Open Merocyanine Research Articles

Light‐Triggered Self‐Assembly of a Spiropyran‐Functionalized Dendron into Nano‐/Micrometer‐Sized Particles and Photoresponsive Organogel with Switchable Fluorescence

AbstractThe synthesis, self‐assembly, and spectroscopic investigations of spiropyran (SP)‐functionalized dendron 1 are reported. Under UV light irradiation, assembly of 1 into nano‐/microparticles occurs due to the transformation of the closed form of SP into the open merocyanine (MC) form. The formation of these nano‐/microparticles is confirmed by transmission electron microscopy (TEM) and dynamic light scattering (DLS) experiments in addition to the confocal laser scanning microscopy (CLSM) measurements. These nano‐/microparticles exhibit relatively strong red emission. It is interesting to note that the direct cooling of the toluene/benzene solution of 1 to 0 °C leads to gel formation. Multivalent π–π interactions due to the dendron in 1 may be the driving‐force for the gelation. The UV light irradiation cannot destroy the gel phase, and in fact, the gel–gel transition is successfully realized. The purple‐blue gel exhibits relatively strong red fluorescence; moreover, the fluorescence can be reversibly switched by alternating UV and visible light irradiation. The results clearly indicate that the MC form after aggregation becomes more stable and fluorescent.

Photochromism of Spiropyran−Cyclodextrin Inclusion Complexes on Au(111)

This paper reports on the switching characteristics of a spiropyran−cyclodextrin inclusion complex on a Au(111) surface. The switching behavior in acetone/water and ethanol/water solution is compared with that of the complex as thin film (∼100 layers) on gold and with the complex in a microcrystalline form. For the inclusion complex, we find a significantly increased switching time from the closed spiro to the open merocyanine conformation as compared to a solution of pure spiropyran. The thin film has a similar switching time as the solution of the inclusion complex. For the microcrystalline form, on the other hand, the switching times are considerably shorter. The back reaction exhibits a switching time for the solution of the inclusion complex which is similar to the pure spiropyran solution whereas the microcrystalline sample switches faster and the thin film of the inclusion complex slower than all the other ones. Furthermore, reversible switching of an adsorbed layer of the complex on gold could be observed. The switching characteristics can be explained in a model considering steric and compressive strain.

Photophysical and Redox Properties of Dinuclear Ru and Os Polypyridyl Complexes with Incorporated Photostable Spiropyran Bridge

Aimed at creating a true photoswitchable energy transfer system, four dinuclear complexes containing ruthenium(II) and osmium(II) metal centers bridged by spiropyran-type linkers were designed and investigated. The bridge in its closed spiropyran form was shown to be a good insulator for energy transfer between the Ru-bpy donor and the Os-bpy acceptor (bpy = 2,2'-bipyridine). On the basis of properties of previously reported photochromic nitrospiropyrans substituted with a single polypyridine metal center, conversion of the bridge to the open merocyanine form was envisaged to result in efficient electronic energy transfer by a sequential ("hopping") mechanism. In contrast to the expectations, however, the studied closed-form dinuclear complexes remained stable independently of their photochemical or electrochemical activation. This difference in reactivity is attributed to the replacement of the nitro group by a second polypyridine metal center. We assume that these changes have fundamentally altered the excited-state and redox properties of the complexes, making the ring-opening pathways unavailable.

Synthesis, structure, and the photomagnetic effect in crystals of 1,3,3,7′-tetramethylspiro[indoline-2,2′-2H-pyrano[3,2-f]quinolinium] tris(oxalato)chromate(III)

Single crystals of 1,3,3,7′-tetramethylspiro[indoline-2,2′-2H-pyrano[3,2-f]quinolinium] tris(oxalato)chromate(III), in which the cation exists in the open merocyanine form, (Mc)3Cr(ox)3·11.56H2O (ox is C2O42−), were obtained for the first time. The molecular and crystal structure of the salt was established by X-ray diffraction. The crystal structure consists of two types of layers (cationic and anionic). In the cationic layer, the Mc+ cations are parallel to each other and form a close-packed arrangement. The anionic layer consists of oxalatochromate complexes and water molecules linked together by hydrogen bonds. The salt does not display photochromic properties typical of salts of cationic spiropyrans. However, UV irradiation (355 nm) causes a change in the average spin of the sample from 3/2 to 1 and a change in the Weiss constant from −7 to 7 K.

Laser-induced phase change in Langmuir films observed using nanosecond pump-probe Brewster angle microscopy

A technique is described to observe transient events in thin interfacial films and monolayers. p-polarized light has minimum reflectivity at the Brewster angle. When an interface is viewed with light that is both incident and reflected at the Brewster angle the resulting image is dark. However, small refractive index changes can increase the reflectivity producing a high-contrast image of an altered interface with a dark background level. Using this phenomenon, with imaging optics, photo-induced phase change in Langmuir films was monitored. Two synchronized 5-ns pulsed lasers were used in the pump–probe configuration to induce changes at an air–liquid interface and to monitor the resulting morphology changes at selected time delays after photo-excitation. The photo-responsive layers were made from photochromic spiropyrans, having long aliphatic chain substituents. When irradiated with UV light the closed form of the molecule converts to a more planar ring open merocyanine form. This results in the layers changing their morphology in order to accommodate the new molecular form.

Synthesis and Photochromic Properties of Poly(4-vinylpyridine-co-styrene) with Pendant Spiroxazine Moieties

An ionic polymer with pendant photochromic moieties was prepared by the reaction of poly(4-vinylpyridine-co-styrene) and 1,3,3-trimethyl-6'-bromohexyloxyspiro[2H]-indol-2,3'-[3H]-naphth[2,1-b][1,4]oxazine. The photochromic reaction in question is caused by the reversible heterolytic cleavage of the C(spiro)-O bond under UV irradiation, yielding the colored form that can return to the colorless form by ring closure under visible light irradiation or in dark. The visible range absorption of the corresponding polymer increased gradually by UV irradiation which is ascribed to the generation of the open merocyanine form from the closed spiro form. The ionic conductivity of polymer increased upon UV irradiation, which brought about the generation of zwitterions form, and subsequently decreased in dark, which brought about the generation of closed spiro form. Sufficient reversibility was found in this polymer and this response was completely synchronized with that in the absorbance changes.

Deuterated dicondensed indolinobenzospiropyran formed from the reaction of Fischer base-d2 and salicylaldehyde: Mechanism involving a carbinol intermediate

Abstract From the deuterium content ( x -value) of deuterated molecules obtained from the reaction of an excess of Fischer base- d 2 with salicylaldehydes, it is found that the formation of the dicondensed indolinobenzospiropyrans occurred via a carbinol intermediate, rather than an open merocyanine form of spiropyran. 1 H NMR behavior supported this proposed mechanism, indicating that a carbinol intermediate was simultaneously, and not consecutively, formed prior to the transformation to both spiropyrans and dicondensed indolinobenzospiropyran.

Synthesis, Photophysical, Photochemical, and Redox Properties of Nitrospiropyrans Substituted with Ru or Os Tris(bipyridine) Complexes

Photochromic nitrospiropyrans substituted with 2,2'-bipyridine (bpy), [Ru(bpy)3]2+, and [Os(bpy)3]2+ groups were synthesized, and their photophysical, photochemical, and redox properties investigated. Substitution of the spiropyran with the metal complex moiety results in strongly decreased efficiency of the ring-opening process as a result of energy transfer from the excited spiropyran to the metal center. The lowest excited triplet state of the spiropyran in its open merocyanine form is lower in energy than the excited triplet MLCT level of the [Ru(bpy)3]2+ moiety but higher in energy than for [Os(bpy)3]2+, resulting in energy transfer from the excited ruthenium center to the spiropyran but inversely in the osmium case. The open merocyanine form reduces and oxidizes electrochemically more easily than the closed nitrospiropyran. Like photoexcitation, electrochemical activation also causes opening of the spiropyran ring by first reducing the closed form and subsequently reoxidizing the corresponding radical anion in two well-resolved anodic steps. Interestingly, the substitution of the spiropyran with a Ru or Os metal center does not affect the efficiency of this electrochemically induced ring-opening process, different from the photochemical path.

Photo-and thermochromic properties of 1′,3′,3′-trimethyl-6-nitro-8-pyridiniomethylspiro[2H-[1]benzopyran-2,2′-indoline] chloride in the crystalline state

1′,3′,3′-Trimethyl-6-nitro-8-pyridiniomethylspiro[2H-[1]benzopyran-2,2′-indoline] chloride (1) was synthesized. Single crystals (crystal hydrates) of the open merocyanine form of compound 1 were obtained under the dark conditions, and their crystal structure was studied. On irradiation with visible light compound 1 crystallizes from propan-2-ol in the polycrystalline closed form. The spectral and photochemical properties of compound 1 in the polycrystalline state and the kinetic characteristics on thermal transformation of the closed form to the open form were studied.

Highly stable and fluorescent switching spirooxazines

Two novel photochromic spirooxazines, SO- NA1 and SO- NA2, containing a naphthalimide unit were synthesized. The imide group of naphthalimide unit is incorporated at the naphthoxazine fragment, thus giving strong electron-withdrawing effect favoring the long-lived merocyanine (MC) in the dark giving good colorability in solution. Remarkably, their open merocyanine (MC) forms exhibit significantly long lifetimes, almost three magnitudes longer than that of unsubstituted spironaphthoxazine ( 1). Moreover, the fluorescence of naphthalimide unit can be switched on and off by photoinduced conversion between the open and closed forms.

Synthesis and properties of ionic conjugated polymer with spiroxazine moiety

A noble conjugated ionic polymer with photochromic spiroxazine moiety, poly[2-ethynyl- N-hexyloxyspiroxazine pyridinium bromide] (PEHSPB) 6 was prepared by the activated polymerization of 2-ethynylpyridine 5 with 1,3,3-trimethyl-6′-bromohexyloxyspiro[2H]-indol-2,3′-[3H]-naphth[2,1- b][1,4]oxazine 4 without any additional initiator or catalyst. The polymerization reaction was performed in DMF solvent at the temperature of 120 °C. As the polymerization proceeded, the reaction solution became more viscous and black in colour. The polymer yield and inherent viscosity were 75% and 0.15 dL/g, respectively. The instrumental analysis data on the chemical structure of PEHSPB 6 indicated that the present PEHSPB 6 have a conjugated polymer backbone system with the spiroxazine-hexyl substituents. X-ray diffraction analyses of the polymers indicated that the present polymers are mostly amorphous. The photoluminescence peak is located at 515 nm corresponding to the photon energy of 2.41 eV. Upon UV irradiation, closed spiroxazine form is transformed into the open merocyanine form that reverts back to the colourless spiro form in dark.

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 & Sons, Ltd.

Ground- and First-Excited-Singlet-State Electric Dipole Moments of Some Photochromic Spirobenzopyrans in Their Spiropyran and Merocyanine Form

The dipole moments of four photochromic derivatives of indolinospirobenzopyran have been studied in their spiro and their open merocyanine form, in dioxane. Determinations have been done by electrooptical absorption measurements (EOAM). To prepare either a pure spiropyran or a pure merocyanine form solution, the experimental setup had to be modified by addition of a second light beam. In this way thermal equilibration between the spiropyran and the merocyanine form and switching from one form to the other induced by the measuring light beam could be avoided. The ground-state dipole moment of the spiro form amounts to (10−15) × 10-30 C m and increases strongly to about 60 × 10-30 C m with the excitation to the first excited singlet state. The ground-state dipole moment of the merocyanine form is much larger and amounts to (50−60) × 10-30 C m. In contrast to the spiro form it decreases to about 40 × 10-30 C m, when exciting to the first excited singlet state of the merocyanine form.

Photochromism and solvatochromism of push–pull or pull–push spiroindolinenaphthoxazines

The photochromic and solvatochromic behaviour of 17 variously substituted spiroindoline naphthoxazines has been investigated in cyclohexane, toluene, acetonitrile and methanol in fluid solution. Specific parameters such as the wavelengths and the molar absorption coefficients of the closed spiro- and open merocyanine forms, together with the photocoloration and photobleaching quantum yields, rate constants and activation energies of thermal fading have been determined under continuous monochromatic irradiation. It has been shown that most of these parameters are closely related to the electron distribution on the highly conjugated photomerocyanines and on their quinoidal or zwitterionic character. Depending on the acceptor or donor properties and on the position of their substituents, three classes of compounds have been detected.

Picosecond time-resolved spectroscopy of the photocolouration reaction of photochromic naphthoxazine-spiro-indolines

The photochemical formation of the open merocyanine forms of several naphthoxazine-spiro-indolines in different solvents have been studied using both picosecond transient absorption (PTA) and picosecond time-resolved resonance Raman (PTR3) methods. The PTA studies have established the presence of several metastable species in the photochemical formation of the coloured merocyanine form of these photochromic compounds. The primary photochemical step occurs on the sub-ps timescale and is followed by the formation of a cisoid intermediate over the next 6–30 ps. This cisoid species then isomerises to the equilibrated distribution of transoid isomers of the merocyanine form with a lifetime that is dependent upon both solvent viscosity and polarity as well as the nature of the substituents on the naphthalene part of the molecule. However, the rate of this cis → trans isomerisation is unaffected on changing the N-alkyl group on the indoline part of the molecule from a methyl to an isobutyl group. The PTR3 studies have demonstrated that, in butan-1-ol, there are at least three different transient species with characteristic vibrational spectra which evolve with different lifetimes to give the final isomeric distribution over the first few ns of the reaction. In cyclohexane it is likely that the formation of a single species is being probed, which is fully developed after the first 200 ps of the reaction. This single species has a spectrum which is the same as the equilibrated steady-state resonance Raman merocyanine spectrum. It is likely that the evolution in the PTR3 spectra obtained here, in butan-1-ol, results from an equilibration of initially formed transoid merocyanine isomers to give a more stable distribution in this polar hydrogen-bonded solvent. Such an equilibration appears to be unnecessary in a non-polar solvent such as cyclohexane and it is suggested that this is because the transoid isomer, initially formed, is already in its most stable form.

Comparison of Photochromic Behaviour Between Spiroxazines and Spiropyrans: Theoretical Calculations of Ground and Excited States

MOPAC/AM1 and CNDO/S calculations have been performed on a spiro[indoline-naphthoxazine], a spiro[indoline-naphthopyran] and their open merocyanine(s) form. The energies were fully optimized and the bond orders, charge distribution and energies of both ground and excited states evaluated. The theroretical absorption spectra were calculated and found in good agreement with the experimental ones.

Surface-Enhanced Resonance Raman Studies of Spiropyrans (Bips and Derivatives)

Abstract The merocyanine form of BIPS and three of its nitro derivatives were studied by surface—enhanced Raman spectroscopy. It was found that in case of the nitro derivates, the SERS spectra originated from the unprotonated open merocyanine forms, even if the solution was acidified. In case of 6-nitro BIPS, there exist two enantiomers of the closed spiro form with different properties. The photoinduced merocyanine form of the photoactive enantiomer is different from the thermal ring opening product of the photoinactive enantiomer.

Water Soluble Organic Photochromes: Sulphonated Spiro (Indolinonaphthoxazines)

Abstract Two photochromic disulphonated spiro (indolino-naphthoxazines) have been synthesized. Both compounds crystallize in the open merocyanine form. The molar extinction coefficients of the coloured forms have been calculated and the rate constant kb for the thermal bleaching measured. The presence of sulphonic groups lowers the kb value by two orders of magnitude with respect to that of the unsubstituted compound.

Modeling of Spiropyran: Calculation Of Absorption Spectra

AbstractSemiempirical computational chemistry techniques were used to investigate the absorption spectra of 1′-isopropyl-3′,3′-dimethyl-indoline-spiro-6-nitro-8-methoxy-benzopyran. The spectra of both the closed (spiropyran) and open (merocyanine) forms were calculated using the INDO/CI method. Spectroscopic calculations showed the effects, both electronic and steric, that substituents such as hydrogen, methoxy and thiophene have on the absorption spectrum. Knowledge of the absorption spectra as a function of the choice and placement of substituents and molecular geometry will potentially allow optical properties to be optimized for specific applications.

Photochromic spironaphthoxazines: A theoretical study

AbstractPhotochromic spironaphthoxazines and their open merocyanine forms were studied via semiempirical Hartree–Fock SCF quantum chemical calculations. The most stable of the merocyanine isomers were found to have a cis–trans–cis conformation and the zwitterionic form is, generally, more stable than is the quinoidal tautomer. The calculated activation energy for the thermal bleaching reaction is in good agreement with the experimental one.