photoexcited CdSe QD to the N719 dye but provide the most favorable conditions for the hole transfer. Overall, our computational work provides an insightful view about the surface chemistry of CdSe regarding the donor–acceptor interaction, energy level alignment, and charge transfer between CdSe and dye molecule, which can guide the rational design of QD-based materials for energy conversion applications

Relativistic GVVPT2 Multireference Perturbation Theory Description of the Electronic States of Y₂ and Tc₂

The multireference generalized Van Vleck second-order perturbation theory (GVVPT2) method is used to describe full potential energy curves (PECs) of low-lying states of second-row transition metal dimers Y₂ and Tc₂, with scalar relativity included via the spin-free exact two-component (sf-X2C) Hamiltonian. Chemically motivated incomplete model spaces, of the style previously shown to describe complicated first-row transition metal diatoms well, were used and again shown to be effective. The studied states include the previously uncharacterized 2¹Σ_g⁺ and 3¹Σ_g⁺ PECs of Y₂. These states, together with 1¹Σ_g⁺, are relevant to discussion of controversial results in the literature that suggest dissociation asymptotes that violate the noncrossing rule. The ground state of Y₂ was found to be X⁵Σ_u^– (similar to Sc₂) with bond length <i>R</i>_e = 2.80 Å, binding energy <i>D</i>_e = 3.12 eV, and harmonic frequency ω_e = 287.2 cm^–1, whereas the lowest 1¹Σ_g⁺ state of Y₂ was found to lie 0.67 eV above the quintet ground state and had spectroscopic constants <i>R</i>_e = 3.21 Å, <i>D</i>_e = 0.91 eV, and ω_e = 140.0 cm^–1. Calculations performed on Tc₂ include study of the previously uncharacterized relatively low-lying 1⁵Σ_g⁺ and 1⁹Σ_g⁺ states (i.e., 0.70 and 1.84 eV above 1¹Σ_g⁺, respectively). The ground state of Tc₂ was found to be X³Σ_g^– with <i>R</i>_e = 2.13 Å, <i>D</i>_e = 3.50 eV, and ω_e = 336.6 cm^–1 (for the most stable isotope, Tc-98) whereas the lowest ¹Σ_g⁺ state, generally accepted to be the ground state symmetry for isovalent Mn₂ and Re₂, was found to lie 0.47 eV above the X³Σ_g^– state of Tc₂. The results broaden the range of demonstrated applicability of the GVVPT2 method