Ultrafast dynamics of a spiropyran in water

The reversible switching of a water-soluble spiropyran compound is recorded over 1 ns by means of femtosecond vis-pump/vis- and IR-probe spectroscopy under aqueous conditions. Our investigations reveal that the photochemical conversion from the closed spiropyran to the open merocyanine takes 1.6 ps whereas the reversed photoreaction is accomplished within 25 ps. The combination of time-resolved and steady-state observations allows us to reveal central parts of the reaction pathway leading to either form. The enhanced water solubility, its fast and efficient switching behavior, and its stability against hydrolysis over a time range of several weeks make this compound an attractive and versatile tool for biological applications.     

Ultrafast bidirectional photoswitching of a spiropyran

We report on bidirectional photochemical switching of 6,8-dinitro-1',3',3'-trimethylspiro[2H-1-benzopyran-2,2'-indoline] (6,8-dinitro-BIPS) between the ring-closed spiropyran and the ring-open merocyanine form. This is studied by femtosecond three-color pump-repump-probe experiments. Both ring opening and ring closure are photoinduced. Completion of an entire cycle, consisting of opening and subsequent closure, can be achieved within 40 ps. A much shorter time (<6 ps) is needed for the converse cycle, consisting of initial ring closure and subsequent ring opening. Furthermore, we perform pump-probe experiments with ultraviolet/visible pump and visible/mid-infrared probe pulses for an unambiguous spectroscopic identification of the open and closed molecular forms. Following visible excitation of the ring-open molecules, ultrafast ring closure is observed directly in the mid-infrared. The quantum efficiencies for ring opening and ring closure starting from the respective equilibirum states are determined to be approximately 9% and 40%. These results show that 6,8-dinitro-BIPS is an ultrafast bidirectional molecular switch exhibiting a high quantum efficiency.     

Ultrafast photoinduced reflectivity transients in doped manganite

The temperature and magnetic field dependence of ultrafast photoinduced spin and quasiparticle relaxation dynamics is reported in La(0.67)Ca(0.33)MnO(3) and LaMnO(3) single crystals and thin films. Both manganites reveal an unusually slow ( approximately 10 micros) carrier relaxation process attributed to the spin-lattice relaxation in localized states. The quasiparticle dynamics is governed by the temperature- and magnetic field-dependent pseudogap in La(0.67)Ca(0.33)MnO(3), and by the temperature-independent Jahn-Teller gap in LaMnO(3). The loss of spectral weight near the Fermi level in La(0.67)Ca(0.33)MnO(3) strongy affects the quasiparticle relaxation dynamics as temperature increases from below T(C). Our results show that the coupled dynamics of charge, spin and lattice is strongly correlated with the distinct gap structures in these manganites.     

Polarized normal-incidence cavity ring-down spectroscopy: probing spiropyran photochromism in thin PMMA films and toluene

A normal-incidence geometry, polarization-resolved cavity ring-down spectroscopy technique (polarized NICRDS) is described to probe the polarized absorbance of surface-adsorbed thin films and short-path length liquid samples. The technique is demonstrated by a kinetic study of the photochromic behavior of the spiropyran dye 6,8-dibromo-1', 3'-dihydro-1', 3', 3'-trimethylspiro[2H-1-benzopyran-2, 2'-(2H)-indole]. The technique is shown potentially to have monolayer coverage sensitivity and can measure the angular orientation distribution of analyte molecules. The photochromic kinetics of 6,8-dibromoBIPS in toluene solution were qualitatively consistent with a previous study of this molecule using conventional absorption spectroscopy. The absorption polarizations and slow ring-closing kinetics measured in a thin poly(methyl methacrylate) film are consistent with a strong interaction of the spiropyran and merocyanine forms with the polymer matrix.     

Ultrafast UV-mid-IR investigation of the ring opening reaction of a photochromic spiropyran

We present a femtosecond UV-mid-IR pump-probe study of the photochemical ring-opening reaction of the spiropyran 1',3',3',-trimethylspiro-[-2H-1-benzopyran-2,2'-indoline] (also known as BIPS) in tetrachloroethene, using 70 fs UV excitation pulses and probing with 100 fs mid-IR pulses. The time evolution of the transient IR absorption spectrum was monitored over the first 100 ps after UV excitation. We conclude that the merocyanine product is formed with a 28 ps time constant, contrasting with a 0.9 ps time constant obtained in previous investigations where the rise of absorption bands at visible wavelengths were associated with product formation. We deduce from the observed strong recovery of the spiropyran IR absorption bleaches that, in tetrachloroethene, the main decay channel for the S(1) excited state of the spiropyran BIPS, is internal conversion to the spiropyran S(0) state with a quantum yield of > or = 0.9. This puts an upper limit of 0.1 to the quantum yield of the photochemical ring-opening reaction.     

Polymethylmethacrylate-silica hybrid thin films heavily doped with spiropyran and exhibiting visual photochromism and high mechanical and chemical durability

Perhydropolysilazane (PHPS)—polymethylmethacrylate (PMMA)—spiropyran (SP) films were prepared by spin-coating using a solution of a mass ratio, PHPS:PMMA:SP:xylene = 0.8:0.2:0.25:3.6. The SP-doped PHPS-PMMA films were then exposed to the vapor from aqueous ammonia at room temperature, resulting in the formation of 1.7 μm thick, deep-red colored SP-doped PMMA-silica hybrid films. Due to the high concentration of SP, which could be achieved by the hydrophobic nature of PHPS and xylene, the photochromic color changes were visualized in spite of the small film thickness. The films had pencil hardness higher than 9H, and no SP leaching was observed even when the films were soaked in xylene for 24 h. Thus, thin films that exhibit visual photochromism with high mechanical and chemical durability could be prepared, which can never been achieved by conventional sol–gel methods.     

Ultrafast exciton dynamics in CdSe quantum dots studied from bleaching recovery and fluorescence transients

We have performed ultrafast absorption bleach recovery and fluorescence upconversion measurements ( approximately 100 fs time resolution) for three CdSe samples, with nanoparticle diameters of 2.7, 2.9, and 4.3 nm. The two types of experiments provide complementary information regarding the contributions of the different processes involved in the fast relaxation of electrons and holes in the CdSe quantum dots. Transient absorption and emission experiments were conducted for the 1S [1Se-1S3/2(h)] transition, 1S(e) and 1S3/2(h) representing the lowest electron (e) and hole (h) levels. The bleach recovery of the 1S transition shows a approximately 400-500 fs initial rise, which is followed by a size-dependent approximately 10-90 ps decay and finally a long-lived (approximately ns) decay. The fluorescence upconversion signal for the 1S transition shows quite different temporal behavior: a two times slower rise time (approximately 700-1000 fs) and, when the fluorescence upconversion signal has risen to about 20% of its maximum intensity, the signal displays a slight leveling off (bend), followed by a continued rise until the maximum intensity is reached. This bend is well reproducible and power and concentration independent. Simulations show that the bend in the rise is caused by a very fast decay component with a typical time of about 230-430 fs. Considering that the 1S quantum dot excitation is comprised of five exciton substates (F=+/-2, +/-1L, 0L, +/-1U, and 0U), we attribute the disparity in the rise of the bleaching and emission transients to the results from the dynamics of the different excitons involved in respectively the bleaching and fluorescence experiments. More specifically, in transient absorption, population changes of the F=+/-1U excitons are probed, in emission population effects for the F=+/-2 ("dark") and the F=+/-1L ("bright") exciton states are monitored. It is discussed that the fast (approximately 400-500 fs) rise of the bleach recovery is representative of the feeding of the F=+/-1U exciton (by filling of the 1S(e) electron level) and that the slower (approximately 700-1000 fs) feeding of the emissive +/-2, +/-1L excitons is determined by the relaxation of the hole levels within the 1S3/2 fine structure. Finally, the approximately 230-430 fs component, typical of the bend in the fluorescence transient, is attributed to the thermalization of the close-lying +/-2 ("dark") and +/-1L ("bright") excitons.     

Ultrafast dynamics of photochemical radical formation from

The excited-state dynamics and photochemistry of [Re(R)(CO)3(dmb)] (R=Me, Et); dmb=4,4'-dimethyl-2,2'-bipyridine) in CH2Cl2 have been studied by time-resolved visible absorption spectroscopy on a broad time scale ranging from approximately 400 fs to a few microseconds, with emphasis on the femtosecond and picosecond dynamics. It was found that the optically prepared Franck-Condon 1MLCT (singlet metal-to-ligand charge transfer) excited state of [Re(R)(CO)3(dmb)] undergoes femtosecond branching between two pathways (< or =400 fs for R=Me; approximately 800 fs for R=Et). For both methyl and ethyl complexes, evolution along one pathway leads to homolysis of the Re-R bond via a 3SBLCT (triplet sigma-bond-to-ligand charge transfer) excited state, from which [Re(S)(CO)3(dmb)]* and R* radicals are formed. The other pathway leads to an inherently unreactive 3MLCT state. For [Re(Me)(CO)3(dmb)], the 3MLCT state lies lowest in energy and decays exclusively to the ground state with a lifetime of approximately 35 ns, thereby acting as an excitation energy trap. The reactive 3SBLCT state is higher in energy. The quantum yield (0.4 at 293 K) of the radical formation is determined by the branching ratio between the two pathways. [Re(Et)(CO)3(dmb)] behaves differently: branching of the Franck-Condon state between two pathways still occurs, but the 3MLCT excited state lies above the dissociative 3SBLCT state and can decay into it. This shortens the 3MLCT lifetime to 213 ps in CH2Cl2 or 83 ps in CH3CN. Once populated, the 3SBLCT state evolves toward radical photoproducts [Re(S)(CO)3(dmb)]* and Et*. Thus, population of the 3MLCT excited state of [Re(Et)(CO)3(dmb)] provides a second, delayed pathway to homolysis. Hence, the quantum yield is unity. The photochemistry and excited-state dynamics of [Re(R)(CO)3(dmb)] (R=Me, Et) complexes are explained in terms of the relative ordering of the Franck-Condon, 3MLCT, and 3SBLCT states in the region of vertical excitation and along the Re-R reaction coordinate. A qualitative potential energy diagram is proposed.     

Ultrafast IR spectroscopy of the short-lived transients formed by UV excitation of cytosine derivatives

A strong infrared band at 1574 cm(-1) is observed following 267 nm excitation of 2'-deoxycytidine (tau = 37 +/- 4 ps) or 2'-deoxycytidine 5'-monophosphate (tau = 33 +/- 4 ps); this band is provisionally attributed to an 1n(N)pi* state and is absent for cytosine.     

Complementarity in bimolecular photochromism

Irradiating 2,3,6,7-tetraphenylanthracene in the presence of 9,10-dimethylanthracene leads to exclusive formation of the cross-dimer. No photochemical reaction is observed when either of these chromophores is irradiated in the absence of the other.     

Structural observation of photochromism

The reversible formation of a long-lived, coloured F-centre has been observed in small structural changes delineated by neutron diffraction.     

Preparation,photochromism, and complex formation with metal ions of pyrazolylazomethine derivative of spirooxazine

New hybrid spirooxazine has been prepared, based on amino-substituted spironaphthoxazine and pyrazolone. Photochromism and photoinduced complex formation with metal ions of the prepared compound has been studied. The effects of solvent polarity and the metal ion nature on the spirooxazine photochromism and the complex formation ability have been elucidated.     

Dithienylcyclopentene-functionalised subphthalocyaninatoboron complexes: photochromism, luminescence modulation and formation of self-assembled monolayers on gold

Subphthalocyaninatoboron (SubPc) complexes bearing six peripheral n-dodecylthio substituents and an apical photochromic dithienylperfluorocyclopentene unit were prepared. The photoinduced isomerisation of the apical substituent from the open to the ring-closed form significantly influences the photoluminescence of the covalently attached SubPc unit, which is more efficiently quenched by the ring-closed form. Films on gold were fabricated from these multifunctional conjugates and characterised by near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). The results are in accord with the formation of self-assembled monolayers based on dome-shaped SubPc-based anchor groups. Their chemisorption is primarily due to the peripheral n-dodecylthio substituents, giving rise to covalently attached thiolate as well as coordinatively bound thioether units, whose alkyl chains are in an almost parallel orientation to the surface.     

Ultrafast dynamics of 2E state formation in Cr(acac)3

Femtosecond time-resolved absorption spectroscopy has been used to elucidate the excited-state dynamics associated with formation of the (2)E excited state in a Cr(III) transition metal complex. Cr(acac)(3) (where acac is the deprotonated monoanion of acetylacetone) exhibits monophasic decay kinetics with tau = 1.1 +/- 0.1 ps following excitation into the lowest-energy ligand-field absorption band; the time constant is found to be independent of both excitation and probe wavelength across the entire (4)A(2) --> (4)T(2) absorption envelope. The lack of a significant shift in the excited-state absorption spectrum combined with the observed spectral narrowing is consistent with an assignment of this process as vibrational cooling (k(vib)) in the (2)E state. The data on Cr(acac)(3) indicate that intersystem crossing associated with the (4)T(2) --> (2)E conversion occurs at a rate k(ISC) > 10(13) s(-)(1) and furthermore competes effectively with vibrational relaxation in the initially formed (4)T(2) state. Excitation into the higher energy (4)LMCT state (lambda(ex) = 336 nm) gives rise to biphasic kinetics with tau( 1) = 50 +/- 20 fs and tau( 2) = 1.2 +/- 0.2 ps. The slower component is again assigned to vibrational cooling in the (2)E state, whereas the subpicosecond process is attributed to conversion from the charge-transfer to the ligand-field manifold. In addition to detailing a process central to the photophysics of Cr(III), these results reinforce the notion that the conventional picture of excited-state dynamics in which k(vib) > k(IC) > k(ISC) does not generally apply when describing excited-state formation in transition metal complexes.     

Photoswitched DNA-binding of a photochromic spiropyran

The dramatically different DNA-binding properties of the two isomeric forms of a photochromic spiropyran have been demonstrated, enabling photoswitched DNA binding. The closed, UV-absorbing form shows no signs of interaction with DNA. Upon UV exposure the spiropyran is isomerized to the open form that binds to DNA by intercalation. The process is fully reversible as the corresponding dissociation process is induced by visible light.     

Thermally induced paramagnetism of spiropyran salts

An increase in the effective magnetic moments and linewidths in ESR spectra caused by the presence of crystallization water molecules in crystals of tris(oxalato)chromium(III) complexes with spiropyran cations Sp⁺ was found. Dehydration (rehydration) of crystals causes a reversible decrease (increase) in thermally induced paramagnetism of Sp⁺. Thermally induced paramagnetism of crystals appears in the compounds synthesized in oxygen atmosphere but is not found in the compounds synthesized under argon atmosphere. The contributions of chromium oxalates and thermally excited spiropyran molecules to the magnetization were separated using ESR spectroscopy. The enthalpies of the biradical states of some types of spiropyran cations in crystals were determined.     

Novel photochromism of differently-linked bis-benzopyrans

The unique photochromic bis-chromene 5 incorporates the structural attributes of both 3 and 4 . UV-vis irradiation of 5 leads to a dark brown colour, which is formed by mixing the purple and red colours observed for the photolysates of 3 and 4 , respectively.