Two-Photon Absorption Properties of Chromophores in Micelles: Electrostatic Interactions

Two-photon absorption (2PA) cross sections of neutral Coumarin 485 (C485) and anionic Coumarin 519 (C519^–) solubilized in Triton X-100 (Tx-100, neutral), sodium dodecyl sulfate (SDS, anionic), and cetyltrimethylammonium bromide (CTAB, cationic) are reported. The objective of the study is to probe the influence of local electrostatic fields in micelles on the 2PA properties of chromophores. The 2PA measurements have shown that the cross sections of neutral C485 are unchanged in different micellar environments, although the local micropolarities of chromophores are different. On the other hand, the 2PA cross sections of C519^– are unchanged or slightly decreased in Tx-100 and SDS micelles when compared to water while 100% increase in 2PA cross sections was observed for C519^– in CTAB micelles. The enhancement in 2PA cross section is attributed to the electrostatic fields arising in the Stern layer of CTAB, where C519^– is solubilized. The titration measurements have shown that the 2PA enhancement is due to the organized medium only and not because of the simple association of C519^– and the quaternary ammonium group of CTAB. From the analysis, local electric field of 0.7 ± 0.3 MV/cm is estimated for the Stern layer of CTAB

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

Energy Gap Law for Exciton Dynamics in Gold Cluster Molecules

The energy gap law relates the nonradiative decay rate to the energy gap separating the ground and excited states. Here we report that the energy gap law can be applied to exciton dynamics in gold cluster molecules. Size-dependent electrochemical and optical properties were investigated for a series of <i>n</i>-hexanethiolate-protected gold clusters ranging from Au₂₅ to Au₃₃₃. Voltammetric studies reveal that the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO–LUMO) gaps of these clusters decrease with increasing cluster size. Combined femtosecond and nanosecond time-resolved transient absorption measurements show that the exciton lifetimes decrease with increasing cluster size. Comparison of the size-dependent exciton lifetimes with the HOMO–LUMO gaps shows that they are linearly correlated, demonstrating the energy gap law for excitons in these gold cluster molecules

Au₂₇₉(SR)₈₄: The Smallest Gold Thiolate Nanocrystal That Is Metallic and the Birth of Plasmon

We report a detailed study on the optical properties of Au₂₇₉(SR)₈₄ using steady-state and transient absorption measurements to probe its metallic nature, time-dependent density functional theory (TDDFT) studies to correlate the optical spectra, and density of states (DOS) to reveal the factors governing the origin of the collective surface plasmon resonance (SPR) oscillation. Au₂₇₉ is the smallest identified gold nanocrystal to exhibit SPR. Its optical absorption exhibits SPR at 510 nm. Power-dependent bleach recovery kinetics of Au₂₇₉ suggests that electron dynamics dominates its relaxation and it can support plasmon oscillations. Interestingly, TDDFT and DOS studies with different tail group residues (−CH₃ and −Ph) revealed the important role played by the tail groups of ligands in collective oscillation. Also, steady-state and time-resolved absorption for Au₃₆, Au₄₄, and Au₁₃₃ were studied to reveal the <i>molecule-to-metal</i> evolution of aromatic AuNMs. The optical gap and transient decay lifetimes decrease as the size increases

Temperature-Dependent Absorption and Ultrafast Exciton Relaxation Dynamics in MAu₂₄(SR)₁₈ Clusters (M = Pt, Hg): Role of the Central Metal Atom

Temperature-dependent and ultrafast transient absorption measurements were carried out to probe the optical properties and exciton relaxation dynamics in metal-doped (Pt and Hg) Au₂₅ clusters. Optical absorption and electrochemistry results have shown that the Pt-doped cluster has a distinctly different HOMO–LUMO gap than that of Au₂₅, while the gap did not change much for Hg-doped Au₂₅. A decrease in temperature had resulted in much sharper absorption features as well as an increased number of absorption peaks, enhanced oscillator strength, and a shift in the energy maximum to higher energies for all metal-doped Au₂₅ clusters. Interestingly, the peaks observed for Pt and Hg-doped clusters are very different from that of undoped Au₂₅ cluster, suggesting that the altered structures play a crucial role on their optical properties. From the analysis of absorption peak shifts, higher phonon energies of 67 ± 8 meV were determined for Pt- and Hg-doped Au₂₅ clusters when compared to 43 ± 6 meV for undoped Au₂₅. The larger phonon energies suggest stronger coupling of core-gold and shell-gold and are explained by contraction of metal-doped clusters. Ultrafast transient absorption results have shown that Pt-doping lead to faster excited state relaxation, where more than 70% of the created electron–hole pairs recombine within 20 ps. However, Hg-doping and undoped Au₂₅ relax to shell gold and recombination takes a much longer time. The results are consistent with energy gap law, where the smaller energy gap for PtAu₂₄ led to faster exciton relaxation. An interesting correlation between the spin–orbit coupled transitions and bleach maximum was observed, which can be ascribed to exciton localization in Au₁₂-icosahedron

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

Au₂₁S(SAdm)₁₅: An Anisotropic Gold Nanomolecule. Optical and Photoluminescence Spectroscopy and First-Principles Theoretical Analysis

We introduce a class of gold nanomolecules exhibiting anisotropy as a major feature by reporting steady-state and time-resolved photoluminescence and anisotropy measurements and in-depth theoretical analysis of energetics and optical response of a recently synthesized Au₂₁S­(SAdm)₁₅ nanomolecule (SAdm = adamantanethiol). Starting from single-crystal X-ray data showing that Au₂₁S­(SAdm)₁₅ exhibits a symmetry-broken structure, we unambiguously demonstrate how this translates into a striking anisotropy of its properties, for example, of its (chiro)­optical absorption spectrum of great promise for sensing, optoelectronic, and electrochemical applications, and argue about the abundance and general significance of this class of compounds

Au₂₁S(SAdm)₁₅: Crystal Structure, Mass Spectrometry, Optical Spectroscopy, and First-Principles Theoretical Analysis

Here we report X-ray crystal structure, spectroscopic and theoretical characterization of Au₂₁S­(SAdm)₁₅ (SAdm = adamantanethiol). Single-crystal X-ray diffraction shows that the Au₂₁S­(SAdm)₁₅ nanomolecule exhibits a Au₁₂ cuboctahedral core missing one vertex surrounded by a single μ₃ sulfur atom (sulfide), five bridging thiols, two additional Au atoms, one monomeric Au­(SR)₂, and two trimeric Au₃(SR)₄ staples, with the two trimeric staples being linked through a Au₂(SR) unit with a thiolate ligand in μ₄ coordination. Compositional, electronic, optical, and structural features of this compound are clarified via nanoelectrospray ionization mass spectrometry (nESI-MS), matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS), low-temperature UV–vis spectroscopy, and first-principles analysis

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

Ultrafast Interfacial Charge-Transfer Dynamics in a Donor-π-Acceptor Chromophore Sensitized TiO₂ Nanocomposite

The dynamics of interfacial charge transfer across (<i>E</i>)-3-(5-((4-(9H-carbazol-9-yl)­phenyl)­ethynyl)­thiophen-2-yl)-2-cyanoacrylic acid (CT-CA) and TiO₂ nanocomposites was studied with femtosecond transient absorption, fluorescence upconversion, and molecular quantum dynamics simulations. The investigated dye, CT-CA is a push–pull chromophore that has an intramolecular charge-transfer (ICT) excited state and binds strongly with the surface of TiO₂ nanoparticles. Ultrafast transient absorption and fluorescence measurements, in both solution and thin film samples, were carried out to probe the dynamics of electron injection and charge recombination. Multiexponential electron injection with time constants of <150 fs, 850 fs, and 8.5 ps were observed from femtosecond fluorescence measurements in solution and on thin films. Femtosecond transient absorption measurements show similar multiexponential electron injection and confirm that the picosecond electron injection component arises from the excited ICT state of the CT-CA/TiO₂ complex. Quantum dynamics calculations also show the presence of a slow component (30%) in the electron injection dynamics although most of the electron injection (70%) takes place in less than 20 fs. The slow component of electron injection, from the local ICT state, is attributed to the energetic position of the excited state, which is close to, or slightly below, the conduction band edge. In addition, the transient bleach of CT-CA on the TiO₂ surface is shifted to longer wavelengths when compared to its absorption spectrum and the transient bleach is further shifted to longer wavelengths with charge recombination. These features are attributed to transient Stark shifts that arise from the local electric fields generated at the dye/TiO₂ interface due to charge-transfer interactions