Spin-orbit TDDFT electronic structure of diplatinum(II,II) complexes

Area of Science:

• Inorganic Chemistry
• Photochemistry
• Computational Chemistry

Background:

• Diphosphonate-bridged platinum(II) dimers, such as [Pt2(μ-P2O5H2)4](4-) (Pt(pop)) and its perfluoroborated derivative [Pt2(μ-P2O5(BF2)2)4](4-) (Pt(pop-BF2)), are known for their ability to drive redox reactions via electronic excited states.
• Understanding the excited-state dynamics, including fluorescence and phosphorescence, is key to optimizing their reactivity for applications in catalysis and photoredox transformations.

Purpose of the Study:

• To perform spin-orbit (SO) time-dependent density functional theory (TDDFT) calculations to elucidate the electronic structure and excited-state properties of Pt(pop) and Pt(pop-BF2).
• To correlate the calculated electronic properties with the observed absorption spectra and photophysical behavior (fluorescence and phosphorescence) of these platinum complexes.

Main Methods:

• Spin-orbit (SO) time-dependent density functional theory (TDDFT) calculations were employed to model the electronic absorption spectra and excited states of the platinum complexes.
• Calculations were performed on both ground-state and optimized excited-state geometries to assess the influence of structural changes on electronic properties.
• Analysis focused on the nature of singlet and triplet excited states, their energy ordering, spin-orbit interactions, and pathways for intersystem crossing (ISC).

Main Results:

• Both Pt(pop) and Pt(pop-BF2) exhibit distinct fluorescence and phosphorescence originating from singlet and triplet excited states, respectively, with a significant energy gap between them.
• The lowest triplet excited state (3)dσ*pσ is split by spin-orbit interactions with higher-lying singlet states, influencing intersystem crossing efficiency.
• Calculations indicate that intersystem crossing is facilitated by second-order spin-orbit interactions and potentially influenced by structural distortions, which are less hindered in Pt(pop) compared to the more rigid Pt(pop-BF2).

Conclusions:

• The electronic structure and excited-state properties, including the energetic isolation of emissive states and spin-orbit coupling, are well-described by SO-TDDFT calculations.
• The observed photophysical properties and differences in intersystem crossing rates between Pt(pop) and Pt(pop-BF2) can be rationalized by their electronic structures and excited-state dynamics.
• These findings provide a fundamental understanding of the photophysical processes governing the reactivity of these platinum complexes, paving the way for targeted catalyst design.