Carola Kryschi

Reaction dynamics of photochromic dithienylethene derivatives

The reaction dynamics of the photochromic ring-opening reaction of 1,2-bis 5-formyl-2-methyl-thien-3-yl perfluoro- . cyclopentene CHO-BMTFP in dichloromethane solution was investigated using femtosecond transient absorption spec- troscopy. The data were analyzed in terms of a model potential and single-electron density matrices, which were calculated . using the collective electronic oscillator CEO approach and the INDOrS semiempirical Hamiltonian. The S -S and 01 S -S transitions of the closed isomer were resonantly excited using 120 fs pump pulses at 610 and 410 nm, respectively. A 02 temporally delayed white light continuum probe pulse monitors the decay of the S or S state as well as the recovery of the 12 S state. Within the first picosecond after excitation, CHO-BMTFP was observed to undergo a fast structural relaxation 0 along the S potential energy surface into a minimum constituting a precursor of the ring-opening process. The rather long 1 lifetime of the precursor, t s 13 ps, was consistent with the calculated potential barrier in front of the conical intersection 2 with the S potential energy surface, which may arise from stabilization of the nearly planar closed isomer by an efficiently 0

Coupling of Chromophores: Carotenoids and Photoactive Diarylethenes – Photoreactivity versus Radiationless Deactivation

New photochromic carotenoid-like chromophores containing the dithienylperfluorocyclopentene fragment have been synthesized and characterized by UV/Vis spectroscopy. The quantum yields of the photochromic forward and back reactions of these compounds have been determined and are found to decrease sharply with increasing chain length of the substituent. This decrease in quantum yield can be rationalized in terms of a decrease of the excitation density at the central photoreactive unit and of the excited state lifetime, which is known to shorten in carotenoids with increasing chain length.

Femtosecond transient absorption studies on photochromism of dithienylethene derivates

Abstract The kinetics of the photochromic ring-opening reaction of benzoyl-phenyl-ethenyl substituted 1,2-bis[2-methylthien-3-yl]perfluorocyclopentene(bpe-BMTFP) in solution was investigated using fs transient absorption spectroscopy. While resonant excitation of the S0-S1 transition of the closed isomer is performed with a 100 fs pump pulse, the decay of the S1 state as well as the recovery of the S0 state were monitored using a white-light continuum probe pulse. Within the first 500 fs after excitation bpe-BMTFP is observed to undergo fast structural relaxations into two minima of the S1 potential-energy surface which differ significantly in their lifetimes τL and τdis. While the long lifetime of about τdis = 9 ps is assigned to the potential minimum reached by the disrotatory twist motion, the other minimum with τL = 1.9 ps is ascribed to the prestate of the ring-opening process.

The mechanism of singlet to triplet transitions of pentacene guests in p-terphenyl and benzoic acid crystals

Abstract In this paper we examine the effect of the different site configurations on the S 1 ⇝T 1 intersystem crossin (ISC) of pentacene in p -terphenyl (ptp) and benzoic acid (bza) crystals by measuring site-selectively the fluorescence quantum yield of pentacene as function of temperature in the range 4.2–140 K. The local structures of the four sites (O 1 , O 2 , O 3 , O 4 ) for pentacene in ptp are simulated using the Buckingham (exp-6) potential. The present study provides a rationalization for the temperature dependent ISC rates of pentacene at O 1 and O 2 in ptp and pentacene in bza as well as for the enhanced, temperature independent ISC rates of pentancene at O 3 and O 4 in ptp.

Vibronically induced intersystem crossing in pentacene in p-terphenyl and benzoic acid crystals

Abstract The temperature behaviour of pentacene fluorescence of p-terphenyl: pentacene and benzoic acid: pentacene is investigated by site-selectively measuring the fluorescence quantum yield Φ F of pentacene in the four sites (O 1 , O 2 , O 3 , O 4 ) in p-terphenyl (ptp) and in the single site (O 0 ) in benzoic acid (bza) varying the temperature from 4.2 to 140 K. A theoretical analysis of the experimental data yields that the temperature dependences of Φ ptp F (O 1 ), Φ ptp F (O 2 ) and Φ bza F (O 0 ) arise from a vibronically induced intersystem crossing due to an out-of-plane vibrational mode of pentacene.

A spectroscopic study of the growth and redox processes of electrodeposited polybithiophene films

Electrochemically prepared polybithiophene films were characterized during polymerization and as a function of the redox potential via in situ measurements of absorption and fluorescence spectra. Ex situ measurements of the fluorescence of undoped films were performed as a function of temperature and film thickness. The absorption spectra were observed to depend significantly on the film thickness, showing that the morphology of the films changes during the polymerization, while the fluorescence spectra are independent of the film thickness and attributed to the longest-conjugation-length segments in the film. The absorption spectra of the p-doped polymer films were obtained from reflectance and transmission spectra recorded during the polymerization and reduction process. The shape of the spectra and, in particular, the position of the extinction maximum, that we observe at 692 nm, differs significantly from previously reported results. On the other hand, the energy of the extinction maximum is close to the calculated singlet excitation energies of oligothiophene bications.

Proton tunnelling reactions in pentacene doped benzoic acid crystals

Abstract Optical excitation of pentacene, doped into a benzoic acid crystal matrix, induces a reversible proton transfer reaction between the host and the guest. This reaction occurs at low temperatures and leads to the creation of defects corresponding to the displacement, from its regular position, of an acid proton of the host matrix. The formation and evolution of these defects is monitored via the electronic S 0 → S 1 transition of the pentacene guest. Here we report and discuss a variety of measurements made with the aim of obtaining information about the first step of the reaction. It is shown that the rate for the first step of this reaction is reduced by a factor of about 10 4 upon deuteration of the host matrix, demonstrating that this step occurs by tunnelling. Other unsuccessful experiments (optical, ESR, magnetic field effects) made to identify the first intermediate of the reaction as well as molecular orbital calculations of potential intermediates are also reported briefly. It is shown that the formation of the pentacene cation, protonated in the centre position, is consistent with all observations. This species is proposed as the most likely first intermediate of the reaction.

Optical spectroscopic study of the domain structure of triclinic p-terphenyl

Abstract The relative intensities, at 4.2 K, of fluorescence lines attributed to tetracene guests in different sites of a p -terphenyl host crystal are shown to depend on the rate of cooling of the crystal between 240 and 140 K across the disorder-order phase transition. This observation reflects the kinetics of the formation of the domain structure in the low-temperature phase, which affects the distribution of guest molecules located within domains and within domains boundaries.

Phonon‐induced site relaxation of tetracene guests in p‐terphenyl single crystals

Site relaxation processes of tetracene‐h12 and tetracene‐d12 guests in p‐terphenyl crystals are examined using high‐resolution fluorescence and fluorescence excitation spectroscopy. Two of the four sites, the so‐called O3 and O4 sites, undergo this process and their phonon sidebands observed in the excitation spectra are characterized by an exceptionally long and well resolved progression of a low‐frequency (pseudo‐) local phonon mode superimposed on a broad background. Perdeuteration of tetracene results in a frequency reduction of the progression forming mode which identifies this local phonon as a guest librational mode. The latter is suggested to induce the site relaxation via guest reorientation processes. The intensity distribution in the phonon sideband of the O3 site indicates a reorientational angle of ∼21° about the L‐axis for the tetracene guest in the excited singlet state.

Doping influence on the domain structure in p-terphenyl crystals: microscopy and calorimetry study

Abstract The order-disorder phase transition of p -terphenyl from the monoclinic high-temperature (disordered) to the triclinic low-temperature (ordered) phase is associated with a lowering of the symmetry. As a consequence, a ferroelastic and an antiphase domain structure coexist in the ordered phase. The formation of the ferroelastic domain structure in tetracene doped and undoped p -terphenyl was observed under polarization microscopy to occur in a narrow range around the phase transition temperature. The shape and size of domains were found not to depend on the guest concentration but on the crystal quality. On the other hand, experiments with differential scanning calorimetry show that even small guest concentrations (10 −3 mol/mol) may decrease the phase transition temperature, indicating that tetracene guests favour conformational disorder in the triclinic phase.