Femtosecond Infrared Spectroscopy Resolving the Multiplicity of High-Spin Crossover States in Transition Metal Iron Complexes.

Tuning the photophysical properties of iron-based transition-metal complexes is crucial for their employment as photosensitizers in solar energy conversion. For the optimization of these new complexes, a detailed understanding of the excited-state deactivation paths is necessary. Here, we report femtosecond transient mid-IR spectroscopy data on a recently developed octahedral ligand-field enhancing [Fe(dqp)] () complex with dqp = 2,6-diquinolylpyridine and prototypical [Fe(bpy)] ().

Charge Transfer from Photoexcited Semiconducting Single-Walled Carbon Nanotubes to Wide-Bandgap Wrapping Polymer.

As narrow optical bandgap materials, semiconducting single-walled carbon nanotubes (SWCNTs) are rarely regarded as charge donors in photoinduced charge-transfer (PCT) reactions. However, the unique band structure and unusual exciton dynamics of SWCNTs add more possibilities to the classical PCT mechanism. In this work, we demonstrate PCT from photoexcited semiconducting (6,5) SWCNTs to a wide-bandgap wrapping poly-[(9,9-dioctylfluorenyl-2,7-diyl)--(6,6')-(2,2'-bipyridine)] (PFO-BPy) via femtosecond transient absorption spectroscopy.

Diffusion-Controlled Singlet Fission in a Chlorinated Phenazinothiadiazole by Broadband Femtosecond Transient Absorption.

Singlet fission (SF) is a process by which one excited singlet state yields two triplet states upon close interaction with a ground-state chromophore of the same kind. This photoreaction was first observed in solid state and has important implications in organic photovoltaics. Singlet fission was also reported in concentrated solutions, where the need for diffusion of the reaction partners slows the dynamics.

Ultrafast Singlet Fission in Rigid Azaarene Dimers with Negligible Orbital Overlap.

Singlet fission (SF) has the potential to boost solar energy conversion. Research has focused on designing new strategies to tune the electrochemistry, photophysics, and device architecture at the molecular level to improve the efficiency of SF sensitizers. These studies indicate that SF efficiency strongly depends on morphology, packing, and chemical structure.

Ultrafast Singlet Fission and Intersystem Crossing in Halogenated Tetraazaperopyrenes.

Charge carrier multiplication via singlet fission into two triplet states has the potential to increase efficiencies of photovoltaics by one-third due to the reduction of thermalization losses. In the present work, we investigate tetraazaperopyrenes, a class of -heteropolycyles, as suitable singlet fission candidates. Using a combined experimental and theoretical approach, fundamentally different mechanisms for triplet formation in solution and thin film are identified.

Tailoring Ultrafast Singlet Fission by the Chemical Modification of Phenazinothiadiazoles.

Quantum chemistry and time-resolved spectroscopy are applied to rationalize how singlet fission (SF) is affected by systematic chemical modifications introduced into phenazinothiadiazoles (PTD). Substitution of the terminal aromatic ring of TIPS-tetracene by a thiadiazole group leads to a considerable change in the relative energies of its S and T states. Thus, in contrast to TIPS-tetracene, SF becomes exothermic for various PTD derivatives, which show S-2T energy differences as high as 0.

Substituting Coumarins for Quinolinones: Altering the Cycloreversion Potential Energy Landscape.

The light-activated cleavage of cyclobutane-based systems via [2 + 2] cycloreversions, such as thymine and coumarin dimers, is an important but still poorly understood ultrafast photochemical reaction. Systems displaying reversible cycloreversion have found various uses in cross-linked polymers, enhancing gas adsorption affinities in inorganics, and light-activated medical therapies. We report the identification of a heterogeneous mode of cycloreversion for a rarely examined coumarin analogue system.

Dissociation of a strong acid in neat solvents: diffusion is observed after reversible proton ejection inside the solvent shell.

Strong-acid dissociation was studied in alcohols. Optical excitation of the cationic photoacid N-methyl-6-hydroxyquinolinium triggers proton transfer to the solvent, which was probed by spectral reconstruction of picosecond fluorescence traces. The process fulfills the classical Eigen-Weller mechanism in two stages: (a) solvent-controlled reversible dissociation inside the solvent shell and (b) barrierless splitting of the encounter complex.

Photoisomerization around a fulvene double bond: coherent population transfer to the electronic ground state?

Photoisomerization around a central fulvene-type double bond is known to proceed through a conical intersection at the perpendicular geometry. The process is studied with an indenylidene-dihydropyridine model compound, allowing the use of visible excitation pulses. Transient absorption shows that 1) stimulated emission shifts to the red and loses oscillator strength on a 50 fs timescale, and 2) bleach recovery is highly nonexponential and not affected by solvent viscosity or methyl substitution at the dihydropyridine ring.

Two competitive routes in the lactim-lactam phototautomerization of a hydroxypyridine derivative cation in water: dissociative mechanism versus water-assisted proton transfer.

Ground-state tautomerism and excited-state proton-transfer processes of 2-(6'-hydroxy-2'-pyridyl)benzimidazolium in H2O and D2O have been studied by means of UV-vis absorption and fluorescence spectroscopy in both steady-state and time-resolved modes. In the ground state, this compound shows a tautomeric equilibrium between the lactim cation, protonated at the benzimidazole N3, and its lactam tautomer, obtained by proton translocation from the hydroxyl group to the pyridine nitrogen. Direct excitation of the lactam tautomer leads to its own fluorescence emission, while as a result of the increase of acidity of the OH group and basicity at the pyridine N upon excitation, the lactim species undergoes a proton translocation from the hydroxyl group to the nitrogen, favoring the lactam structure in the excited state.

Femtosecond S3-->S1 conversion and structural reorganisation of trans-3-phenylprop-2-enaldehyde and derivatives in solution.

Photoinduced excited-state relaxation of trans-3-phenylprop-2-enaldehyde (cinnamaldehyde) and three derivatives was studied in hexane and acetonitrile with the pump-supercontinuum-probe technique. Transient spectra were measured with 50 fs resolution in the range 260-660 nm after S3<--S0 excitation at 288 nm. The early spectra reveal an ultrafast, 0.