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Near-Infrared Spectrum of the First Excited State of Au₂.

ABSTRACT: Au₂ ⁺ is a simple but crucial model system for understanding the diverse catalytic activity of gold. While the Au₂ ⁺ ground state (X² Σ_g ⁺ ) is understood reasonably well from mass spectrometry and computations, no spectroscopic information is available for its first excited state (A² Σ_u ⁺ ). Herein, we present the vibrationally resolved electronic spectrum of this state for cold Ar-tagged Au₂ ⁺ cations. This exceptionally low-lying and well isolated A² Σ_(u) ⁺ ←X² Σ_(g) ⁺ transition occurs in the near-infrared range. The observed band origin (5738 cm^-1 , 1742.9 nm, 0.711 eV) and harmonic Au-Au and Au-Ar stretch frequencies (201 and 133 cm^-1 ) agree surprisingly well with those predicted by standard time-dependent density functional theory calculations. The linearly bonded Ar tag has little impact on either the geometric or electronic structure of Au₂ ⁺ , because the Au₂ ⁺ ⋅⋅⋅Ar bond (∼0.4 eV) is much weaker than the Au-Au bond (∼2 eV). As a result of 6 s←5d excitation of an electron from the antibonding σ_u ^* orbital (HOMO-1) into the bonding σ_g orbital (SOMO), the Au-Au bond contracts substantially (by 0.1 Å).

SUBMITTER: Forstel M

PROVIDER: S-EPMC8596823 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:While the influence of proquinoidal character upon the linear absorption spectrum of low optical bandgap ?-conjugated polymers and molecules is well understood, its impact upon excited-state relaxation pathways and dynamics remains obscure. We report the syntheses, electronic structural properties, and excited-state dynamics of a series of model highly conjugated near-infrared (NIR)-absorbing chromophores based on a (porphinato)metal(ii)-proquinoidal spacer-(porphinato)metal(ii) (PM-Sp-PM) structural motif. A combination of excited-state dynamical studies and time-dependent density functional theory calculations: (i) points to the cardinal role that excited-state configuration interaction (CI) plays in determining the magnitudes of S1 ? S0 radiative (kr), S1 ? T1 intersystem crossing (kISC), and S1 ? S0 internal conversion (kIC) rate constants in these PM-Sp-PM chromophores, and (ii) suggests that a primary determinant of CI magnitude derives from the energetic alignment of the PM and Sp fragment LUMOs (?EL). These insights not only enable steering of excited-state relaxation dynamics of high oscillator strength NIR absorbers to realize either substantial fluorescence or long-lived triplets (?T1 > ?s) generated at unit quantum yield (?ISC = 100%), but also crafting of those having counter-intuitive properties: for example, while (porphinato)platinum compounds are well known to generate non-emissive triplet states (?ISC = 100%) upon optical excitation at ambient temperature, diminishing the extent of excited-state CI in these systems realizes long-wavelength absorbing heavy-metal fluorophores. This work highlights approaches to: (i) modulate low-lying singlet excited-state lifetime over the picosecond-to-nanosecond time domain, (ii) achieve NIR fluorescence with quantum yields up to 25%, (iii) tune the magnitude of S1-T1 ISC rate constant from 109 to 1012 s-1 and (iv) realize T1-state lifetimes that range from ?0.1 to several ?s, for these model PM-Sp-PM chromophores, and renders new insights to evolve bespoke photophysical properties for low optical bandgap ?-conjugated polymers and molecules based on proquinoidal conjugation motifs.

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Near-Infrared Spectrum of the First Excited State of Au₂.

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