Ultrafast bidirectional photoswitching of a spiropyran

We report on bidirectional photochemical switching of 6,8-dinitro-1',3',3'-trimethylspiro[2H-1-benzopyran-2,2'-indoline] (6,8-dinitro-BIPS) between the ring-closed spiropyran and the ring-open merocyanine form. This is studied by femtosecond three-color pump-repump-probe experiments. Both ring opening and ring closure are photoinduced. Completion of an entire cycle, consisting of opening and subsequent closure, can be achieved within 40 ps. A much shorter time (<6 ps) is needed for the converse cycle, consisting of initial ring closure and subsequent ring opening. Furthermore, we perform pump-probe experiments with ultraviolet/visible pump and visible/mid-infrared probe pulses for an unambiguous spectroscopic identification of the open and closed molecular forms. Following visible excitation of the ring-open molecules, ultrafast ring closure is observed directly in the mid-infrared. The quantum efficiencies for ring opening and ring closure starting from the respective equilibirum states are determined to be approximately 9% and 40%. These results show that 6,8-dinitro-BIPS is an ultrafast bidirectional molecular switch exhibiting a high quantum efficiency.     

Oxidative electrochemical switching in dithienylcyclopentenes, part 1: effect of electronic perturbation on the efficiency and direction of molecular switching

The electro- and spectroelectrochemical properties of dithienylhexahydro- and dithienyhexafluorocyclopentenes are reported. The large effect of variation in the central cyclopentene moieties on the redox properties of the dithienylcyclopentenes is in striking contrast to the minor effect on their photochemical properties. The electronic properties of the oxidised compounds in the +1 and +2 oxidation state are reported, and the possibility of electrochemical cyclisation and cycloreversion were explored by UV/Vis spectroelectrochemistry. The efficiency of electrochemical switching is found to be dependent both on the central cyclopentene unit and on the nature of the substituents at C5 of the thienyl rings. For the hexahydrocyclopentene-based compounds oxidative ring closure of the ring-open form is observed, while for the hexafluorocyclopentene-based compounds oxidative ring opening of the ring-closed form is observed. However, the introduction of electroactive groups such as methoxyphenyl allows oxidative ring closure to occur in the hexafluoro compounds. The effect of electrolyte, solvent and temperature on the spectroelectrochemical properties were examined, and the switching process was found to be sensitive to the donor properties of the solvent/electrolyte system employed. In addition, thermally activated reversible isomerisation of the dicationic closed form was observed. The driving force for electrochemical ring opening and closure appears to be dependent on the relative stabilisation of the dicationic ring-open and ring-closed states. This study provides insight into the factors which determine the direction of cyclisation.     

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.     

Redox-assisted ring closing reaction of the photogenerated cyclophanediene form of bis(ferrocenyl)dimethyldihydropyrene with interferrocene electronic communication switching

We synthesized ferrocene-attached dimethyldihydropyrene (DHP) derivatives and investigated their photochemical and redox behaviors. For bis(ferrocenylethynyl)dimethyldihydropyrene (1), reversible photoisomerization between the closed DHP form (1c) and the open CPD form (1o) occurred in high yields upon alternate irradiation of visible (578 nm) light and UV (303 nm) light, whereas no photoisomerization proceeded for bis(pentamethylferrocenylethynyl)dimethyldihydropyrene (2). 1 exhibited reversible switching of electronic communication between the ferrocene (Fc) moieties by photoisomerization of the DHP moiety and demonstrated a novel ring closing reaction induced by oxidation of the Fc moieties. The magnitude of electronic communication, deltaE0' (the difference between the redox potentials of two Fc's), was 63 mV in 1c and 16 mV in 1o, indicating that the electronic communication through the spacer is enhanced in the more developed pi-conjugation of the DHP moiety. The rate constants of the ring closing reaction from 1o+ to 1c+ and from 1o2+ to 1c2+ were estimated at 3.7 and 0.50 s(-1), respectively, by the simulation of cyclic voltammograms.     

Conrotatory photochemical ring opening of alkylcyclobutenes in solution. A test of the hot ground-state mechanism.

Quantum yields for photochemical ring opening of six alkylcyclobutenes have been measured in hexane solution using 228-nm excitation, which selectively populates the lowest pi,R(3s) excited singlet states of these molecules and has been shown previously to lead to ring opening with clean conrotatory stereochemistry. The compounds studied in this work-1,2-dimethylcyclobutene (1), cis- and trans-1,2,3,4-tetramethylcyclobutene (cis- and trans-5), hexamethylcyclobutene (8), and cis- and trans-tricyclo[6.4.0.0(2,7)]dodec-1(2)-ene (cis- and trans-9)-were selected so as to span a broad range in molecular weight and as broad a range as possible in Arrhenius parameters for thermal (ground-state) ring opening. RRKM calculations have been carried out to provide estimates of the rate constants for ground-state ring opening of each of the compounds over a range of thermal energies from 20 000 to 49 000 cm(-1). These have been used to estimate upper limits for the quantum yields of ring opening via a hot ground-state mechanism, assuming a value of k(deact) = 10(11) s(-1) for the rate constant for collisional deactivation by the solvent, that internal conversion to the ground state from the lowest Rydberg state occurs with close to unit efficiency, and that ergodic behavior is followed. The calculated quantum yields are significantly lower than the experimental values in all cases but one (1). This suggests that the Rydberg-derived ring opening of alkylcyclobutenes is a true excited-state process and rules out the hot ground-state mechanism for the reaction.     

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.     

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.     

10-甲基吖啶苯酚酯衍生物的合成、表征及化学发光特性

吖啶-9-羧酸苯酚酯是一类效率较高的化学发光试剂.其结构一般有两部分组成,吖啶环发光部分和9-羧酸苯酚酯离去基团.本文对离去基团进行修饰,分别在离去基团苯酚部分的2,5或2,6位引入取代基CF₃、NO₂、Br、CH₃,合成了4个新吖啶酯衍生物,并对它们的分子结构进行了表征.4个新化合物和模型化合物(无取代基)比较,均表现出较好的化学发光效率;发光动力学基本符合文献报道规律.而且化学发光法对其水解稳定性考察显示,取代基的电性和位阻是影响稳定性的两大因素.     

Amino substituted bisketenes: generation, structure, and reactivity

Hitherto unknown diamino-substituted bisketenes with both free (14) and tethered (16) amino substituents have been generated by using laser flash photolysis for ring opening of the corresponding cyclobutenediones. The time-resolved kinetics of ring closure of the amino bisketenes back to the cyclobutenediones were measured by IR or UV spectroscopy, and give first-order rate constants which vary by a factor of 7.5x10(4), and the bis(Me2N) bisketene 14 is the most reactive in ring closure that has been reported. Rate constants for ring closure of these and previously observed bisketenes vary by a factor of 10(13). The dialkylamino bisketenes 16 (R=Me, n-Bu) with tethered substituents and restricted geometries are less reactive than the bis(Me2N) bisketene 14 by factors of 1700 and 540, respectively. Computational results obtained with DFT methods suggest angle strain in the tethered cyclobutenediones 15 inhibits facile cyclization of bisketenes 16.     

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.     

Benzimidazole Derivatives: Synthesis,Physical Properties,and n‐Type Semiconducting Properties

A series of new benzimidazole derivatives were synthesized by the solid‐state condensation and direct sublimation (SSC‐DS) method and their physical properties were investigated. The reaction yields and product stability were significantly affected by the identity of the diamine and anhydride substituents. On the other hand, the substituents of the benzimidazole ring allowed fine tuning of the emission maxima, fluorescence quantum yields, and redox potentials. The HOMO–LUMO levels were estimated by cyclic voltammetry in film on indium tin oxide (ITO) and compared with values obtained by other methods. The described benzimidazoles showed high crystallinity, which is attributed to a high planarity and interactions between carbon and heteroatoms. These compounds showed n‐type semiconducting behavior in organic field‐effect transistors (OFETs). Optimized devices for fluorinated NTCBI (naphthalene tetracarboxylic bisbenzimidazole) showed respectable electron mobilities of ～10^?2 cm² V^?1 s^?1.     

Synthesis of azafluorenones and related compounds using deprotocupration–aroylation followed by intramolecular direct arylation

The efficiency of the deprotocupration–aroylation of 2-chloropyridine using lithiocuprates prepared from CuX (X=Cl, Br) and LiTMP (TMP=2,2,6,6-tetramethylpiperidido, 2 equiv) was investigated. CuCl was identified as a more suitable copper source than CuBr for this purpose. Different diaryl ketones bearing a halogen at the 2 position of one of the aryl groups were synthesized in this way from azines and thiophenes. These were then involved in palladium-catalyzed ring closure: substrates underwent expected CH-activation-type arylation to afford fluorenone-type compounds, and were also subjected to cyclization reactions leading to xanthones, notably in the presence of oxygen-containing substituents or reagents.     

Spectral-luminescence properties and basicities of 2-(4-pyridyl)-5-(4-r-phenyl)oxazoles

The spectral-luminescence and acid-base properties of 2-(4-pyridyl)-5-(4-R-phenyl)oxazoles were studied. Significant -electron conjugation between the individual fragments exists in the molecules of these compounds in solutions at room temperature. Bathochromic and bathofluoric effects, which are particularly appreciable for electron-donor substituents, are observed when substituents R are introduced into the 5-phenyl ring. Ethanol solutions of the investigated compounds have high fluorescence quantum yields which, for some of them, are close to unity. Electron-donor substituents R increase the basicity of the pyridine nitrogen atom.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 479–482, April, 1985.     

反-1,2-双[2-(5-取代苯基恶唑基)]环丙烷的合成与光性能的研究

合成了8个反 -1,2-双[2-(5-取代苯基恶唑基)]环丙烷和2个1,2-双[2-取代苯基恶唑基)]乙烷,其中9个为新化合物.讨论了化合物的结构与其电子光谱及荧光量子产率间的关系.发现恶唑环与三元环间存在一定程度的共轭,并解释了上述化合物荧光量子产率较低的现象.     

An expedient synthesis of mellitic triimides

Heating of the solid ammonium salts obtained from treatment of mellitic acid with 3 equiv of a primary amine yields trisubstituted mellitic triimides via dehydration and imide ring closure. This surprisingly simple synthetic approach is amenable to incorporation of alkyl, aryl, and amino acid ester substituents, thereby opening broad access to a family of C(3)-symmetric organic electron acceptors.     

Cyclohexenylphenyldiazene: a simple surrogate of the azobenzene photochromic unit

We have carried out an experimental and computational study on the ground- and excited-state photochemical and photophysical properties of (1-cyclohexenyl)phenyldiazene (CPD), a species formally derived from azobenzene in which one of the phenyl rings is replaced by a 1-cyclohexene substituent. The results show that CPD does substantially behave like azobenzene, but with a higher (approximately 70%) Phi(Z-->E) (npi*) photoisomerization quantum yield, calling for CPD as an effective alternative of azobenzene itself with new functionalization possibilities. By use of state-of-the-art ab initio CASPT2//CASSCF minimum energy path computations, we have identified the most efficient decay and isomerization routes of the absorbing singlet (pipi*), S1 (npi*), T1, and S0 states of CPD. The resulting mechanistic scheme agrees with experimental findings and provides a rationale of the observed photoisomerization quantum yields. Furthermore, this study provides a deep insight on the photophysical and photochemical properties of compounds based on the -N=N- double bond which supplies a general model for the photoreactivity of azobenzene-type compounds in general. This is expected to be a useful guideline for the design of novel photoreactive azo compounds.     

Ring Reconstruction on a Trichalcogenasumanene Buckybowl: A Facile Approach to Donor–Acceptor‐Type [5‐6‐7] Fused Planar Polyheterocycles

The transformation of trichalcogenasumanene buckybowls into donor–acceptor‐type [5‐6‐7] fused polyheterocycles is disclosed. The strategy involves a highly efficient ring‐opening of the flanking benzene upon oxidation at room temperature, and facile ring closure by functional‐group transformation. Crystallographic studies indicate that the resulting [5‐6‐7] fused polyheterocycles possess a planar conformation owing to the release of ring strain by expansion of one of the six‐membered flanking rings to the seven‐membered one. Additionally, the [5‐6‐7] fused polyheterocycles bear electron‐withdrawing groups, which reduce the HOMO–LUMO energy gap, and display broad absorption bands extending to λ=590 nm. Consequently, these compounds show strong red emission with fluorescence quantum yields of up to 38 %.     

Toward a mechanistic understanding of the photochromism of dimethyldihydropyrenes

A mechanism accounting for the photochromic properties of dimethyldihydropyrene (DHP)/metacyclophanediene (CPD) is proposed based on high-level CASSCF/CASPT2 ab initio calculations. The very low ring-opening quantum yield observed upon irradiation of DHP is explained by the quenching of the photoisomerization channel by internal conversion of the initially excited zwitterionic state to the lowest excited state minimum, where the system can fluoresce or decay back to the ground state via another internal conversion. On the other hand, irradiation of CPD leads to the formation of a biradical excited state minimum along the ring-closure reaction path. Internal conversion back to the ground state can then occur at a well-characterized conical intersection, leading to the DHP photoproduct. Thus, our results point unambiguously toward a stepwise mechanism involving the formation of a biradical intermediate on a singlet excited state. The fact that this intermediate does not correspond to the lowest excited state minimum is mainly responsible for the inefficiency of this system.     

Porphyrin, phthalocyanine and porphyrazine derivatives with multifluorenyl substituents as efficient deep-red emitters

The synthesis and photophysical properties are described for a series of porphyrin, phthalocyanine and pyrazinoporphyrazine derivatives which bear four or eight peripheral fluorenyl substituents as antennae. Representative examples are 5,10,15,20-tetra(9,9-dihexyl-9H-fluoren-2-yl)porphyrin (2), 5,10,15,20-tetrakis[4-(9,9-dihexyl-9H-fluoren-2-yl)phenyl]porphyrin (3), 2,3,9,10,16,17,23,24-octakis(9,9-dihexyl-9H-fluoren-2-yl)-29H,31H-phthalocyanine (8) and 2,3,9,10,16,17,23,24-octakis[4-(9,9-dihexyl-9H-fluoren-2-yl)phenyl]-29H,31H-tetrapyrazinoporphyrazine (9). Palladium-mediated Suzuki-Miyaura cross-coupling reactions have been key steps for attaching the substituents. The compounds are deep-red emitters: lambda(max)(em)=659 (3), 737 (8) and 684 nm (9). Their absorption and emission spectra, their fluorescence lifetimes and quantum yields are correlated with the structures of the macrocycles and the substituents. The solution fluorescence quantum yields of porphyrin derivatives substituted with fluorene (2-4) and terphenyl substituents (7) (Phi(f)=0.21-0.23) are approximately twice that of tetraphenylporphyrin. For phthalocyanine derivative 8, Phi(f) was very high (0.88). Specific excitation of the fluorene units of 8 produced emission from both of them (lambda(max)=480 nm) and also from the phthalocyanine core (lambda(max)=750 nm), indicating a competitive rate of energy transfer and radiative decay of the fluorenes. Organic light-emitting devices (OLEDs) were made by spin-coating techniques by using a polyspirobifluorene (PSBF) copolymer as the host blended with 3 (5 wt. %) in the configuration ITO/PEDOT:PSS/PSBF copolymer:3/Ca/Al. Deep-red emission (lambda(max)=663 nm; CIE coordinates x=0.70, y=0.27) was observed with an external quantum efficiency of 2.5 % (photons/electron) (at 7.5 mA cm(-2)), a low turn-on voltage and high emission intensity (luminance) of 5500 cd m(-2) (at 250 mA/ m(2)).