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Photoswitches are widely investigated molecules because upon exposure to selected light irradiation, they are able to undergo structural, and hence optical, changes. To fully exploit their responsiveness to irradiation, the quantum efficiency of the forward and back reactions is a fundamental parameter, whose accurate determination is critical. In this work, the spectral evolution of a biomimetic switch, which undergoes E/Zphotoinduced isomerization, is spectrophotometrically examined. The minimal spectral differences between the Eand Zforms inhibit the accurate determination of the quantum efficiency of the E–Zand Z–Ephotoconversions. We present a kinetic analysis of the E–Zspectral evolution, which enables us to determine quantum yields of the forward and back reactions; the data show that the back reaction has a higher efficiency (0.60) than the forward process (0.49), evidencing that irradiation conditions have to be carefully selected to accumulate the Zisomer. The method has been further used to determine the E–Zand Z–Equantum yield in the deprotonated form of the biomimetic switch; the obtained values demonstrate that under basic conditions, the back reaction is relatively more efficient than the forward one even for the deprotonated species. The detailed analysis enables us to establish the conditions to monitor the fluorescence of the Eand Zforms, setting the basis for a more sensitive detection of the photoswitch and its transformation.

Optical technologies allowing modulation of neuronal activity at high spatio-temporal resolution are becoming paramount in neuroscience. In this respect, azobenzene-based photoswitches are promising nanoscale tools for neuronal photostimulation. Here we engineered a light-sensitive azobenzene compound (Ziapin2) that stably partitions into the plasma membrane and causes its thinning through trans-dimerization in the dark, resulting in an increased membrane capacitance at steady state. We demonstrated that in neurons loaded with the compound, millisecond pulses of visible light induce a transient hyperpolarization followed by a delayed depolarization that triggers action potential firing. These effects are persistent and can be evoked in vivo up to 7 days, proving the potential of Ziapin2 for the modulation of membrane capacitance in the millisecond timescale, without directly affecting ion channels or local temperature.

Photochromic diarylethenes represent one of the most important classes of molecular switches, and their fatigue resistance is reported in several papers. Previous studies have demonstrated that the presence of perfluorocyclopentene and methyl in the 4-positions of dithienylethenes improve the switching resistance. However, general guidelines to correlate chemical structure to fatigue resistance have not been found yet. In this work, we provide a different thought in the description of the fatigue resistance of diarylethenes, which is related to the light absorbed by the two isomeric states during photoconversion. For two series of 1,2-diarylethenes, which differ from the aromatic rings without bearing any electroactive substituent, it turns out that the fatigue resistance depends on the dose of light absorbed by the colored form rather than on the specific molecular structure, namely, the presence of specific molecular building blocks.

A recently synthesized and structurally characterized bisdiarylethene containing a benzobis(imidazole) core substituted with two aniline moieties has been investigated from spectroscopic, thermodynamic, and photochemical points of view, using both experimental and theoretical tools. Due to the presence of four basic centers and of two photochromic diarylethenes in the molecular structure, the solutions of this compound change color upon acid addition and/or UV irradiation. Neutralization and/or visible irradiation lead to bleaching. In a water/dioxane (1/1, v/v) mixture, pKavalues and absorption spectra of the open and closed neutral and protonated species, as well as quantum yields of the photochromic cyclization and cycloreversion processes, have been determined at selected pH/H0values by spectrophotometric and photostationary spectrokinetic methods. Time-dependent density functional theory simulations have been carried out and provide insight into the protonation sites and photochromic properties. The results show that the four acidity constants overlap within a restricted pH/H0interval (4.5/–1.5) generating spectra of the colorless and colored forms that progressively shift toward the red. The possibility of continuously tuning the color of the solutions by external stimuli, such as irradiation and acidification, generates a multiswitchable acidichromic and photochromic material.

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-cisopen form in less than one picosecond and then to the metastable transoid-cisstructure 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).

A spectrokinetic study of the photophysical and photochemical properties of a series of photochromes bearing thienyl (T1), dithienyl (T2) and terthienyl (T3) groups linked to a naphthopyran in position 3 (sp3carbon) 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 T3groups 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 T3group 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.

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 (kΔ> 102s−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 (ΦC1= 0.42). Visible light irradiation converts C1 (Φ_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.

In this work a completely novel photochromic mechanism exhibited by an indeno-fused naphthopyran (chromene) is presented. The studied molecule is the first case of a chromene-type compound exhibiting both photochromism and thermochromism. These phenomena were investigated in ethanol. Thermochromism was detected by monitoring absorption spectra in the 285–320 K temperature range: the enthalpy of the thermocoloration reaction was determined (ΔH = 25 kJ mol−1) and the equilibrium constant was estimated (K_eq ≈ 10−3). For the photochemical investigation, carried out in the 200–270 K temperature range, continuous monochromatic light was used for excitation. The photocoloration was observed under two distinct stimulations: UV-irradiation (λ_exc = 303 nm), which is normally used for photochromic chromene activation, and visible-irradiation (λ_exc = 574 nm) of the thermally equilibrated solution. Two coloured species are involved in the photochemical and thermal processes. One of them, P, is photochemically produced by UV irradiation of the colourless form, while the other, T, is present in thermal equilibrium with the closed form. When the thermally equilibrated solution is irradiated with visible light, T is converted to P. This system not only results in a cyclic on–off chromogenic device, which can be switched-on by UV radiation, but it also exhibits on–off functionality using lower energy visible light. Such behaviour makes this photochromic system exceptionally efficient upon exposure to sunlight.

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 &lt; 240 K), since the rate of the back thermal reaction (bleaching) is very fast (k_Δ > 102 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 (Φ_C1 = 0.42). Visible light irradiation converts C1 (Φ_−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.

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.

In this article the analytical integration of kinetic equations describing the dynamic behavior of reversible photoreactions has been undertaken. Photochemical systems giving rise to competing thermal and photochemical steps are kinetically analyzed; the rate law is set up and analytical solutions are obtained under precisely defined boundary conditions. More complicated kinetic systems, where several species interact both thermally and photochemically, are also investigated. The kinetic treatment leads to a system of coupled differential equations which are amenable to analytical solutions, under the appropriate experimental and boundary conditions. The usefulness of the equations developed is illustrated by their application to some spirooxazine and chromene photochromic systems: two examples are described in detail. © 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 303–313, 1999