Light and pH tunable luminescence in a photochromic bisdiarylethene.

In this work the luminescence of a bisdiarylethene, containing a benzobis(imidazole) core substituted with two aniline moieties, has been investigated. In previous research, it was found that both acidification and irradiation reversibly triggered colour changes of this compound, thus generating a multi-responsive acidichromic and photochromic system. Intense fluorescence emission, which was detected in several organic solvents, can be an additional light driven signal. In a dioxane/water (1 : 1, v/v) mixture, intensity and spectral position of luminescence have been found to drastically depend on the pH/H0 values of the solutions (pH 5/H0-2 range) due to subsequent protonations (four steps) as the acidity of the solution changes. Alternated irradiations with UV and visible light lead to a decrease and increase, respectively, of the fluorescence intensity, due to the photochromic reaction producing a non-fluorescent compound. Quantum yields and lifetimes of fluorescence were determined as a function of the acidity. The results indicate that protonation shifts the emission to the red and decreases its intensity. The possibility of tuning the colour and intensity of luminescence by both acidification and irradiation generates a multi-switchable "fluorochromic" material.

Tetra- and tri-thienyl ethenes: new fluorescent photochromic compounds.

In this work the photochemistry and photophysics of two new ethenic compounds, a tetra-thienylethene (tetrakis) and a tri-thienylethene (triakis), have been investigated by steady state and time-resolved UV-vis absorption and luminescence spectroscopies. The quantum yields of the UV-photoinduced ring-closure reaction (Φ(O→C)) and of the visible-stimulated cycloreversion reaction (Φ(C→O)) were determined as a function of the temperature in 3-methylpentane. The trends of Φ(O→C) and Φ(C→O) with temperature suggest that a barrier exists to the ring opening reaction which hampers the cycloreversion in a rigid matrix at 80 K. For the closure reaction, a negative effect of the temperature on Φ(O→C) reflects an increased concentration of the more easily isomerizable antiparallel conformer with decreasing the temperature. Quantum yields and lifetimes of fluorescence, observed for tetrakis in both the colourless and coloured forms and for triakis only for the coloured form, were determined. The solvent and temperature effects on fluorescence were investigated. Nanosecond time-resolved experiments showed the formation of a triplet transient from tetrakis which could be a precursor of the main reaction photoproduct. For triakis no transient was detected.

Ultrafast excited-state dynamics in some spirooxazines and chromenes. Evidence for a dual relaxation pathway.

A great variety of technological applications makes photochromism a currently appealing theme for basic studies. In this work, excited state dynamics of two spirooxazines and two naphthopyrans, that upon UV irradiation undergo thermally reversible conversion to coloured photomerocyanines, have been investigated by using pump-probe techniques (femtosecond time resolution). The breakage of the C-O bond, involved in the photoreaction, has been found to occur within a few hundreds of femtoseconds producing a first transient that evolved on picosecond time-scale to the most stable isomer through a number of intermediates that depended on the solvent and the structure of the photochrome. The peculiar behaviour of one of the molecules studied (1,3-dihydro-3,3-dimethyl-1-isobutyl-6'-(2,3-dihydro-1H-indol-1-yl)spiro [2H-indole-2,3'-3H-naphtho[2,1-b][1,4]oxazine]) has been investigated in depth in various media because it revealed an unusual dual photochemistry pathway. This finding is traced to reactivity of π,π* and ICT excited states whose relative populations are controlled by the polarity of the solvent.

Fluorescence spectroscopy: a powerful technique for the noninvasive characterization of artwork.

After electronic excitation by ultraviolet or visible radiation, atoms and molecules can undergo thermal or radiative deactivation processes before relaxing to the ground state. They can emit photons with longer wavelengths than the incoming exciting radiation, that is, they can fluoresce in the UV-vis-near-infrared (NIR) range. The study of fluorescence relaxation processes is one of the experimental bases on which modern theories of atomic and molecular structure are founded. Over the past few decades, technological improvements in both optics and electronics have greatly expanded fluorimetric applications, particularly in analytical fields, because of the high sensitivity and specificity afforded by the methods. Using fluorimetry in the study and conservation of cultural heritage is a recent innovation. In this Account, we briefly summarize the use of fluorescence-based techniques in examining the constituent materials of a work of art in a noninvasive manner. Many chemical components in artwork, especially those of an organic nature, are fluorescent materials, which can be reliably used for both diagnostic and conservative purposes. We begin by examining fluorimetry in the laboratory setting, considering the organic dyes and inorganic pigments that are commonly studied. For a number of reasons, works of art often cannot be moved into laboratories, so we continue with a discussion of portable instruments and a variety of successful "field applications" of fluorimetry to works of cultural heritage. These examples include studies of mural paintings, canvas paintings, tapestries, and parchments. We conclude by examining recent advances in treating the data that are generated in fluorescence studies. These new perspectives are focused on the spectral shape and lifetime of the emitted radiation. Recent developments have provided the opportunity to use various spectroscopic techniques on an increasing number of objects, as well as the ability to fully characterize very small amounts of sample, either in a laboratory setting or on site. Thus, a new technological highway is open to scientists; it is still difficult to navigate but offers an enormous potential for investigating objects without touching them. Fluorescence spectroscopy is one of the most important of these techniques.

Excited-state properties of a photochromic spirooxazine: double pathways for both fluorescence emission and camphorquinone-sensitized reaction.

In this article, we report a study on the singlet and triplet excited-state properties of a spirooxazine (1,3-dihydro-3,3-dimethyl-1-isobutyl-6'-(2,3-dihydro-1H-indol-1-yl)spiro[2H-indole-2,3'-3H-naphtho[2,1-b][1,4]oxazine]). The singlet state of this molecule is photoreactive: upon UV light stimulation, it produces a colored merocyanine that thermally reverts to the starting compound. A double-way radiative relaxation path was found for singlet-state excitation. Experimental observations on the absorption and fluorescence spectra were in excellent agreement with TD-DFT calculations for the singlet state. The triplet state, which could not be directly populated by intersystem crossing from the singlet, when reached by energy transfer from a suitable sensitizer (camphorquinone), yielded the colored merocyanine with quantum yield close to unity. However, the donor/acceptor interaction also originated a new photochromic system as a consequence of the competition of hydrogen abstraction with energy transfer in the interplay of the sensitizer with the substrate. The newly produced photochrome was structurally, spectrally, and photochemically characterized. It exhibited excellent colorability in both directly excited and triplet-sensitized photoreactions by virtue of high photoreaction quantum yield and rather slow bleaching rate of the colored form but also underwent significant degradation in the presence of oxygen that led to the destruction of the photochromic functionality.

New thermally irreversible and fluorescent photochromic diarylethenes.

A photokinetic investigation is here carried out on four newly synthesized diarylethenes with the aim to test their performance as photoreversible chromogenic and light emitting materials. The pentatomic ring, which fixes these diarylethenes in the cis conformation, contains a Si atom or a PO group. The 1,2 positions at the ethenic bond are symmetrically substituted with thienyl or benzothienyl groups. The results are compared with those for the structurally related and widely studied 1,2-bis(2-methyl-benzo[b]thiophen-3-yl)perfluorocyclopentene (BTF6), investigated here under the same experimental conditions. Spectra of the colorless and colored forms and photoreaction quantum yields were determined; temperature, excitation wavelength and viscosity effects were explored. Compounds containing benzothienyl substituents were found to be good bistable photochromes, with high photochemical yields of both the cyclization and cycloreversion reactions, and to display appreciable fluorescence emission from the colored forms, which is a rare and desirable property for photochromes. In contrast, the molecules not bearing the benzene condensed rings were found unsuitable as photochromes because of side degradation processes occurring in competition with cyclization.

Photochromic, thermochromic, and fluorescent spirooxazines and naphthopyrans: a spectrokinetic and thermodynamic study.

The photochromic and thermochromic behavior of four commercially available Reversacol dyes are presented. The compounds studied belong to the class of spirooxazines and naphthopyrans, which are typically thermoreversible photochromic molecules. On stimulation with UV light, these compounds become colored and exhibit spectra which extend over the whole visible region. Increasing the temperature causes spontaneous coloration (thermochromism). Herein, absorption and fluorescence spectra, molar absorption coefficients of the colorless and colored forms, fluorescence and photochemical quantum yields, and kinetic parameters of thermal bleaching (rate constant, frequency factor, and activation energy) are determined in acetonitrile solution. The thermal ground-state reaction is exhaustively described in terms of thermodynamic parameters (equilibrium constant, free energy, enthalpy, and entropy). Temperature effects on photochemical and thermal colorabilities are evaluated. The results indicate that the two spirooxazines are good photochromes below room temperature, whereas they are efficient thermochromic compounds above room temperature. Naphthopyrans are better photochromes but worse thermochromic compounds than spirooxazines.

Static and dynamic interaction of a naturally occurring photochromic molecule with bovine serum albumin studied by UV-visible absorption and fluorescence spectroscopy.

In this work, the interaction of a naturally occurring chromene, flindersine (FL), and bovine serum albumin (BSA) has been investigated by UV-vis absorption and fluorescence spectroscopy, time-resolved lifetime measurements, steady state photochemistry, and semiempirical calculations. The interplay of FL with tryptophan (Trp) has been studied in parallel. The interaction of FL with BSA causes fluorescence quenching of BSA through both static and dynamic quenching mechanisms. FL binds BSA with a stoichiometry that varies from 1.09:1 to 0.80:1 as the temperature increases from 293 to 308 K. The reaction is characterized by negative enthalpy (deltaH degrees = -193 kJ mol(-1)) and negative entropy (deltaS degrees = -550 J K(-1) mol(-1)), indicating that the predominant forces in the FL-BSA complex are hydrogen bonding and van der Waals forces. The binding distance between the protein and the photochrome was calculated as 2.5 nm, according to the Foerster theory on resonance energy transfer. The effect of FL concentration on the BSA fluorescence was analyzed according to the maximum entropy method. FL also quenches the emission of Trp with a mechanism that, based on the experimental evidence, excludes both static and dynamic effects. An alternative relaxation pathway, consisting in an electron transfer from a prefluorescent state of Trp to FL, is put forward. The photobehavior of FL is affected by the interplay with BSA but not with Trp. When FL is complexed with BSA, it becomes a more fluorescent and more reactive species. Semiempirical calculations of the lowest optically active electronic transitions of hypothetical FL photoproducts suggest the most likely structure for the photoproduct.

Effects of the exciting wavelength and viscosity on the photobehavior of 9- and 9,10-bromoanthracenes.

The widely investigated photobehaviors of 9-bromo and 9,10-di-bromoanthracenes have been revisited here to clarify the competition among different relaxation paths of their lowest two electronic excited states. The results obtained show that these two molecules exhibit a parallel photobehavior, which depends on the excited electronic and vibronic transition, the medium viscosity, and the temperature. The first electronic state of either of these does not exhibit photochemistry in fluid solution or rigid matrices (80 K). The fluorescence emission occurs with a very low quantum yield (approximately 10(-2)) at room temperature but with a very high quantum yield (0.9 to approximately 1) at 80 K. When exciting in the second electronic transition, the fluorescence intensity is lower than when exciting in S1 at both room temperature and low temperature due to competition with the observed photocleavage of the C-Br bond. The reaction yield decreases as the temperature decreases and depends on the viscosity of the solvent; the higher the viscosity, the lower the observed yield of photochemistry. Temperature and viscosity effects are a consequence of the fact that the radicals produced by C-Br bond breakage cannot escape from the solvent cage and, moreover, quickly recombine within the cage giving the appearance that no photochemistry occurred. The presence of photochemistry in S(2) and its absence in S(1) is principally due to the fact that S(2) has a pi,sigma* character in the C-Br bond, whereas the S(1) state has its origin from a pi,pi* delocalized configuration.

Effects of proximity on the relaxation dynamics of flindersine and 6(5H)-phenanthridinone.

The role played by the carbonyl group in the antenna system of a naturally occurring photochromic chromene, flindersine (FL), has been experimentally investigated and compared with that of a carbonyl group present in a structurally related unreactive heterocyclic compound, 6(5H)-phenanthridinone (PH). Through stationary and time-resolved absorption and emission techniques, the excited-state relaxation dynamics after UV irradiation were determined for FL and PH. The presence of a carbonyl group in both compounds entails the existence of two close-lying, strongly coupled electronic excited states, having n,pi* and pi,pi* character, respectively. Their coupling can be modulated by a careful choice of the solvent proticity and temperature. Moreover, in the case of strong coupling between the n,pi* and pi,pi* states, we have proved that the relaxation dynamics can involve transitions in which the upper of the coupled states acts as an intermediate for radiationless decay, bypassing the lowest emissive state, whereby the fluorescence quantum yield becomes a function of the excitation wavelength.

Photoinduced processes in dipyrrolyl-perfluoro-cyclopentenes.

The photobehavior of five photochromic dipyrrolyl-perfluoro-cyclopentenes was studied by steady state and time-resolved absorption spectroscopy. The quantum yields of the UV-photoinduced ring-closing reaction (coloration) and the visible-stimulated cycloreversion reaction (bleaching) were measured. Kinetic and thermodynamic parameters of thermal bleaching were also determined. Nanosecond time-resolved experiments showed formation of a transient, which was not a precursor of the reaction photoproduct. This transient was tentatively assigned to a radical cation formed by direct photoionization through a short-lived triplet state. The nature of the transient species was supported by photoinduced electron transfer to electron acceptors.

The ring-opening reaction of chromenes: a photochemical mode-dependent transformation.

The efficiency of the photochemical ring-opening of chromenes (or benzopyrans) depends on the vibronic transition selected by the chosen excitation wavelength. In the present work, ab initio CASPT2//CASSCF calculations are used to determine the excited-state ring-opening reaction coordinate for 2H-chromene (C) and 2,2-diethyl-2H-chromene (DEC) and provide an explanation for such an unusual mode-dependent behavior. It is shown that excited-state relaxation and decay occur via a multimodal and barrierless (or nearly barrierless) reaction coordinate. In particular, the relaxation out of the Franck-Condon involves a combination of in-plane skeletal stretching and out-of-plane modes, while the second part of the reaction coordinate is dominated exclusively by a different out-of-plane mode. Population of this last mode is shown to be preparatory with respect to both C-O bond breaking and decay via an S(1)/S(0) conical intersection. The observed mode-dependent ring-opening efficiency is explained by showing that the vibrational mode corresponding to the most efficient vibronic transition has the largest projection onto the out-of-plane mode of the reaction coordinate. To support the computationally derived mechanism, we provide experimental evidence that the photochemical ring-opening reaction of 2,2-dimethyl-7,8-benzo(2H)chromene, that similarly to DEC exhibits a mode-dependent photoreaction, has a low ( approximately 1 kcal mol(-1)) activation energy barrier.

Effect of oligothiophene substituents on the photophysical and photochromic properties of a naphthopyran.

A spectrokinetic study of the photophysical and photochemical properties of a series of photochromes bearing thienyl (T(1)), dithienyl (T(2)) and terthienyl (T(3)) groups linked to a naphthopyran in position 3 (sp(3) carbon) or 8 (naphtho moiety) has been carried out. The effect of the number and position of the thienyl groups on the excited state properties of the six compounds in the singlet and triplet manifold has been investigated by stationary and pulsed fluorimetric techniques, laser flash photolysis and kinetic analysis of the colouration/decolouration processes upon continuous irradiation. On increasing the length of the oligothiophene moiety, the contribution of fluorescence to the deactivation processes becomes detectable, while the photocolouration yield decreases. Moreover, only for compounds with T(3) groups was the triplet absorption observed by laser flash photolysis; its decay did not contribute to photocolouration. The competition between physical (fluorescence emission and intersystem crossing) and chemical relaxations markedly affects the behaviour of the compound bearing the T(3) group at the naphtho moiety, which shows the most efficient intersystem crossing and fluorescence emission while does not lead to photoproduction of the coloured open form. A parallel study of the NMR spectra of the compounds bearing one thiophene group allowed two and four photomerocyanine-type photoproducts to be identified for the naphtho-substituted and pyran-substituted compounds, respectively. The compound with a thiophene group at the naphtho moiety was studied in more detail at different temperatures. On the basis of the combined optical and NMR spectrometric results, a reasonable mechanism of its photochromic behaviour is proposed.

Dynamics of the excited states of chromenes studied by fast and ultrafast spectroscopies.

In this paper, the photodynamics of three chromenes (2,2-spiro-adamantyl-7,8-benzo(2H)chromene, 2,2-diphenyl(2H)chromene and 2,2-diphenyl-5,6-benzo(2H)chromene) has been investigated by nano- and femtosecond time resolved techniques in hydrocarbon solutions at room temperature. Using pump-probe techniques, ultrafast steps of molecular dynamics characterizing the photoresponse of chromenes have been investigated: the breakage of the C-O bond was found to occur within a few hundred femtoseconds producing a short-lived transient that converts to the cisoid-cis open form in less than one picosecond and then to the metastable transoid-cis structure within a few picoseconds. The effect of different excitation wavelengths on the formation rate of the first shortest-lived transient of 5,6DPBC is in agreement with the model previously proposed to explain the wavelength dependence of the reaction and emission quantum yields in photostationary experiments. The results obtained in the nanosecond time domain show that the triplet marginally participates in the photoreaction for only one of the studied compounds (2,2-spiro-adamantyl-7,8-benzo(2H)chromene).

The complex photochromic behaviour of 5,6-benzo(2H)dimethylchromene in 3-methylpentane solution.

The photokinetic behaviour of 5,6-benzo(2H)dimethylchromene was investigated in 3-methylpentane solution by spectrophotometric methods using monochromatic irradiation wavelengths in the UV and visible regions. This molecule only exhibits photochromism at low temperature (T < 240 K), since the rate of the back thermal reaction (bleaching) is very fast (k > 10(2) s(-1)) at ambient temperature. The colour-forming kinetics were followed under continuous irradiation using UV light (318 nm) in two low temperature ranges (160-140 and 235-219 K). In the 160-140 K interval, the temperature is low enough to reduce the thermally activated processes and UV irradiation produces one coloured form, C1 (phi(C1) = 0.42). Visible light irradiation converts C1 (phi(-C1) = 0.10) to a photoproduct, P, which is thermally and photochemically stable in that temperature range. In the 235-219 K temperature interval, both thermal and photochemical processes were detected: two coloured forms, C1 and C2, are produced in consecutive steps, one of them is thermoreversible, whereas the other is photoreversible. Quantum yields of the photoreactions and kinetic parameters (rate coefficient and activation energy) of the thermal ring-closure reaction were determined. Based on the results obtained, a plausible reaction mechanism is proposed.

Comprehensive photokinetic and NMR study of a biphotochromic supermolecule involving two naphthopyrans linked to a central thiophene unit through acetylenic bonds.

A photophysical and photochemical study of a biphotochromic compound where two naphthopyran units are linked by an acetylene-thiophene-acetylene bridge has been carried out in toluene. Both fluorescence and intersystem crossing to the triplet manifold were found to compete with the photocoloration process. Two photoproducts (transoid-trans and transoid-cis stereoisomers), absorbing at approximately 480 nm and corresponding to the opening of a single photochromic unit, were detected by spectrophotometric analysis after short irradiation time in diluted solution and identified by 1H-nuclear magnetic resonance (NMR) spectroscopy. After prolonged irradiation at 228 K of highly concentrated solutions (up to 3 x 10(-3) mol dm(-3)), two additional isomers, absorbing at approximately 550 nm, were formed. Their NMR spectra indicate the opening of both photochromic units. An interesting effect of selective vibronic excitation was found, showing that the photoreaction is favored at excited vibronic levels to the detriment of the radiative relaxation.

Effects of the environment on the photochromic behaviour of a novel indeno-fused naphthopyran.

The photochromism of a new chromene-type compound, an indeno-fused naphthopyran, was studied under steady irradiation in solvents of different polarity and/or proticity, in microheterogeneous systems (micelles and gel) and in a nematic liquid crystal. The solutions change from colourless to coloured upon UV irradiation, due to cleavage of the carbon-oxygen pyran bond. The photoreaction is thermally reversible. Spectra, molar absorption coefficients of the colourless and coloured forms, quantum yield of photocolouration and kinetic parameters of the thermal bleaching (rate constant and activation energy) were determined. Compared with other chromenes, the spectra of both the coloured and colourless forms are red-shifted and the coloured form exhibits a marked positive solvatochromism. The photocolourability is good, even at ambient temperature, and is better in a polar and/or protic medium where the entropy loss due to solvent reorganisation around the transition state decreases the rate of the bleaching process, Thus, the best media for colouration are ethanol and the liquid crystals, In the gel (Aerosol-OT [sodium bis(2-ethylhexyl)sulfosuccinate] isooctane-animal gelatin-water) and microemulsion (AOT-isooctane-water) the non-polar ground state molecules (in both the open and closed forms) occupy the hydrophobic sites and therefore the behaviour is similar to that observed in the pure hydrocarbon.