Thermal Reversion Research Articles

Oligothiophene Functionalized Dimethyldihydropyrenes I: Syntheses and Photochromicity

The syntheses of 2,7-di-tert-butyldimethyldihydrobenzo[e]pyrenes with thienyl (6), terthienyl (7), and pentathienyl (14) side chains at the 4,5- positions, ter- and pentathienyl side chains at the 4-position with ter- (39) and pentathienylcarbonyl (40) side chains at the 10- and 11-positions, 2-naphthoyl-7-tert-butyldimethyldihydropyrenes with ter- (53), penta- (54), and septithienyl (55) side chains at the 4,9-positions are described. These compounds are all photochromic and open to the corresponding cyclophanedienes with long wavelength (>490 nm) light, and as such, the conjugative path could change considerably, making them suitable to investigate as potentially switchable conducting molecules. In this paper, the syntheses and the photochemical and thermal isomerizations are studied; in the accompanying paper, the electrochemical and conductive properties are studied. Here, a comparison of the relative opening rates to that of the benzo[e]pyrene 4 (with no thienyl substituents) is made, and all of the above photochromes show considerably enhanced photo-opening of the DHPs to the CPDs. As examples, 14, 40, and 54 were cycled between the open and closed forms, and no decomposition was observed; however, when 54 was irradiated for 40 h with 254 nm light, some radicals did form, which enhanced the thermal closing rate, and so extensive irradiation with short wave UV is better avoided. The thermal closing reactions were also studied, and all of the above compounds close faster than benzo-CPD 4', though for the highly photochromic ter- and pentathienyl benzo-CPDs 39' and 40', the rate was not too enhanced from that of 4' and so are probably the best compromise between fast photochromicity and slow thermal reversion.

Photochromic Dye-Doped Conjugated Polymer Nanoparticles: Photomodulated Emission and Nanoenvironmental Characterization

We present studies of fluorescence photomodulation and solvatochromism in nanoparticles of the conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) doped with a photochromic spirooxazine dye. The fluorescence properties of doped nanoparticles with dyes in the spirooxazine form are identical to those in undoped control nanoparticles. UV irradiation converts some of the dyes to their visible-absorbing merocyanine form, which is an efficient quencher of MEH-PPV fluorescence. The fluorescence intensity of the nanoparticles drops to less than 10% of its initial value and recovers when the merocyanines undergo thermal reversion to spirooxazines. The fluorescence modulation can be cycled many times without fatigue or photodegradation, and the degree of quenching is linear with merocyanine concentration. The photochromic conversion can also be used as a probe of the environment within the nanoparticles as both the kinetics of the thermal merocyanine-to-spirooxazine conversion and...

Photochromism of Spiropyran in Ionic Liquids: Enhanced Fluorescence and Delayed Thermal Reversion

Photochromism of spiropyran and thermal reversion of MC in a series of imidazolium-based ionic liquids (ILs) were investigated via fluorescence and UV-vis absorption spectra. The fluorescence emission maxima (lambda(em)) of the colored merocyanine (MC) form is polarity dependent in all ILs and fluorescence intensity enhancement was first observed in ILs with respect to that observed in organic solvents (e.g., in [HOMIm][PF(6)], nearly 32-fold enhancement than ethanol). Moreover, the thermal decay of MC in all ILs is uniformly first order, and in comparison with polar solvents possessing comparable polarity (such as ethanol and acetonitrile), a significant retardation of the decay rate and increase of half-lifetime of MC form was observed, in particular at elevated temperature (e.g., at 313 K, MC decay with k of 5.19 x 10(-4) and 34.7 x 10(-4) s(-1) in [BMMIm][BF(4)] and ethanol, respectively). Abnormal thermodynamics of the thermal decay of MC in ILs were observed, activation energies in less polar ILs (88-97 kJ mol(-1)) have a larger value than in polar ILs (70-82 kJ mol(-1)), opposite that in organic solvents (70-80 kJ mol(-1) in nonpolar solvents and around 100 kJ mol(-1) in polar solvents), and all ILs exclusively exhibit minus activation entropies (-85.4 to -24.5 J K(-1) mol(-1)), similar to that of nonpolar solvents. Desolvation due to the conformational change during the thermal reversion between MC and the transition state may be the key to interpret this experimental result.

Thermal reversion of spirooxazine in ionic liquids containing the [NTf2]− anion

We have investigated the kinetic and thermodynamic parameters of thermal reversion of the spirooxazine 1,3,3-trimethyl-5'-(2-benzothiazolyl)-spiroindoline-2,3'-naphtho(2,1-b)(1,4) oxazine (SO) in molecular solvents and ionic liquids containing the [NTf2]- anion. E(T)30 and Kamlet-Taft parameter studies were employed to interpret IL polarity and attempt to rationalise the kinetic-polarity relationship of MC --> SO relaxation. The observed thermal relaxation of SO within ionic liquids was slower than that of molecular solvents with similar polarity, indicating a greater degree of interactions between the ionic liquid ions and the zwitterionic MC isomer, which led to increased lifetimes for the MC-ion complexes (19.6 s in acetonitrile and 90.9 s in [P6,6,6,14][NTf2]). Thermodynamic parameters of activation implied enhanced ordering of SO in ILs and its subsequent effect on thermal reversion of SO. Activation parameters found MC --> SO relaxation was more temperature dependent in polar protic ILs such as [bmIm]+ than aprotic ILs such as [P6,6,6,14][NTf2] with deltaS++ values of 40 J K mol(-1) and -8.6 J K mol(-1), respectively. Activation energies were generally higher in ILs (83.9-97 kJ mol(-1)) compared to molecular solvents (67.8-89.8 kJ mol(-1)), which was reflected in longer lifetimes of MC due to greater energy barriers of relaxation to SO. Pre-metathesis cleaning of precursor salts was found to be necessary in order to obtain spectroscopic grade ILs for physicochemical analysis using solvatochromic probe dyes. Inconsistencies in the polarity-kinetic relationship further reinforced the belief that measuring polarity of ILs may not be possible with solvatochromic probe dyes.

Photophysical and Duplex‐DNA‐Binding Properties of Distamycin Dimers Based on 4,4′‐ and 2,2′‐Dialkoxyazobenzenes as the Core

Distamycin-based tetrapeptide (1) was covalently tethered to both ends of the central dihydroxyazobenzene moiety at either the 2,2' or 4,4' positions. This afforded two isomeric, distamycin-azobenzene-distamycin systems, 2 (para) and 3 (ortho), both of them being photoisomerizable. Illumination of these conjugates in solution at approximately 360 nm induced photoisomerization and the time course of the process was followed by UV/Vis and (1)H NMR spectroscopy. The kinetics of the thermal reversion at various temperatures of cis to trans isomers of the conjugates obtained after photoillumination were also examined. This afforded the respective thermal-activation parameters. Both the molecular architecture and the location of the substituent around the core azobenzene determined the rate and activation-energy barrier for the cis-to-trans back-isomerization of these conjugates in solution. Duplex-DNA binding of the conjugates and the changes in DNA-binding efficiency upon photoisomerization was also examined by CD spectroscopy, thermal denaturation studies, and a Hoechst displacement assay. The conjugate 2 showed higher DNA-binding affinity and a greater change in the DNA-binding efficiency upon photoisomerization compared with its 2,2'-disubstituted counterpart. The experimental findings were substantiated by using molecular-docking studies involving each conjugate with a model duplex d[(GC(AT)(10)CG)](2) DNA molecule.

Isomerization in Fluorescent Protein Chromophores Involves Addition/Elimination

The green fluorescent protein (GFP) chromophore undergoes both photochemical and thermal isomerizations. Typically, the Z form is more stable and undergoes photochemical conversion to the E form followed by thermal reversion over a period of seconds or minutes. Although the mechanism of the thermal reversion has been the subject of some investigations, the surprisingly low activation energy for this process has not sparked any controversy. We now show that the chromophore is surprisingly stable in both E and Z forms and that the facile thermal reversion is the result of a novel nucleophilic addition/elimination mechanism. This observation may have implications for the intervention of such processes, as well as blinking and kindling, in fluorescent proteins.

Photoresponsive Arylether Dendrimers with Azobenzene Core and Terminal Vinyl Groups

Photoresponsive arylether dendrimers Bis-azo-Gn(3,5) 1a-1c and Bis-azo-Gn(3,4,5) 2a-2c (n = 1-3) with an azobenzene unit at the core and several vinyl groups (3,5-bis(but-3-enyloxy)phenyl groups or 3,4,5-tris(but-3- enyloxy)phenyl groups) at the periphery have been prepared. Their structures and reversible trans-cis isomerization behaviors have been investigated by <TEX>$^1H$</TEX>-NMR, <TEX>$^{13}C$</TEX>-NMR, MALDI-TOF-Mass, and UV-vis spectra. All six azobenzene-cored dendrimers carried out very fast trans <TEX>$\rightarrow$</TEX> cis photoisomerization on irradiation of 350 nm light and reached to the photostationary state within 180 s. During the dark incubation, slow thermal back reversion from cis to trans form is observed for all six dendrimers and is completed within 3 days for 1a-1c and 1 day for 2a-2c. Isomerization efficiency decreases with increasing generation. However, the initial reaction rates of both trans <TEX>$\rightarrow$</TEX> cis photochemical isomerization and cis <TEX>$\rightarrow$</TEX> trans thermal isomerization increases significantly with increasing generation for dendrimers for 1a-1c but only slightly for 2a-2c.

Sugar-Induced Blue Membrane: Release of Divalent Cations during Phase Transition of Purple Membranes Observed in Sugar-Derived Glasses

The formation of blue membrane from purple membranes (PM) has been observed in glassy films made from PM and various sugars. The phase transition of PM at about 70 degrees C causes the complexation of divalent cations to be weakened. The vicinal diol structures in sugars are capable to complex divalent cations and delocalize them throughout the matrix as long as its glass transition temperature is lower than the phase transition temperature of PM. The loss of divalent cations from bacteriorhodopsin (BR), the only protein in PM, causes the formation of blue membrane (BM), which is accompanied by a loss of beta-sheet structure observable in the infrared spectrum. Glassy sugars are particular useful to observe this transition, as sugar entrapment does not restrict conformational changes of BR but rather retards them. The material obtained was named sugar-induced blue membrane (SIBM). The formation of SIBM is inhibited by the addition of divalent cations. Furthermore, SIBM is reverted immediately to PM by addition of water. A characteristic time dependence of the thermal reversion of SIBM to PM proves that the phase transition of PM triggers the release and uptake of divalent cations and the corresponding color change.

The Effect of Addition of Fluorescent Moieties to Dihydropyrenes: Enhancing Photochromicity and Fluorescence Monitoring

A series of dihydropyrenes with appending fluorescent moieties were synthesized with the objective of increasing the photochromic efficiency for this class of compounds and to establish how suitable fluorescence would be to follow their photochromic behavior. The ring opening quantum yields of dihydropyrenes with aroyl substituents at the 4-position showed increased ring opening quantum yields without a decrease of the half-life for the thermal reversion of the less stable open isomer, the metacyclophanediene to the dihydropyrene. The fluorescence of the appending naphthoyl or pyrenoyl moieties was not suitable to follow the photochromic cycling of the dihydropyrenes. However, the emission detected above 600 nm of the closed isomer of the dihydropyrene moiety was shown to be a good monitoring method for the photochromic cycling.

Photoresponsive Azobenzene-cored Dendrons with Terminal Vinyl Groups

Azobenzene-cored dendrons having the photoisomerizable azobenzene core and terminal vinyl groups have been prepared. Absorption bands of azobenzene-cored dendrons are similar except more intense 280 nm band in higher generation azobenzene dendron. All three azobenzene-cored dendrons show reversible photoisomerization similar to simple azobenzene, irrespective of the generation of dendron. On 350 nm irradiation, absorption band around 344 nm decreases and 436 nm band increases. Photoisomerization reactions are very fast for all three azobenzene-cored dendron and the reaction efficiency is dependent of its generation and solvent. In the dark, slow thermal reversion to original absorption spectrum is observed.

Generation and reactions of new ether and acetal functionalized tricyclo[3.3.0.0 3,7]oct-1(5)-ene derivatives. DSC and NMR studies on the [2+2] retrocycloaddition of several cyclobutane dimers

Two new highly pyramidalized tricyclo[3.3.0.0 3,7]oct-1(5)-ene derivatives containing ether and acetal functionalities have been generated, trapped as Diels–Alder adducts and dimerized. The initially obtained diene dimers were photochemically converted into cyclobutane derivatives. The thermal reversion of several cyclobutane derivatives to the corresponding dienes has been studied by 1H NMR, ab initio calculations and DSC. For the first time, transannular additions of bromine and iodine to a diene dimer of this series have been observed.

Reversible Three-State Switching of Luminescence: A New Twist to Electro- and Photochromic Behavior

The fully reversible three-state blue/red/off emission from photo-/electrochromic substituted bis-thiaxanthylidenes is reported. The blue luminescence of the most stable (anti-folded) conformer of dimethyl- and dimethoxy-bis-thiaxanthylidene can be switched off by photochemical conversion to the meta-stable (syn-folded) conformer and switched on again by thermal reversion to the anti-folded state. The red luminescence of the bis-thiaxanthylium dication can be switched on by oxidation at approximately 1.0 and 1.2 V vs SCE of the syn- and anti-folded conformers respectively and switched off or to blue by reduction at approximately 0.35 V vs SCE.

Effect of Heat Treatment and Thermochemical Treatment on Linear Recovery Property of TiNi Shape Memory Alloy

A new concept of linear recovery behavior in TiNi shape memory alloy (SMA) is proposed in this article. Effects of heat treatment and thermochemical treatment including “Ceriuming” and “Samariuming” on the linear recovery property of TiNi SMA were investigated through measuring the thermal reversion curves. The results indicates that the linear recovery properties can be modified by both heat treatment and thermochemical treatment.

Assignment of the Vibrational Modes of the Chromophores of Iodopsin and Bathoiodopsin: Low-Temperature Fourier Transform Infrared Spectroscopy of 13C- and 2H-Labeled Iodopsins

To investigate the chromophore structures of iodopsin and its low-temperature photoproducts, we have assigned their vibrational bands in the Fourier transform infrared (FTIR) spectra using iodopsin samples that were reconstituted with a series of (13)C- and deuterium-labeled retinals. The analyses of the vibrational bands in the fingerprint and hydrogen-out-of-plane (HOOP) regions indicated that the structure of the chromophores in the iodopsin system differs near their centers from those in the rhodopsin system. Compared to rhodopsin, the chromophore of the batho intermediate of iodopsin is twisted in the C(12) to C(14) regions but is more planar around C(11) region. The large amount of twisting was reduced by removing the chloride ion from the iodopsin, suggesting that this twisting hinders the relaxation of the torsion near C(11) necessary for the transition to the lumi intermediate and thus results in the thermal reversion of the batho intermediate back to the iodopsin. From the analyses of the C=NH and C=ND stretching bands, we conclude that the displacement of the Schiff base region upon photoisomerization of the chromophore is restricted, as is the case for rhodopsin. These results indicated that iodopsin's chromophore has a unique structure near its center and that this difference is enhanced by the binding of chloride nearby.

Spiropyran-Merocyanine Equilibrium in Presence of Organic Acids and Bases

ABSTRACT Manipulation of the rate of thermal reversion of N-functionalized merocyanines (MC) to the corresponding spiropyran isomers in the dark, was demonstrated by using both organic acid and base additives. In the presence of organic bases, increase or decrease of the rate of reaction was dependent on the solvent used. When organic acids were used, in all cases the reaction rate decreased when compared with the system containing no acid. Similar behavior was also observed in polymer films. This approach is of interest for applications in the design of optical switches and memory with enhanced stability of the stored information.

Photorearrangement of α-Azoxy Ketones and Triplet Sensitization of Azoxy Compounds

[reaction: see text] Although some aspects of azoxy group radical chemistry have been investigated, unhindered alpha-azoxy radicals remain poorly understood. Here we report the generation of alpha-azoxy radicals under mild conditions by irradiation of alpha-azoxy ketones 4a,b. These compounds undergo alpha-cleavage to yield radicals 5a,b, whose oxygen atom then recombines with benzoyl radicals to produce presumed intermediate 15. Formal Claisen rearrangement gives alpha-benzoyloxyazo compounds 8a,b, which are themselves photolabile, leading to both radical and ionic decomposition. The ESR spectrum of 5a was simulated to extract the isotropic hyperfine splitting constants, which showed its resonance stabilization energy to be exceptionally large. Azoxy compounds have been found for the first time to be good quenchers of triplet excited acetophenone, the main sensitized photoreaction of 7Z in benzene being deoxygenation. While this reaction has been reported previously, it was always in hydrogen atom donating solvents, where chemical sensitization occurred. The principal direct irradiation product of 4bZ and model azoxyalkane 7Z is the E isomer, whose thermal reversion to Z is much faster than that of previously studied analogues.

New benzopyranocarbazoles: synthesis and photochromic behaviour

The synthesis of three new benzopyranocarbazoles (=[indole]naphthopyrans) from hydroxybenzo[ a]carbazoles is described. The photochromic properties of these novel compounds were investigated under flash photolysis and continuous irradiation. Compared to known [indole]benzopyrans these new compounds showed a significant bathochromic shift in the spectra of the open forms, an increase in colourabilities and slower ring closure kinetics. The photochromic behaviour of compound 4 has been further investigated. Continuous near-UV irradiation led to the formation of one photoisomer (TC) that was subsequently partially converted, to the other (TT). Thermal reversion of the preirradiated system to the original form was only partial and followed a monoexponential decay involving the back-conversion of the TC-isomer to the uncoloured closed form (CF). The thermally stable TT-isomer could only be photobleached with visible light. This process was shown to proceed through a fast photoconversion TT→TC followed by the thermal path TC→CF. Thermal relaxation of the activated system was also studied at various temperatures.

The control of the cholesteric pitch by some azo photochemical chiral switches.

A few chiral azo compounds, which undergo reversible photochemical switching, are presented. Of these, the most interesting contain the binaphthyl moiety and belong to the C2 (derivatives 1 and 2) or C1 symmetry group (derivatives 3 and 4). These binaphthyl compounds display intense CD and high beta values. Photochemical switching has profound effects on both the CD and beta values of these compounds; in the case of compound 3, the sign of beta changes upon isomerisation. Compound 2 has, to our knowledge, the highest beta of the switches reported in the literature and also seems the most interesting owing to its fast response to photochemical stimuli. Nematic phases can be transformed into cholesteric phases with reflection bands in the visible region by doping with reasonable amounts of 1 and 2. The reflection colours can be changed reversibly by photoisomerisation of the switches. Thermal reversion of the colourless UV photostationary state to the green isomeric EE state or to intermediate coloured states is temperature dependent. This can allow the thermal history of a sample to be traced.

Solvation effects on spirooxazine to merocyanine thermal reversion kinetics in acetonitrile–water binary mixtures

AbstractIn solution, the photochromic molecule 5‐methoxyspiro[indole‐phenanthrolinoxazine] (SPO) is in equilibrium with its corresponding merocyanine open isomer. In pure acetonitrile, there is about 22.5% of the open form, whereas in acetonitrile–water binary solvents this proportion increases to more than 60% when the water molar fraction χ(H2O) is around 0.80. Irradiation in the visible range leads to a photobleached steady state in which the proportion of the merocyanine open isomer decreases significantly. When irradiation is switched off, a slow thermal reversion of the closed spirooxazine to the open merocyanine takes place until the initial equilibrium position is reached. Kinetic analysis of a whole set of relaxation curves recorded at various water contents was used to establish a model involving multiple equilibria between three merocyanines in different solvation states (solvated by acetonitrile, solvated by an acetonitrile–water complex and hydrated). Their relative proportions, which are dependent on the binary mixture composition, were determined, in addition to their corresponding visible spectra. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Improved Durability of Norbornadiene Derivatives Using Antioxidized Reagent

Abstract It was found that improvement in durability of NBDs against degradation during repeated cycles of photoisomerization of NBDs to QCs and thermal reversion of QCs to NBDs could be achieved by an antioxidized reagent. Hindered amine derivative was the most effective.