A Multiscale Free Energy Method Reveals an Unprecedented Photoactivation of a Bimetallic Os(II)-Pt(II) Dual Anticancer Agent

The photoreactivity of relatively large transition metal complexes is often limited to the description of the static potential energy surfaces of the involved electronic states. While useful to grasp some...

Photophysical properties of N719 and Z907 dyes, benchmark sensitizers for dye-sensitized solar cells, at room and low temperature

Two benchmark sensitizers used for dye-sensitized solar cells, ruthenium polypyridyl N719 and Z907 dyes were investigated with spectroscopic methods as steady-state absorption, time-gated phosphorescence and femto-/nanosecond time-resolved transient absorption at room temperature and at 160 K. Aim of this study was to perform comprehensive photophysical study of dye excited singlet and triplet metal-to-ligand charge transfer (MLCT) states including states lifetimes, dependency on temperature and dye concentration and obtain detailed information on the excitation decay pathway. Transient absorption and phosphorescence decay data provided a clearer picture of the dynamics of the excited MLCT states. Based on data analysis, the excitation decay pathway consists of rapid intersystem crossing to the triplet MLCT state that undergoes state solvation and vibrational relaxation. It was demonstrated that the lifetime of the fully relaxed triplet MLCT is also strongly dependent on dye concentration for both molecules, providing a viable explanation for a large inconsistency seen in previous studies.

Critical Role of Protons for Emission Quenching of Indoline Dyes in Solution and on Semiconductor Surfaces

By combining time-correlated single photon counting (TCSPC) measurements, density functional theory (DFT), and time-dependent DFT (TD-DFT) calculations, we herein investigate the role of protons, in solutions and on semiconductor surfaces, for the emission quenching of indoline dyes. We show that the rhodanine acceptor moieties, and in particular the carbonyl oxygens, undergo protonation, leading to nonradiative excited-state deactivation. The presence of the carboxylic acid anchoring group, close to the rhodanine moiety, further facilitates the emission quenching, by establishing stable H-bond complexes with carboxylic acid quenchers, with high association constants, in both ground and excited states. This complexation favors the proton transfer process, at a low quencher concentration, in two ways: bringing close to the rhodanine unit the quencher and assisting the proton release from the acid by a partial-concerted proton donation from the close-by carboxylic group to the deprotonated acid. Esterification of the carboxylic group, indeed, inhibits the ground-state complex formation with carboxylic acids and thus the quenching at a low quencher concentration. However, the rhodanine moiety in the ester form can still be the source of emission quenching through dynamic quenching mechanism with higher concentrations of protic solvents or carboxylic acids. Investigating this quenching process on mesoporous ZrO₂, for solar cell applications, also reveals the sensitivity of the adsorbed excited rhodanine dyes toward adsorbed protons on surfaces. This has been confirmed by using an organic base to remove surface protons and utilizing cynao-acrylic dye as a reference dye. Our study highlights the impact of selecting such acceptor group in the structural design of organic dyes for solar cell applications and the overlooked role of protons to quench the excited state for such chemical structures.

Excited-State Properties of Metal-Free ((Z)-2-Cyano-3-(4-((E)-2-(6-(4-methoxyphenyl)-9-octyl-9H-carbazol-3-yl)vinyl)phenyl)acrylic Acid and (E)-2-Cyano-3-(4-((E)-4-(diphenylamino)styryl)phenyl)acrylic Acid) and Ru-Based (N719 and Z907) Dyes and Photoinduced Charge Transfer Processes in FTO/TiCl4/TiO2/Dye Photoanodes Fabricated by Conventional Staining and Potential-Assisted Adsorption

Excited-state properties of two novel metal-free custom-made dyes D2d [(Z)-2-cyano-3-(4-((E)-2-(6-(4-methoxyphenyl)-9-octyl-9H-carbazol-3-yl)vinyl)phenyl)acrylic acid] and T-SB-C [(E)-2-cyano-3-(4-((E)-4-(diphenylamino)styryl)phenyl)acrylic acid] and two commercially available Ruthenium-based N719 and Z907 dyes were investigated with application of time-resolved absorption and emission. Singlet excited state lifetimes of D2d and T-SB-C were determined in acetonitrile and are 1.4 and 2.45 ns, respectively. The ³MLCT state lifetimes of N719 and Z907 dyes determined in methanol are 9.25 and 8.85 ns, respectively. Subsequently, photoexcited processes like electron injection and charge recombination were studied for those dyes adsorbed on the FTO/TiCl₄/TiO₂ photoanodes and fabricated via a conventional staining technique and innovative potential-assisted fast dye staining method. The dynamics of the spectro-temporal data was determined with application of single-wavelength and global fitting. All dye-TiO₂ systems showed fast picosecond injection of excited electrons to the conduction band of the TiO₂ layer and in complex multiphasic charge recombination processes. The dynamics of those processes is not altered by the dye adsorption method.

Short- and Long-Range Solvation Effects on the Transient UV-Vis Absorption Spectra of a Ru(II)-Polypyridine Complex Disentangled by Nonequilibrium Molecular Dynamics

Evidence of subtle effects in the dynamic reorganization of a protic solvent in its first- and farther-neighbor shells, in response to the sudden change in the solute's electronic distribution upon excitation, is unveiled by a multilevel computational approach. Through the combination of nonequilibrium molecular dynamics and quantum mechanical calculations, the experimental time evolution of the transient T₁ absorption spectra of a heteroleptic Ru(II)-polypyridine complex in ethanol or dimethyl sulfoxide solution is reproduced and rationalized in terms of both fast and slow solvent re-equilibration processes, which are found responsible for the red shift and broadening experimentally observed only in the protic medium. Solvent orientational correlation functions and a time-dependent analysis of the solvation structure confirm that the initial, fast observed red shift can be traced back to the destruction-formation of hydrogen bond networks in the first-neighbor shell, whereas the subsequent shift, evident in the [20-500] ps range and accompanied by a large broadening of the signal, is connected to a collective reorientation of the second and farther solvation shells, which significantly changes the electrostatic embedding felt by the excited solute.

Multispectral multidimensional spectrometer spanning the ultraviolet to the mid-infrared

Multidimensional spectroscopy is the optical analog to nuclear magnetic resonance, probing dynamical processes with ultrafast time resolution. At optical frequencies, the technical challenges of multidimensional spectroscopy have hindered its progress until recently, where advances in laser sources and pulse-shaping have removed many obstacles to its implementation. Multidimensional spectroscopy in the visible and infrared (IR) regimes has already enabled respective advances in our understanding of photosynthesis and the structural rearrangements of liquid water. A frontier of ultrafast spectroscopy is to extend and combine multidimensional techniques and frequency ranges, which have been largely restricted to operating in the distinct visible or IR regimes. By employing two independent amplifiers seeded by a single oscillator, it is straightforward to span a wide range of time scales (femtoseconds to seconds), all of which are often relevant to the most important energy conversion and catalysis problems in chemistry, physics, and materials science. Complex condensed phase systems have optical transitions spanning the ultraviolet (UV) to the IR and exhibit dynamics relevant to function on time scales of femtoseconds to seconds and beyond. We describe the development of the Multispectral Multidimensional Nonlinear Spectrometer (MMDS) to enable studies of dynamical processes in atomic, molecular, and material systems spanning femtoseconds to seconds, from the UV to the IR regimes. The MMDS employs pulse-shaping methods to provide an easy-to-use instrument with an unprecedented spectral range that enables unique combination spectroscopies. We demonstrate the multispectral capabilities of the MMDS on several model systems.