Unusual Solvent Effects on Optical Properties of Bi-Icosahedral Au₂₅ Clusters

Temperature-dependent and time-resolved absorption measurements were carried out to understand the influence of solvent hydrogen bonding on the optical properties of bi-icosahedral [Au₂₅(PPh₃)₁₀(C₆S)₅Cl₂]²⁺ (bi-Au₂₅) clusters. Theoretical calculations have shown a low energy absorption maximum that is dominated by the coupling of the two Au₁₃ icosahedra, as well as a high energy absorption arising from the individual Au₁₃ icosahedra that make up the bi-Au₂₅ clusters. Temperature-dependent absorption measurements were carried out on bi-Au₂₅ in aprotic (toluene) and protic (ethanol and 2-butanol) solvents to elucidate the cluster–solvent hydrogen bonding interactions. In toluene, both the low and high energy absorption bands shift to higher energies consistent with electron–phonon interactions. However, in the protic solvents, the low energy absorption shows a zigzag trend with decreasing temperature. In contrast, the high energy absorption in protic solvents shifts monotonically to higher energy similar to that of toluene. Also at the temperature where the zigzag trend was observed, new absorption peaks emerged at higher energy region. The results are attributed to the hydrogen bonding of the solvent with Au–Cl leading to a disruption of the coupling of icosahedra, which is reflected in unusual trends at the low energy absorption. However, at the transition temperature, the hydrogen bonding solvents distort the icosahedrons so much so that the symmetry of Au₁₃ icosahedron is lifted leading to new absorption peaks at high energy. The transition happens at the dynamic crossover temperature where the solvent attains high density liquid status. Femtosecond time-resolved absorption measurements have shown similar dynamics for bi-Au₂₅ in ethanol and toluene with slower vibrational cooling in ethanol. However, the nanosecond transient measurements show significantly longer lifetime for bi-Au₂₅ in ethanol that suggest the solvent does have an influence on the exciton recombination

Temperature-Dependent Optical Absorption Properties of Monolayer-Protected Au₂₅ and Au₃₈ Clusters

The temperature dependence of electronic absorption is reported for quantum-sized monolayer-protected gold clusters (MPCs). The investigations were carried out on Au₂₅L₁₈ (L = SC₆H₁₃) and Au₃₈L₂₄ (L = SC₂H₄Ph) clusters, which show discrete absorption bands in the visible and near-infrared region at room temperature and with a decrease in temperature: (i) the optical absorption peaks become sharper with the appearance of vibronic structure, (ii) the absorption maximum is shifted to higher energies, and (iii) the oscillator strengths of transitions increased. Smaller temperature dependence of absorption is observed for plasmonic gold nanoparticles. The results of the band gap shifts are analyzed by incorporating electron–phonon interactions using the O’Donnell–Chen model. An average phonon energy of ∼400 cm^–1 is determined, and is attributed to the phonons of semiring gold. The unique property of decreasing oscillator strength with increasing temperature is modeled in the Debye–Waller equation, which relates oscillator strength to the exciton–phonon interaction

Directional Electron Transfer in Chromophore-Labeled Quantum-Sized Au₂₅ Clusters: Au₂₅ as an Electron Donor

Novel Au₂₅(C₆S)₁₇PyS clusters (pyrene-functionalized Au₂₅ clusters) showing interesting electrochemical and optical properties are synthesized and characterized. Significant fluorescence quenching is observed for pyrene attached to Au₂₅ clusters, suggesting strong excited-state interactions. Time-resolved fluorescence upconversion and transient absorption measurements are utilized to understand the excited-state dynamics and possible interfacial electron- and energy-transfer pathways. Electrochemical investigations suggest the possibility of electron transfer from Au₂₅ clusters to the attached pyrene. Fluorescence upconversion measurements have shown faster luminescence decay for the case of pyrene attached to Au₂₅ clusters pointing toward ultrafast photoinduced electron/energy-transfer pathways. Femtosecond transient absorption measurements have revealed the presence of the anion radical of pyrene in the excited-state absorption, suggesting the directional electron transfer from Au₂₅ clusters to pyrene. The rate of forward electron transfer from the Au₂₅ cluster to pyrene is ultrafast (∼580 fs), as observed with femtosecond fluorescence upconversion and transient absorption

Accessibility and Selective Stabilization of the Principal Spin States of Iron by Pyridyl versus Phenolic Ketimines: Modulation of the ⁶<i>A</i>₁ ↔ ²<i>T</i>₂ Ground-State Transformation of the [FeN₄O₂]⁺ Chromophore

Several potentially tridentate pyridyl and phenolic Schiff bases (apRen and HhapRen, respectively) were derived from the condensation reactions of 2-acetylpyridine (ap) and 2′-hydroxyacetophenone (Hhap), respectively, with <i>N</i>-R-ethylenediamine (RNHCH₂CH₂NH₂, Ren; R = H, Me or Et) and complexed in situ with iron­(II) or iron­(III), as dictated by the nature of the ligand donor set, to generate the six-coordinate iron compounds [Fe^II(apRen)₂]­X₂ (R = H, Me; X^– = ClO₄^–, BPh₄^–, PF₆^–) and [Fe^III(hapRen)₂]­X (R = Me, Et; X^– = ClO₄^–, BPh₄^–). Single-crystal X-ray analyses of [Fe^II(apRen)₂]­(ClO₄)₂ (R = H, Me) revealed a pseudo-octahedral geometry about the ferrous ion with the Fe^II–N bond distances (1.896–2.041 Å) pointing to the ¹<i>A</i>₁ (d_π⁶) ground state; the existence of this spin state was corroborated by magnetic susceptibility measurements and Mössbauer spectroscopy. In contrast, the X-ray structure of the phenolate complex [Fe^III(hapMen)₂]­ClO₄, determined at 100 K, demonstrated stabilization of the ferric state; the compression of the coordinate bonds at the metal center is in accord with the ²<i>T</i>₂ (d_π⁵) ground state. Magnetic susceptibility measurements along with EPR and Mössbauer spectroscopic techniques have shown that the iron­(III) complexes are spin-crossover (SCO) materials. The spin transition within the [Fe^IIIN₄O₂]⁺ chromophore was modulated with alkyl substituents to afford two-step and one-step ⁶<i>A</i>₁ ↔ ²<i>T</i>₂ transformations in [Fe^III(hapMen)₂]­ClO₄ and [Fe^III(hapEen)₂]­ClO₄, respectively. Previously, none of the X-salRen- and X-sal₂trien-based ferric spin-crossover compounds exhibited a stepwise transition. The optical spectra of the LS iron­(II) and SCO iron­(III) complexes display intense d_π → p_π* and p_π → d_π CT visible absorptions, respectively, which account for the spectacular color differences. All the complexes are redox-active; as expected, the one-electron oxidative process in the divalent compounds occurs at higher redox potentials than does the reverse process in the trivalent compounds. The cyclic voltammograms of the latter compounds reveal irreversible electrochemical generation of the phenoxyl radical. Finally, the H₂salen-type quadridentate ketimine H₂hapen complexed with an equivalent amount of iron­(III) to afford the μ-oxo-monobridged dinuclear complex [{Fe^III(hapen)}₂(μ-O)] exhibiting a distorted square-pyramidal geometry at the metal centers and considerable antiferromagnetic coupling of spins (<i>J</i> ≈ −99 cm^–1)

Accessibility and Selective Stabilization of the Principal Spin States of Iron by Pyridyl versus Phenolic Ketimines: Modulation of the ⁶<i>A</i>₁ ↔ ²<i>T</i>₂ Ground-State Transformation of the [FeN₄O₂]⁺ Chromophore

Several potentially tridentate pyridyl and phenolic Schiff bases (apRen and HhapRen, respectively) were derived from the condensation reactions of 2-acetylpyridine (ap) and 2′-hydroxyacetophenone (Hhap), respectively, with <i>N</i>-R-ethylenediamine (RNHCH₂CH₂NH₂, Ren; R = H, Me or Et) and complexed in situ with iron­(II) or iron­(III), as dictated by the nature of the ligand donor set, to generate the six-coordinate iron compounds [Fe^II(apRen)₂]­X₂ (R = H, Me; X^– = ClO₄^–, BPh₄^–, PF₆^–) and [Fe^III(hapRen)₂]­X (R = Me, Et; X^– = ClO₄^–, BPh₄^–). Single-crystal X-ray analyses of [Fe^II(apRen)₂]­(ClO₄)₂ (R = H, Me) revealed a pseudo-octahedral geometry about the ferrous ion with the Fe^II–N bond distances (1.896–2.041 Å) pointing to the ¹<i>A</i>₁ (d_π⁶) ground state; the existence of this spin state was corroborated by magnetic susceptibility measurements and Mössbauer spectroscopy. In contrast, the X-ray structure of the phenolate complex [Fe^III(hapMen)₂]­ClO₄, determined at 100 K, demonstrated stabilization of the ferric state; the compression of the coordinate bonds at the metal center is in accord with the ²<i>T</i>₂ (d_π⁵) ground state. Magnetic susceptibility measurements along with EPR and Mössbauer spectroscopic techniques have shown that the iron­(III) complexes are spin-crossover (SCO) materials. The spin transition within the [Fe^IIIN₄O₂]⁺ chromophore was modulated with alkyl substituents to afford two-step and one-step ⁶<i>A</i>₁ ↔ ²<i>T</i>₂ transformations in [Fe^III(hapMen)₂]­ClO₄ and [Fe^III(hapEen)₂]­ClO₄, respectively. Previously, none of the X-salRen- and X-sal₂trien-based ferric spin-crossover compounds exhibited a stepwise transition. The optical spectra of the LS iron­(II) and SCO iron­(III) complexes display intense d_π → p_π* and p_π → d_π CT visible absorptions, respectively, which account for the spectacular color differences. All the complexes are redox-active; as expected, the one-electron oxidative process in the divalent compounds occurs at higher redox potentials than does the reverse process in the trivalent compounds. The cyclic voltammograms of the latter compounds reveal irreversible electrochemical generation of the phenoxyl radical. Finally, the H₂salen-type quadridentate ketimine H₂hapen complexed with an equivalent amount of iron­(III) to afford the μ-oxo-monobridged dinuclear complex [{Fe^III(hapen)}₂(μ-O)] exhibiting a distorted square-pyramidal geometry at the metal centers and considerable antiferromagnetic coupling of spins (<i>J</i> ≈ −99 cm^–1)