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.     

Spectral and luminescent properties of hydroxyazomethines of indoline spiropyrans

Spectral and luminescent properties of novel bifunctional compounds 1–3 based on indole spiropyrans and hydroxyazomethines have been studied in comparison with the properties of the model compounds 5’-substituted spiropyrans and azomethines in organic solvents and PMMA films at 293 and 77 K. Luminescence of compounds 1–3 is due to the presence of the azomethine fragment conjugated with the indoline ring of the spiropyran moiety and determining the long-wavelength absorption band of 1–3. Depending on the solvent, temperature, and the substituent, various combinations of the structures of the chromophore fragments are observed in 1–3: the imine or amine form of the azomethine fragment and the spirocyclic or merocyanine form of the spiropyran fragment.     

Exciplex Emission from a Boron Dipyrromethene (Bodipy) Dye Equipped with a Dicyanovinyl Appendage

The photophysical properties of a prototypic donor–acceptor dyad, featuring a conventional boron dipyrromethene (Bodipy) dye linked to a dicyanovinyl unit through a meso‐phenylene ring, have been recorded in weakly polar solvents. The absorption spectrum remains unperturbed relative to that of the parent Bodipy dye but the fluorescence is extensively quenched. At room temperature, the emission spectrum comprises roughly equal contributions from the regular π, π* excited‐singlet state and from an exciplex formed by partial charge transfer from Bodipy to the dicyanovinyl residue. This mixture moves progressively in favor of the locally excited π, π* state on cooling and the exciplex is no longer seen in frozen media; the overall emission quantum yield changes dramatically near the freezing point of the solvent. The exciplex, which has a lifetime of approximately 1 ns at room temperature, can also be seen by transient absorption spectroscopy, in which it decays to form the locally excited triplet state. Under applied pressure (P<170 MPa), formation of the exciplex is somewhat hindered by restricted rotation around the semirigid linkage and again the emission profile shifts in favor of the π, π* excited state. At higher pressure (170<P<550 MPa), the molecule undergoes reversible distortion that has a small effect on the yield of π, π* emission but severely quenches exciplex fluorescence. In the limiting case, this high‐pressure effect decreases the molar volume of the solute by approximately 25 cm³ and opens a new channel for nonradiative deactivation of the excited‐state manifold.     

A solvent isotope effect upon exciplex photochemistry

For several naphthylalkylamines, change of solvent from methanol to deuterated methanol (CH₃OD) leads to an increase in quantum yield of intramolecular exciplex fluorescence, an increase in exciplex fluorescence lifetime and an increase in naphthalene triplet yield. The solvent effect is explained by the relative abilities of the two solvents to form hydrogen (deuterium) bonds and to protonate (deuterate).     

Solvent switching of intramolecular energy transfer in bichromophoric systems: photophysics of (2,2'-bipyridine)tetracyanoruthenate(II)/pyrenyl complexes

The supramolecular systems [Ru(Pyr(n)bpy)(CN)(4)](2-) (n = 1, 2), where one and two pyrenyl units are linked via two-methylene bridges to the [Ru(bpy)(CN)(4)](2-) chromophore, have been synthesized. The photophysical properties of these systems, which contain a highly solvatochromic metal complex moiety, have been investigated in water, methanol, and acetonitrile. In all solvents, prompt and efficient singlet-singlet energy transfer takes places from the pyrene to the inorganic moiety. Energy transfer at the triplet level, on the other hand, is dramatically solvent dependent. In water, the metal-to-ligand charge transfer (MLCT) emission of the Ru-based chromophore is completely quenched, and rapid (200 ps for n = 1) irreversible triplet energy transfer to the pyrene units is detected in ultrafast spectroscopy. In acetonitrile, the MLCT emission is practically unaffected by the presence of the pyrenyl chromophore, implying the absence of any intercomponent triplet energy transfer. In methanol, triplet energy transfer leads to an equilibrium between the excited chromophores, with considerable elongation of the MLCT lifetime. The investigation of the [Ru(Pyr(n)bpy)(CN)(4)](2-) systems in methanol provided a very detailed and self-consistent picture: (i) The initially formed MLCT state relaxes toward equilibrium in 0.5-1.3 ns (n = 1, 2), as monitored both by ultrafast transient absorption and by time-correlated single photon counting. (ii) The two excited chromophores decay with a common lifetime of 260-450 ns (n = 1, 2), as determined from the decay of MLCT emission (slow component) and of the pyrene triplet absorption. (iii) These equilibrium lifetimes are fully consistent with the excited-state partition of 12-6% MLCT (n = 1-2), independently measured from preexponential factors of the emission decay. Altogether, the results demonstrate how site-specific solvent effects can be used to control the direction of intercomponent energy flow in bichromophoric systems.     

PHOTOPHYSICS STUDIES OF ORGANIC COMPOUNDS——NANOSECOND LASER PHOTOLYSIS AND TRANSIENT ABSORPTION SPECTRA STUDIES OF PYRENE-AROMATIC AMINES

Nanosecond time-resolved transient absorption spectra of pyrene-triphenylamine, pyrenediphenylamine, pyrene-N, N-dibenzylaniline systems in various solvents have been investigated. In nonpolar solvent, pyrene-triphenylamine exciplex and pyrene-N, N-dibenzylaniline exciplex were observed directly. In acetonitrile, the pyrene anion radical, triphenylamine and diphenylamine cation radicals were detected. On the basis of the present experimental facts, the mechanism of fluorescence quenching processes of pyrene in polar and nonpolar solvents has been established.     

一些新的芘衍生物的合成及光物理研究

本文合成了一系列新的芘衍生物，用紫外光谱和稳态荧光光谱方法研究了它们的光物理性质。结果表明，取代烷基几乎不影响芘的吸收光谱，而使芘的荧光光谱振动精细结构消失或部分消失；取代甲酰基的芘衍生物的荧光发射强烈依赖于溶剂的性质，极性溶剂中荧光光谱无精细结构，且荧光发射峰随介质极性的增加而红移。     

Electric field effects on photoinduced electron transfer processes of methylene-linked compounds of pyrene and N,N-dimethylaniline in a polymer film

Time-resolved measurements of the electric-field-induced change in fluorescence intensity have been made for methylene-linked compounds of pyrene and N,N-dimethylaniline (DMA) doped in a polymer film. The lifetime of the fluorescence emitted from the locally excited state of pyrene chromophore becomes shorter in the presence of electric field (F), when the dopant concentration is high. The lifetime of the excipelx fluorescence resulting from the photoinduced electron transfer (PIET) from DMA to the excited state of pyrene chromophore between different molecules also becomes shorter in the presence of F. Based on the simulation of the electric field effect on fluorescence decay, the mechanism of intermolecular PIET between DMA and pyrene chromophore in a polymer film is discussed.     

Exciplex Formation and Excited State Deactivation of Difluoroborondipyrromethene (Bodipy) Dyads

Two series of geometrically‐related dyads are discussed based on the difluoroborondipyrromethene (Bodipy) unit, and incorporating covalently attached hydroquinone/quinone groups. These units are anchored directly, or via a phenylene spacer, to the Bodipy core at the meso position in one series ( BD‐MHQ , BD‐MQ , BD‐MPHQ , BD‐MPQ ), but for the second series the attachment site is the 2‐position ( BD‐SHQ , BD‐SQ , BD‐SPHQ , BD‐SPQ ). The compounds show various levels of fluorescence depending on the oxidation state of the appended group and the substitution pattern. In non‐polar solvents such as toluene, diethyl ether and dichlorobenzene, the S₁ state deactivation of the Bodipy unit in BD‐SPQ and BD‐MPQ is dominated by ^{1, 3}exciplex formation, which has not been reported for Bodipy derivatives so far. In the latter molecule, the decay of the exciplex is divided between population of the Bodipy triplet state (13 %–21 %) and ground state reformation. This partitioning is not seen for the side‐on substituted derivative, BD‐SPQ , and only ground state reformation is observed following decay of the exciplex. This difference in behavior is explained by the radical‐pair inter‐system‐crossing mechanism, which more effectively operates in BD‐MPQ because of the orthogonality of the donor‐acceptor units. In the more polar solvent CH₃CN all the quinone derivatives show fast formation of the charge‐separated state (k_CS) followed by slower charge recombination (k_CR). The ratio k_CS/k_CR≤80.     

Proton transfer reaction of a new orthohydroxy Schiff base at room temperature and 77 K

One new orthohydroxy Schiff base, 2-(N-benzyl-alpha-iminoethyl)naphthol (BEIN) has been synthesized. The proton transfer reaction of BEIN has been investigated by means of absorption, steady state and time resolved fluorescence spectroscopy in different solvents at room temperature and 77K. The behavior of BEIN in ethanol and water, has been studied in neutral, acidic and basic conditions. Excited state intramolecular proton transfer (ESIPT) is evidenced by a large Stokes shifted ( approximately 11,000 cm-1) fluorescence in solid crystalline media at room temperature. We present the observation of phosphorescence both in non-polar and protic solvents at 77K. The observed decay dynamics of the phosphorescence and delayed fluorescence indicates that the triplet state can be attributed to the cis-keto form. The molecular structures are determined by B3LYP/6-31G** calculation. From theoretical study it is suggested that the strengthening of hydrogen bond result from the steric repulsion of the phenyl ring. The presence of benzene ring increases the proton transfer barrier in case of BEIN compared to previously studied 7-ethylsalicylidenebenzylamine (ESBA).     

Induction of Columnar and Smectic Phases for Spiropyran Derivatives: Effects of Acidichromism and Photochromism

Liquid‐crystalline (LC) properties have been induced in a number of spiropyran derivatives by the addition of methanesulfonic acid. Spiropyran derivatives containing one or two gallic acid moieties with one, two, or three long alkyl chains were prepared. Acid‐induced spiro–protonated‐merocyanine isomerization induced mesomorphism for these materials. Equimolar mixtures of methanesulfonic acid and the spiropyran derivatives with one or two dodecyloxy chains exhibited smectic A phases, whereas the spiropyran derivatives containing the gallic acid moiety with three dodecyloxy chains showed hexagonal columnar phases. On the contrary, photoirradiation of the spiropyran compounds in the bulk liquid state did not lead to the induction of mesomorphism, although the merocyanine form was induced. These results suggest that these merocyanine derivatives with ionic and nonionic moieties cannot simply form nanosegregated LC structures. Complex formation of the merocyanine form with methanesulfonic acid may play a key role in the formation of LC molecular assemblies.     

Effect of solvent on the excited-state photophysical properties of curcumin

Photophysical properties of curcumin, 1,7-bis-(4-hydroxy-3-methoxy phenyl)-1,6-heptadiene-2,5-dione, a pigment found in the rhizomes of Curcuma longa (turmeric) have been studied in different kinds of organic solvent and also in Triton X-100 aqueous micellar media using time-resolved fluorescence and transient absorption techniques having pico and nanosecond time resolution, in addition to steady-state absorption and fluorescence spectroscopic techniques. Steady-state absorption and fluorescence characteristics of curcumin have been found to be sensitive to the solvent characteristics. Large change (delta mu = 6.1 Debye) in dipole moments due to photoexcitation to the excited singlet state (S1) indicates strong intramolecular charge transfer character of the latter. Curcumin is a weakly fluorescent molecule and the fluorescence decay properties in most of the solvents could be fitted well to a double-exponential decay function. The shorter component having lifetime in the range 50-350 ps and percent contribution of amplitude more than 90% in different solvents may be assigned to the enol form, whereas the longer component, having lifetime in the range 500-1180 ps with less than 10% contribution may be assigned to the di-keto form of curcumin. Our nuclear magnetic resonance study in CDCl3 and dimethyl sulfoxide-D6 also supports the fact that the enol form is present in the solution by more than about 95% in these solvents. Excited singlet (S1) and triplet (T1) absorption spectrum and decay kinetics have been characterized by pico and nanosecond laser flash photolysis. Quantum yield of the triplet is low (phi T < or = 0.12). Both the fluorescence and triplet quantum yields being low (phi f + phi T < 0.18), the photophysics of curcumin is dominated by the energy relaxation mechanism via the internal conversion process.     

Lifetimes of partial charge transfer exciplexes of 9-cyanophenanthrene and 9-cyanoanthracene

The fluorescence decays of several exciplexes with partial charge transfer have been investigated in solvents of various polarity. The measured lifetimes are found to be in reasonable agreement with the activation enthalpy and entropy of exciplex decay obtained earlier from the temperature dependence of the exciplex emission quantum yields. For exciplexes with 9-cyanophenanthrene substantial contribution of the higher local excited state into the exciplex electronic structure is found and borrowed intensity effect enhances the exciplex emission rate constants.     

A multiswitchable poly(terthiophene) bearing a spiropyran functionality: understanding photo- and electrochemical control

An electroactive nitrospiropyran-substituted polyterthiophene, poly(2-(3,3'-dimethylindoline-6'-nitrobenzospiropyranyl)ethyl 4,4'-didecyloxy-2,2':5',2'-terthiophene-3'-acetate), has been synthesized for the first time. The spiropyran, incorporated into the polymer backbone by covalent attachment to the alkoxyterthiophene monomer units, leads to multiple colored states as a result of both photochemical and electrochemical isomerization of the spiropyran moiety to merocyanine forms as well as electrochemical oxidation of the polyterthiophene backbone and the merocyanine substituents. While electrochemical polymerization of the terthiophene monomer can take place without oxidation of the spiropyran, increasing the oxidation potential leads to complex electrochemistry that clearly involves this substituent. To understand this complex behavior, the first detailed electrochemical study of the oxidation of the precursor spiropyran, 1-(2-hydroxyethyl)-3,3-dimethylindoline-6'-nitrobenzospiropyran, was undertaken, showing that, in solution, an irreversible electrochemical oxidation of the spiropyran occurs leading to reversible redox behavior of at least two merocyanine isomers. With these insights, an extensive electrochemical and spectroelectrochemical study of the nitrospiropyran-substituted polyterthiophene films reveals an initial irreversible electrochemical oxidative ring-opening of the spiropyran to oxidized merocyanine. Subsequent reduction and cyclic voltammetry of the resulting nitromerocyanine-substituted polyterthiophene film gives rise to the formation of both merocyanine π-dimers or oligomers and π-radical cation dimers, between polymer chains. Although merocyanine formation is not electrochemically reversible, the spiropyran can be photochemically regenerated, through irradiation with visible light. Subsequent electrochemical oxidation of the nitrospiropyran-substituted polymer reduces the efficiency of the spiropyran to merocyanine isomerization, providing electrochemical control over the polymer properties. SEM and AFM images support the conclusion that the bulky spiropyran substituent is electrochemically isomerized to the planar merocyanine moiety, affording a smoother polymer film. The conductivity of the freestanding polymer film was found to be 0.4 S cm(-1).     

In silico optimization of merocyanine-spiropyran compounds as second-order nonlinear optical molecular switches

Time-dependent Hartree-Fock and M?ller-Plesset second-order calculations have been used to unravel the relationships between structure and first hyperpolarizability in spiropyran/merocyanine couples and therefore to design efficient second-order nonlinear optical switching compounds. Large first hyperpolarizabilities for the merocyanine form as well as large contrasts of first hyperpolarizability have been obtained when, on the same species, (i) substituents at R(1) and R(2) positions on the phenolate ring of the merocyanine form are strong acceptor and donor substituents, respectively, (ii) the ethylenic bridge is substituted by donor groups, (iii) the other aromatic part of the system is benzimidazolo rather than indolino or benzothiazolo, and (iv) strong donor substituents are placed on the benzimidazolo moiety.     

Donor-acceptor complexes of excited states on silica surface and in hexane solution at liquid nitrogen temperature

It is shown that an exciplex between pyrene (Py) and N,N-dimethylaniline (DMA) forms at low temperature (77 K) on the nonporous silica surface and in a solid hexane solution. The fluorescence lifetime of pyrene in the adsorption state increases when the quencher concentration grows at 77 K. The exciplex fluorescence spectra in the adsorption state shifts to a blue wavelength range when the temperature decreases as a result of the dipole-dipole interaction of the exciplex with the hydroxy groups of the silica surface. On the basis of the model suggested the hydroxyl group concentration on the dehydrated at 770 K silica surface has been estimated.     

Control by Ring Substitution of the Conformation Change Dynamics in Photochromic Polypeptides

Abstract— In order to develop new systems that couple photochromism with molecular conformation change, a series of spiropyrans having different ring substituents were attached to poly(L-glutamic acid). The polypeptides were dissolved in hexafluoroisopropanol and dark adapted so that the dye was in the merocyanine form. Following adaptation by white light and dye photoconversion from the merocyanine to spiropyran forms, polypeptide dark-adaptation kinetics were monitored by circular dichroism (CD) and dye dark adaptation was monitored by UV/ visible. Light adaptation caused a light-induced coil-to-helix transition, with dark adaptation resulting in relaxation back to the coil. The dark-adaptation rate constant measured by UV/visible was equal to that measured by CD, demonstrating close coupling between dye state and polypeptide conformation. By varying the substituents on the spiropyran ring, dark-adaptation half lives were varied from less than a minute to 2 h, representing nearly three orders of magnitude.     

Time resolved spectra of pyrene excimer and pyrene-dimethylaniline exciplex

Time-resolved fluorescence spectra were measured for pyrene and for the pyrene-N,N-dimethylaniline system in the range from 3 nsec to 150 nsec after the excitation by a nitrogen gas laser. By the aid of the spectra, we could follow the formation and decay processes of the pyrene excimer or the pyrene-N,N-dimethlaniline exciplex. The rate constants were determined for the exciplex formation and decay processes.     

RESEARCH NOTE ON THE CHARGE-TRANSFER PATHWAY IN THE MEROCYANINE 540 TRIPLET STATE QUENCHING BY NITROXYL RADICAL

Abstract— Quenching of merocyanine 540 triplet state by nitroxyl radical has been investigated by flash photolysis. An increase of solvent polarity leads to an increase of the quenching rate. This polarity effect shows that merocyanine 540 triplet state is able to react with nitroxyl radical via a charge-transfer process. The data presented in this work are shown to be consistent with previous results obtained in micelles and liposomes.     

Photo-, thermally, and pH-responsive microgels

Microgels with photo-, thermally, and pH-responsive properties in aqueous suspension have been synthesized and characterized using dynamic light scattering and UV-visible spectroscopy. The new route involved first preparing poly(N-isopropylacrylamide) (PNIPAM)-allylamine copolymer microgels and a spiropyran photochrome (SP) bearing a carboxylic acid group. Then the functionalized spiropyran was coupled to the microgel via an amide bond. The dark-equilibrated gel particles feature spiropyran molecules in the polar, merocyanine form. After irradiation of visible light, the particle size becomes smaller because spiropyran changes to the relatively nonpolar, closed spiro form. The PNIPAM-SP microgels undergo a volume phase transition in water from a swollen state to a collapsed state with increasing temperature under all light conditions. However, the transition temperature range of the PNIPAM-SP is much broader than that for the PNIPAM without SP. The PNIPAM-SP microgels are monodisperse and self-assemble into a crystalline lattice while in suspension. The UV-visible spectra of an aqueous suspension of PNIPAM-SP microgel in the dark-adapted, merocyanine form showed both an absorption peak around 512 nm due to the merocyanine (giving a reddish color to the suspension) and two sharp peaks from Bragg diffraction of colloidal crystallites. Upon visible irradiation, the 512-nm band bleached significantly due to spiropyran photoisomerization. The spiropyran photoisomerization and accompanying color changes of the suspension were reversible upon alternating dark, UV, and visible light irradiation. Due to the residues of amine groups, the swelling capability of PNIPAM-SP microgels reduces as the pH value is changed from 7 to 10.