Coumarin 343, C16H15NO4

Preferred Molecular Orientation of Coumarin 343 on TiO2 Surfaces: Application to Dye-Sensitized Solar Cells

The dye···TiO2 interfacial structure in working electrodes of dye-sensitized solar cells (DSCs) is known to influence its photovoltaic device performance. Despite this, direct and quantitative reports of such structure remain sparse. This case study presents the application of X-ray reflectometry to determine the preferred structural orientation and molecular packing of the organic dye, Coumarin 343, adsorbed onto amorphous TiO2. Results show that the dye molecules are, on average, tilted by 61.1° relative to the TiO2 surface, and are separated from each other by 8.2 Å. These findings emulate the molecular packing arrangement of a monolayer of Coumarin 343 within its crystal structure. This suggests that the dye adsorbs onto TiO2 in one of its lowest energy configurations; that is, dye···TiO2 self-assembly is driven more by thermodynamic rather than kinetic means. Complementary DSC device tests illustrate that this interfacial structure compromises photovoltaic performance, unless a suitably sized coadsorbant is interdispersed between the Coumarin 343 chromophores on the TiO2 surface.

Experimental and database-transferred electron-density analysis and evaluation of electrostatic forces in coumarin-102 dye

The electron-density distribution of a new crystal form of coumarin-102, a laser dye, has been investigated using the Hansen-Coppens multipolar atom model. The charge density was refined versus high-resolution X-ray diffraction data collected at 100 K and was also constructed by transferring the charge density from the Experimental Library of Multipolar Atom Model (ELMAM2). The topology of the refined charge density has been analysed within the Bader `Atoms In Molecules' theory framework. Deformation electron-density peak heights and topological features indicate that the chromen-2-one ring system has a delocalized π-electron cloud in resonance with the N (amino) atom. The molecular electrostatic potential was estimated from both experimental and transferred multipolar models; it reveals an asymmetric character of the charge distribution across the molecule. This polarization effect is due to a substantial charge delocalization within the molecule. The molecular dipole moments derived from the experimental and transferred multipolar models are also compared with the liquid and gas-phase dipole moments. The substantial molecular dipole moment enhancements observed in the crystal environment originate from the crystal field and from intermolecular charge transfer induced and controlled by C-H···O and C-H···N intermolecular hydrogen bonds. The atomic forces were integrated over the atomic basins and compared for the two electron-density models.

Role of the medium on the C343 inter/intramolecular hydrogen bond interactions. An absorption, emission, and 1HNMR investigation of C343 in benzene/n-heptane mixtures

C343, a common molecular probe utilized in solvation dynamics experiments, was studied in homogeneous media. Absorption, emission, and (1)HNMR spectroscopies were used to investigate the behavior of C343 in benzene and in benzene/n-heptane mixtures. We demonstrate the implications of the medium polarity, measured as the Kamlet-Taft polarity-polarizability (pi*) parameter, in the C343 inter/intramolecular hydrogen bond (H-bond) interactions and the role that this interaction plays in the dimerization process of the dye. In pure benzene, the dimer prevails because the intermolecular H-bond interaction is favored. On the other hand, as the n-heptane content increases the intramolecular H-bond is the strongest and the C343 monomer is favored. As the polarity of the medium decreases, the solvophobic interaction makes that C343 monomer species experiences a more complicated aggregation process beyond the simple monomer dimer equilibrium present in pure benzene. Thus, the addition of n-heptane to the mixture yields a C343 higher-order aggregates species. Thus, our work reveals the importance that the medium has on the behavior of a widespread dye used as chromophore for very different systems such as homogeneous and microheterogenous media. This is very important since the use of chromophores without understanding its chemistry can induce artifacts into the interpretation of solvation dynamics in heterogeneous environments, in particular, those provided by biological systems such as proteins. Considerable care in choosing and characterizing the system is required to analyze the results fully.

What Can You Learn from a Molecular Probe? New Insights on the Behavior of C343 in Homogeneous Solutions and AOT Reverse Micelles

The behavior of C343, a common molecular probe utilized in solvation dynamics experiments, was studied in homogeneous media and in aqueous and nonaqueous reverse micelles (RMs). In homogeneous media, the Kamlet and Taft solvatochromic comparison method quantified solute-solvent interactions from the absorption and emission bands showing that the solvatochromic behavior of the dye depends not only on the polarity of the medium but also on the hydrogen-bonding properties of the solvent. Specifically, in the ground state the molecule displays a bathochromic shift with the polarity polarizability (pi) and the H-bond acceptor (beta) ability of the solvents and a hypsochromic shift with the hydrogen donor ability (alpha) of the media. The carboxylic acid group causes C343 to display greater sensitivity to the beta than to the pi polarity parameter; this sensitivity increases in the excited state, while the dependence on alpha vanishes. This demonstrates that C343 forms a stable H-bond complex with solvents with high H-bond acceptor ability (high beta) and low H-bond donor character (low alpha). Spectroscopy in nonpolar solvents reveals J-aggregate formation. With information from the Kamlet-Taft analysis, C343 was used to explore RMs composed of water or polar solvents/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/isooctane using absorption, emission, and time-resolved spectroscopies. Sequestered polar solvents included ethylene glycol (EG), formamide (FA), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA). Dissolved in the AOT RM systems at low concentration, C343 exists as a monomer, and when introduced to the RM samples in its protonated form, C343 remains protonated driving it to reside in the interface rather than the water pool. The solvathochromic behavior of the dye depends the specific polar solvent encapsulated in the RMs, revealing different types of interactions between the solvents and the surfactant. EG and water H-bond with the AOT sulfonate group destroying their bulk H-bonded structures. While water remains well segregated from the nonpolar regions, EG appears to penetrate into the oil side of the interface. In aqueous AOT RMs, C343 interacts with neither the sulfonate group nor the water, perhaps because of intramolecular H-bonding in the dye. DMF and DMA interact primarily through dipole-dipole forces, and the strong interactions with AOT sodium counterions destroy their bulk structure. FA also interacts with the Na+ counterions but retains its H-bond network present in bulk solvent. Surprisingly, FA appears to be the only polar solvent other than water forming a "polar-solvent pool" with macroscopic properties similar to the bulk.