Calculation driven synthesis of an excellent dihydropyrene negative photochrome and its photochemical properties

The photochromic properties of dihydropyrenes have been substantially improved by making use of density functional theory (DFT) activation barrier calculations, which suggested that the di-isobutenylcyclophanediene 15' should have a significant barrier to thermal isomerization to the dihydropyrene (DHP) 15, which itself should resist isomerization involving migration of the internal groups to the rearranged dihydropyrene 9 (X = -CH═C(Me)(2)). As a result of these calculations, the synthesis of the colorless cyclophanediene (CPD) 15' was undertaken and achieved from the dinitrile 28 in four steps in 37% overall yield %. The cyclophanediene 15' thermally isomerized to the dihydropyrene 15 at 100 °C with t(1/2) = 4.5 h, giving an extrapolated 20 °C t(1/2) of ～16 y, consistent with the DFT calculations. No evidence for [1,5]-sigmatropic rearrangement in to 9 (X = -CH═C(Me)(2)) was observed on heating to 130 °C. The ring-opening isomerization quantum yields (?(open)) for DHP 15 in to CPD 15' were determined in cyclohexane to be 0.12 ± 0.01, which is three times greater than for the benzoDHP 1. Friedel-Crafts naphthoylation of 15 gave 70% of purple 32, which in toluene showed the largest photochemical ring-opening isomerization quantum yields (?(open)) of 0.66 ± 0.02 for any known dihydropyrene, ～nine times greater than 1 in toluene. The thermal closing of 32' to 32, although faster than for 15', gave a useful extrapolated t(1/2) of ～2 y at 20 °C.     

Fast and stable photochromic oxazines

We have designed and synthesized two photochromic compounds incorporating fused indoline and benzooxazine fragments. Variable-temperature 1H NMR spectroscopy demonstrates that their central [1,3]oxazine ring opens thermally with free energy barriers ranging from 14 to 19 kcal mol(-1). The ring-opened species reverts rapidly to the original isomer and can only be detected after chemical trapping. Specifically, the nucleophilic attack of a hydroxide anion to the indolium cation of the ring-opened species prevents re-isomerization. Laser excitation of both compounds induces the opening of the [1,3]oxazine ring in less than 6 ns with quantum yields up to 0.1. The photoinduced ring opening generates a 4-nitrophenolate chromophore, which absorbs strongly at 440 nm. The photogenerated species reverts to the original form with a lifetime of 22 ns for both compounds. Thus, these transformations can be exploited to interconvert the two isomers of each species with nanosecond switching speeds. Furthermore, thousands of switching cycles can be repeated consecutively without any sign of degradation, even in the presence of molecular oxygen. These processes can be reproduced efficiently in poly(methyl methacrylate) matrixes. Under these conditions, the thermal re-isomerization occurs with biexponential kinetics in submillisecond time scales. In principle, the fast isomerization kinetics and excellent fatigue resistance of both compounds offer the opportunity to modulate rapidly and efficiently a variety of molecular and macroscopic properties. Thus, our molecular design can evolve into the realization of a new family of photochromic compounds and materials with promising photoresponsive character.     

Mechanistic insights into the photochromism of trans-10b,10c-dimethyl-10b,10c-dihydropyrene derivatives

A series of dimethyldihydropyrene derivatives was studied to elucidate the photochemical mechanism associated with the switching between the dimethyldihydropyrene (DHP, closed) and metacyclophanediene (CPD, open) forms of the molecule. Quantum yields of ring opening and closure, fluorescence quantum yields and lifetimes, as well as laser flash photolysis studies were performed to establish the effect of substituents on the switching efficiency. Ring opening of the DHPs occurs from the first singlet excited state. The low quantum yields for the ring opening reaction observed (< or =0.042) are a consequence of the low rate constant (< or =1.7 x 10(7) s(-1)) for this process. The quantum yields for ring closure of the CPD were determined for select compounds and were of the order of 0.1-0.4. These results show that the efficiency for ring opening of this class of compounds is intrinsically low, but can be modulated to some extent by the introduction of substituents. These properties should be taken into account when considering what type of photoswitching devices DHPs might be useful for.     

A fast and stable photochromic switch based on the opening and closing of an oxazine ring

[reaction: see text] We have designed a molecular switch based on the photoinduced opening and thermal closing of an oxazine ring. Ultraviolet excitation of this molecule induces the cleavage of a [C-O] bond to form a p-nitrophenolate chromophore in less than 10 ns with a quantum yield of ca. 0.1. The photogenerated isomer reverts thermally to the original oxazine within 50 ns. Our photochromic switch survives more than 3000 excitation cycles without decomposing, even in air-saturated solutions.     

Synthesis and photochromism of novel unsymmetrical diarylethenes with an azaindole unit

A new class of unsymmetrical photochromic diarylethenes with an azaindole moiety has been firstly synthesized. Their properties, including photochromism, crystal structure, as well as fluorescence, were investigated systematically. The azaindole was connected directly to the central cyclopentene ring as a heteroaryl moiety and available to participate in the photoisomerization reaction. Each of the diarylethenes exhibited favorable photochromism, good thermal stability, remarkable fatigue resistance, and notable fluorescence switches in both solution and solid media. The substituents at the para-position of the terminal benzene ring affected evidently their properties: the electron-donating methoxy could be effective to enhance the cyclization quantum yield, while the electron-withdrawing cyano could shift the absorption maximum to a longer wavelength in both hexane and solid film. The results revealed that the introduction of azaindole moieties and different substituents played an important role in the photoisomerization process of these diarylethenes.     

Conductance switching in diarylethenes bridging carbon nanotubes

The recently reported photoswitching of diarylethene derivative molecules bridging carbon nanotube (CNT) contacts is theoretically analyzed. The short lifetime of the lowest unoccupied molecular orbital (LUMO) indicates that neither the open nor closed form of the molecule can be photoexcited into a charge-neutral excited state for any appreciable length of time preventing photochromic ring opening. Analysis of the highest occupied molecular orbital (HOMO) and LUMO lifetimes also suggests that photoexcitation results in oxidation of the molecules. This either reduces the quantum yield of photochromic ring closing, or it gives rise to the possibility of oxidative ring closing. Analysis of the resistance values and energy levels indicates that the HOMO energy levels of the closed isomers relevant for transport must lie within a few k(B)T of the CNT Fermi level. For armchair contacts, the change in resistance with isomer or substituent group is the result of shifts in the energy level of the molecular HOMO. The coupling of the molecular HOMO to the CNT contacts is insensitive to the isomer type or substituent group. For zigzag CNTs, the conductance is dominated by surface states at the Fermi level on the cut ends of the CNTs so that the conductance is relatively insensitive to the isomer type, and the conductance switching ratio is low. Multiple bridging molecules can interact coherently, resulting in energy splitting, shifting, and interference that cause a nonlinear change in conductance with increasing numbers of molecules. Instead of a factor of 3 increase in conductance expected for three independent channels, a factor of 10(3) increase in conductance is obtained for three bridging molecules.     

P‐Type Photochromism of New Helical Naphthopyrans: Synthesis and Photochemical,Photophysical and Theoretical Study

Two novel helical naphthopyrans have been synthesised. The helical scaffold has the interesting effect of increasing the thermal stability of the transoid‐trans (TT) open isomer formed upon UV irradiation of the closed form (CF), which transforms these naphthopyrans from thermal to photochemical photochromes. The photochromic performance is excellent in both polar and apolar solvents and the conversion percentage from the CF to the TT form can be as high as 92.8 %. We propose a new method to determine the quantum yields of the photochemical processes that lead to transoid‐cis (TC) and TT isomers, and their molar absorption coefficients. The thermal stability of the TT and TC isomers has been studied in different solvents. The quantum yields of fluorescence before and after irradiation, along with the decay lifetimes, have also been measured. TD‐DFT calculations have been performed to determine the relative thermodynamic stability of the species involved in the photochromic mechanism and to rationalise their spectral properties.     

Mechanically controlled FRET to achieve high-contrast fluorescence switching

Organic luminescent materials with the ability to reversibly switch the luminescence when subjected to external stimuli have attracted considerable interest in recent years. However, luminescent materials with mechanochromic and photochromic dual-responsive properties are rarely reported. Hererin, we designed and synthesized a molecule P1 with dipeptide as a spacer to link rhodamine B and spiropyran moieties. P1 exhibited efficient photochromic properties both in solution and solid state. High-contrast independent fluorescence switch was also realized under the stimulus of external force. Moreover, two-step ring opening reaction and subsequent fluorescence resonance energy transfer process between the donor-acceptor pairs within one single molecule achieved successive color switch by mechanical control. Therefore, this behavior of P1 made it a promising candidate for high-contrast and sensitive optical recording and mechanical sensing system.     

Photochromic fluorescent indol-3-yl-substituted maleimides

New hetarylethenes, 3-(indol-3-yl)-4-thienyl(but-1-en-1-yl)-substituted pyrrole-2,5-diones containing coumarin or fluorene substituents on the pyrrole nitrogen atom, were synthesized by reactions of furan-2,5-diones with 9H-fluoren-2-amine and 6-amino-2H-chromen-2-one. Acyclic maleimide isomers showed fluorescence with quantum yields of 0.002 to 0.072. Their irradiation with UV light generates non-fluorescing cyclic isomer. The reverse ring opening occurs in the excited state.     

Synthesis and photochromic properties of molecules containing [e]-annelated dihydropyrenes. Two and three way pi-switches based on the dimethyldihydropyrene-metacyclophanediene valence isomerization

The syntheses of several new simple negative, a simple positive, and multiple negative photochromes containing the dihydropyrene-cyclophanediene photochromic system are described. The photo-openings of the negative photochromes, the [e]-annelated benzo (7), naphtho (9), anthro (11), furano (19), and triphenyleno (15) derivatives of the parent 2,7-di-tert-butyl-trans-10b,10c-dimethyl-dihydropyrene (5), as well as its 4,5-dibromo derivative (13), are described to give the corresponding cyclophanedienes, as well as their photoclosures and thermal closures back to the dihydropyrenes. These are compared to the results obtained for the positive photochrome dibenzo[e,l]dihydropyrene (21) and to the bis(dihydropyreno)chrysene (44) and the (dihydropyrenobenzo)(benzo)metacyclophanediene (47) photochromes, which have more than one photochromic switch present and thus have more than a simple "on-off" state. Thermodynamic data are obtained for the thermal closing reactions. The anthrodihydropyrene (12) has the fastest thermal closing (tau(1/2) = 20 min), while the furanodihydropyrene (19') has the slowest (tau(1/2) = 63 h) at 46 degrees C. An electrochemical readout of the state of the switch is demonstrated for the benzodihydropyrene (7).     

Molecular AND and INHIBIT gates based on control of porphyrin fluorescence by photochromes

A molecular triad consisting of a porphyrin (P) covalently linked to two photochromes-one from the dihydroindolizine family (DHI) and one from the dihydropyrene family (DHP)-has been synthesized and found to act as either a molecular AND logic gate or an INHIBIT gate, depending on the inputs and initial state of the photochromes. The basis of these functions is quenching of porphyrin fluorescence (output of the gates) by the photochromes. The spiro form of DHI does not quench porphyrin fluorescence, whereas its betaine isomer strongly quenches by photoinduced electron transfer. DHP also quenches porphyrin fluorescence, but its cyclophanediene isomer does not. The triad has been designed using suitable energetics and electronic interactions, so that although these quenching phenomena may be observed, independent isomerization of the attached photochromes still occurs. This makes it possible to switch porphyrin fluorescence on or off by isomerization of the photochromes using various combinations of inputs such as UV light, red light, and heat.     

Synthesis of fluorescent diarylethenes having a 2,4,5-triphenylimidazole chromophore

Diarylethenes 1a-4a, having a fluorescent 2,4,5-triphenylimidazole chromophore in the aryl group, were synthesized. Upon excitation of the triphenylimidazole chromophore with 366 nm, 1a-4a underwent photocyclization reactions, and the solutions containing 1a-4a changed color from colorless to red-purple or to blue. The colors disappeared by irradiation with visible (lambda > 480 nm) light. The fluorescence intensity of the solutions also reversibly changed with the photochromic reactions. The fluorescence quantum yields of 1a, 2a, 3a, and 4a were determined to be 4.6, 7.7, 9.1, and 8.4%, respectively. The fluorescence quantum yields decreased with the increase in photocyclization quantum yields.     

On the photochromism of spiro

The recently synthesized spiro[cyclohexadiene-dihydroacridines] consisting of perpendicularly arranged aroylcyclohexadiene and N -methyl-dihydroacridine moieties were found to have photochromic properties. The reversible photoisomerization from the spiro compound toward a colored merocyanine caused by C–C bond cleavage in the cyclohexadiene was studied by stationary and time-resolved measurements of their optical spectra. The course of the absorption under UV and visible irradiation, respectively, and HPLC analysis of the photoproducts result in the determination of excitation energy-dependent quantum yields for the merocyanine formation and, in reverse, the ring closure, as well as degradation. Whereas the thermal back reaction completely recovers the spiro compound ( k ∼ 6.8 × 10⁻⁴ s⁻¹, T = 22°C), degradation of the merocyanine under irradiation at 480 nm has a probability of about 6%. Picosecond-resolved measurements of the fluorescence and the transient absorption show that photoisomerization occurs via the first excited singlet state within 100 ps depending on the activation barrier.     

Novel Bisthienylethene Containing Ferrocenyl‐Substituted Naphthalimide: A Photo‐ and Redox Multi‐Addressable Molecular Switch

A new photochromic bisthienylethene system (BTE? NAFc) is reported in which the ferrocene unit (Fc) is incorporated into a naphthalimide chromophore as the central ethene bridging unit. The incorporated Fc unit in the photochromic system of BTE? NAFc has several effects on optical properties, such as fluorescence‐modulation through photoinduced electron transfer (PET), a decrease in the photochromic cyclization quantum yield, and a selective two‐step oxidation process. The ability to drive ring‐opening and ring‐closing reactions with a secondary redox‐modulation provides increased functionality to the photochromic system. Based on these meaningful photo‐ and redox‐modulation properties, five unprecedented multi‐addressable states (BTE? NAFc, BTE? NAFc⁺, c‐BTE? NAFc, c‐BTE? NAFc⁺, and BTE⁺? NAFc⁺) and gated photochromism are successfully obtained within the unimolecular BTE platform, thus providing deeper insight into photochromic systems as multifunctional outputs.     

A Multi‐Addressable Switch Based on the Dimethyldihydropyrene Photochrome with Remarkable Proton‐Triggered Photo‐opening Efficiency

A series of photochromic derivatives based on the trans‐10b,10c‐dimethyl‐10b,10c‐dihydropyrene (DHP, “closed form”) skeleton has been synthesized and their photoisomerization leading to the corresponding cyclophanediene (CPD, “open form”) isomers has been investigated by UV/Vis and ¹H NMR spectroscopies. Substitution of the DHP core with electron‐withdrawing pyridinium groups was found to have major effects on the photoisomerization efficiency, the most remarkable examples being to enhance the quantum yield of the opening reaction and to allow fast and quantitative conversions at much lower radiant energies. This effect was rationalized by theoretical calculations. We also show that the reverse reaction, that is, going from the open form to the closed form, can be electrochemically triggered by oxidation of the CPD unit and that the photo‐opening properties of pyridine‐substituted DHPs can be efficiently tuned by protonation, the system behaving as a multi‐addressable molecular switch. These multi‐addressable photochromes show promise for the development of responsive materials.     

Photoswitchable fluorescent dyads incorporating BODIPY and [1,3]oxazine components

We designed and synthesized three compounds incorporating a BODIPY fluorophore and an oxazine photochrome within the same molecular skeleton and differing in the nature of the linker bridging the two functional components. The [1,3]oxazine ring of the photochrome opens in less than 6 ns upon laser excitation in two of the three fluorophore-photochrome dyads. This process generates a 3H-indolium cation with a quantum yield of 0.02-0.05. The photogenerated isomer has a lifetime of 1-3 μs and reverts to the original species with first-order kinetics. Both photochromic systems tolerate hundreds of switching cycles with no sign of degradation. The visible excitation of the dyads is accompanied by the characteristic fluorescence of the BODIPY component. However, the cationic fragment of their photogenerated isomers can accept an electron or energy from the excited fluorophore. As a result, the photoinduced transformation of the photochromic component within each dyad results in the effective quenching of the BODIPY emission. Indeed, the fluorescence of these photoswitchable compounds can be modulated on a microsecond time scale with excellent fatigue resistance under optical control. Thus, our operating principles and choice of functional components can ultimately lead to the development of valuable photoswitchable fluorescent probes for the super-resolution imaging of biological samples.     

A Combination of Selective Light Reflection and Fluorescence Modulation in a Cholesteric Polymer Matrix

Summary: The phase behavior and optical properties of a cholesteric ternary copolymer, containing nematogenic phenylbenzoate, cholesteric, and photochromic diarylethene side groups, and its mixture with 2 wt.‐% fluorescent dopant were studied. The investigation of the kinetics of a photochemical opening‐cycle process of the photochromic groups in the cholesteric mixture proved the energy transfer from the fluorescent dopant to the photochromic diarylethene groups. It was shown that the fluorescence intensity of the fluorescent dopant could be controlled by the portion of the “closed” form of the diarylethene groups. During the photocyclization of the photochromic groups a “degeneration” of the selective light reflection of the cholesteric matrix is observed.

Fluorescence‐resonance energy transfer makes possible the process of photosensitization of the back ring‐opening photoreaction of the photochromic diarylethene groups in the cholesteric polymer matrix.     

Photochromism of new diarylethenes bearing both thiazole and benzene moieties

A new class of photochromic diarylethenes bearing both thiazole and benzene moieties has been developed, and the effects of substitution on their properties, including photochromism, fatigue resistance, and fluorescence properties have been investigated. They exhibited good photochromism and functioned as a fluorescence switch upon alternating irradiation with UV and visible light both in solution and in PMMA film. The electron-donating substituents could significantly enhance the cyclization quantum yield and depress the cycloreversion quantum yield whereas the electron-withdrawing groups functionalized an inverse action for these diarylethene derivatives. Relatively big differences exist among the properties of these diarylethenes which may be attributed to the different substituent effects.     

New photochromic diarylethenes with a pyrimidine moiety

A novel class of photochromic diarylethene derivatives based on the hybrid skeleton of six-membered pyrimidine and five-membered thiophene moieties has been firstly synthesized. The substituent effects on their properties, including photochromism, fatigue resistance, and fluorescence, have been systematically investigated. All these diarylethenes showed significant photochromism and notable fluorescence switching properties in both solution and poly(methylmethacrylate) films. The electron-donating substituent enhanced their cyclization quantum yield, fatigue resistance, and fluorescence quantum yield, whereas electron-withdrawing group exhibited contrary effects.     

Ring‐Shaped Silafluorene Derivatives as Efficient Solid‐State UV‐Fluorophores: Synthesis,Characterization, and Photoluminescent Properties

Four ring‐shaped silafluorene‐containing compounds ( 1 – 4 ) were synthesized and characterized as potentially promising monomers for fluorescent polymers. Their optical properties in solution and solid state (thin film and powder) were studied. These compounds have low quantum yields in solution (Φ_fl=0.13‐0.15) with fluorescence maxima at about 355 nm, but high quantum yields in the solid state (powder, Φ_fl=0.35‐0.54) with fluorescence maxima at about 377 and 488 nm. Influence of the substituents and the number of silafluorene units in 1 – 4 on their optical properties was investigated. Extensive study of the X‐ray crystal structures of 1 – 4 was undertaken to analyze and qualitatively estimate the role, extent, and influence of silafluorene moieties’ interactions on solid‐state fluorescent properties. Excited state UV/Vis and theoretical molecular orbital (MO) calculations were performed to explore possible fluorescence mechanisms and differences in quantum yields among these compounds.