Coumarin Photocaging Groups Modified with an Electron‐Rich Styryl Moiety at the 3‐Position: Long‐Wavelength Excitation,Rapid Photolysis,and Photobleaching

A new class of coumarin photocaging groups modified with an electron‐rich styryl moiety at the 3‐position was constructed. The large π‐conjugated structure and stabilization of the carbocation intermediates by electron donors endowed the new photocaging groups with excellent long‐wavelength absorption, large two‐photon absorption cross‐sections, and high uncaging quantum yields. Moreover, the new photocaging groups displayed unique photobleaching properties after photocleavage as a result of the intramolecular cyclization rearrangement of a carbocation intermediate to form five‐membered ring byproducts and block the styryl conjugation at the 3‐position. These superior properties of the new photocaging groups are extremely beneficial for high‐concentration samples and thick specimens, thus extending the application of photocaging groups in many fields.     

New 2,6‐Distyryl‐Substituted BODIPY Isomers: Synthesis,Photophysical Properties,and Theoretical Calculations

A 2,6‐distyryl‐substituted boradiazaindacene (BODIPY) dye and a new series of 2,6‐p‐dimethylaminostyrene isomers containing both α‐ and β‐position styryl substituents were synthesized by reacting styrene and p‐dimethylaminostyrene with an electron‐rich diiodo‐BODIPY. The dyes were characterized by X‐ray crystallography and NMR spectroscopy and their photophysical properties were investigated and analyzed by carrying out a series of theoretical calculations. The absorption spectra contain markedly redshifted absorbance bands due to conjugation between the styryl moieties and the main BODIPY fluorophore. Very low fluorescence quantum yields and significant Stokes shifts are observed for 2,6‐distyryl‐substituted BODIPYs, relative to analogous 3,5‐distyryl‐ and 1,7‐distyryl‐substituted BODIPYs. Although the fluorescence of the compound with β‐position styryl substituents on both pyrrole moieties and one with both β‐ and α‐position substituents was completely quenched, the compound with only α‐position substituents exhibits weak emission in polar solvents, but moderately intense emission with a quantum yield of 0.49 in hexane. Protonation studies have demonstrated that these 2,6‐p‐dimethylaminostyrene isomers can be used as sensors for changes in pH. Theoretical calculations provide strong evidence that styryl rotation and the formation of non‐emissive charge‐separated S₁ states play a pivotal role in shaping the fluorescence properties of these dyes. Molecular orbital theory is used as a conceptual framework to describe the electronic structures of the BODIPY core and an analysis of the angular nodal patterns provides a reasonable explanation for why the introduction of substituents at different positions on the BODIPY core has markedly differing effects.     

Cyanine‐Like Dyes with Large Bond‐Length Alternation

Herein, the synthesis and properties of alkyne‐bridged carbocations, which are analogous in structure to cyanine dyes, are reported. An alkene‐bridged dye, linked at the third position of the indole, was also synthesized as a reference compound. These new carbocations are stable under ambient conditions, allowing characterization by UV/Vis and NMR (¹H and ¹³C) spectroscopies. These techniques revealed a large degree of delocalization of the positive charge, similar to a previously reported porphyrin carbocation. The linear and nonlinear optical properties are compared with cyanine dyes and triarylmethyl cations, to investigate the effects of the bond‐length alternation and the overall molecular geometry. The value of Re(γ), the real part of the third‐order microscopic polarizability, of ?1.3×10^?33 esu for the alkyne‐linked cation is comparable to that of a cyanine dye of similar length. Nondegenerate two‐photon absorption spectra showed that the alkene‐bridged dye exhibited characteristics of cyanines, whereas the alkyne‐bridged dye is reminiscent of octupolar chromophores, such as the triarylmethyl carbocation brilliant green. Such attributes were confirmed and rationalized by quantum chemical calculations.     

A Benzophenone‐Based Photocaging Strategy for the N7 Position of Guanosine

Selective modification of nucleobases with photolabile caging groups enables the study and control of processes and interactions of nucleic acids. Numerous positions on nucleobases have been targeted, but all involve formal substitution of a hydrogen atom with a photocaging group. Nature, however, also uses ring‐nitrogen methylation, such as m⁷G and m¹A, to change the electronic structure and properties of RNA and control biomolecular interactions essential for translation and turnover. We report that aryl ketones such as benzophenone and α‐hydroxyalkyl ketone are photolabile caging groups if installed at the N7 position of guanosine or the N1 position of adenosine. Common photocaging groups derived from the ortho‐nitrobenzyl moiety were not suitable. Both chemical and enzymatic methods for site‐specific modification of N7G in nucleosides, dinucleotides, and RNA were developed, thereby opening the door to studying the molecular interactions of m⁷G and m¹A with spatiotemporal control.     

Bright,Fluorescent Dyes Based on Imidazo[1,2‐a]pyridines that are Capable of Two‐Photon Absorption

A library of imidazo[1,2‐a]pyridines was synthesized by using the Gevorgyan method and their linear and non‐linear optical properties were studied. Derivatives that contained both electron‐donating and electron‐withdrawing groups at the 2 position were comprehensively investigated. Their emission quantum yield ranged between 0.2–0.7 and it was shown to depend on the substitution pattern, most notably that on the phenyl ring. Electron‐donating substituents improved the luminescence performance of these compounds, whereas electron‐withdrawing substituents led to a more erratic behavior. Substitution on the six‐membered ring had less effect on the fluorescence properties. Extension of the delocalization increased the luminescence quantum yield. A new quadrupolar system was designed that contained two imidazo[1,2‐a]pyridine units on its periphery and a 1,4‐dicyanobenzene unit at its center. This system exhibited a large Stokes‐shifted luminescence that was affected by the polarity and rigidity of the solvent, which was ascribed to emission from an excited state with strong charge‐transfer character. This quadrupolar feature also led to an acceptable two‐photon absorption response in the NIR region.     

Luminescent copoly(aryl ether)s with new electron‐transporting bis(3‐(trifluoromethyl)phenyl)‐1,3,4‐oxadiazole or bis(3‐(trifluoromethyl)phenyl)‐4‐(4‐hexyloxyphenyl)‐4H‐1,2,4‐triazole segments

To investigate the effect of trifluoromethyl groups in enhancing electron affinity of aromatic oxadiazole and triazole chromophores, we prepared four new copoly(aryl ether)s ( P1 – P4 ) consisting of bis(3‐(trifluoromethyl) phenyl)‐1,3,4‐oxadiazole (ETO) or bis(3‐(trifluoromethyl)phenyl)‐4‐(4‐hexyloxyphenyl)‐4H‐1,2,4‐triazole (ETT) segments and hole‐transporting segments [2,5‐distyrylbenzene (HTB) or bis(styryl)fluorine (HTF)]. Molecular spectra (absorption and photoluminescence) and cyclic voltammetry were used to investigate their optical and electrochemical properties. The emissions of P1 – P4 are dominated by the hole‐transporting fluorophores with longer emissive wavelengths around 442–453 nm via efficient excitation energy transfer. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of P1 – P4 , estimated from electrochemical data, are ?5.15, ?5.18, ?5.30, ?5.27, ?3.39, ?3.49, ?3.36, and ?3.48 eV, respectively. The LUMO levels of ETO and ETT segments are significantly reduced to ?3.39～?3.36 eV and ?3.48～?3.49 eV, respectively, as compared with ?2.45 eV of P5 containing a 2,5‐diphenyl‐1,3,4‐oxadiazole segment. Moreover, electron and hole affinity can be enhanced simultaneously by introducing isolated hole‐ and electron‐transporting segments in the backbone. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5900–5910, 2004     

First Highly Efficient and Photostable E and C Derivatives of 4,4‐Difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) as Dye Lasers in the Liquid Phase,Thin Films,and Solid‐State Rods

A new library of E‐ and C‐4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) derivatives has been synthesized through a straightforward protocol from commercially available BODIPY complexes, and a systematic study of the photophysical properties and laser behavior related to the electronic properties of the B‐substituent group (alkynyl, cyano, vinyl, aryl, and alkyl) has been carried out. The replacement of fluorine atoms by electron‐withdrawing groups enhances the fluorescence response of the dye, whereas electron‐donor groups diminish the fluorescence efficiency. As a consequence, these compounds exhibit enhanced laser action with respect to their parent dyes, both in liquid solution and in the solid phase, with lasing efficiencies under transversal pumping up to 73 % in liquid solution and 53 % in a solid matrix. The new dyes also showed enhanced photostability. In a solid matrix, the derivative of commercial dye PM597 that incorporated cyano groups at the boron center exhibited a very high lasing stability, with the laser emission remaining at the initial level after 100 000 pump pulses in the same position of the sample at a 10 Hz repetition rate. Distributed feedback laser emission was demonstrated with organic films that incorporated parent dye PM597 and its cyano derivative. The films were deposited onto quartz substrates engraved with appropriate periodical structures. The C derivative exhibited a laser threshold lower than that of the parent dye as well as lasing intensities up to three orders of magnitude higher.     

Ultrafast Photoinduced Charge Separation in Wide‐Band‐Capturing Self‐Assembled Supramolecular Bis(donor styryl)BODIPY–Fullerene Conjugates

A new series of self‐assembled supramolecular donor–acceptor conjugates capable of wide‐band capture, and exhibiting photoinduced charge separation have been designed, synthesized and characterized using various techniques as artificial photosynthetic mimics. The donor host systems comprise of a 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) containing a crown ether entity at the meso‐position and two styryl entities on the pyrrole rings. The styryl end groups also carried additional donor (triphenylamine or phenothiazine) entities. The acceptor host system was a fulleropyrrolidine comprised of an ethylammonium cation. Owing to the presence of extended conjugation and multiple chromophore entities, the BODIPY host revealed absorbance and emission well into the near‐IR region covering the 300–850 nm spectral range. The donor–acceptor conjugates formed by crown ether–alkyl ammonium cation binding of the host–guest system was characterized by optical absorbance and emission, computational, and electrochemical techniques. Experimentally determined binding constants were in the range of 1–2×10⁵ M ^?1. An energy‐level diagram to visualize different photochemical events was established using redox, computational, absorbance, and emission data. Spectral evidence for the occurrence of photoinduced charge separation in these conjugates was established from femtosecond transient absorption studies. The measured rates indicated ultrafast charge separation and relatively slow charge recombination revealing their usefulness in light‐energy harvesting and optoelectronic device applications. The bis(donor styryl)BODIPY‐derived conjugates populated their triplet excited states during charge recombination.     

A Novel Iron‐catalyzed One‐pot Synthesis of 3‐Amino‐1,2,4‐triazoles

A novel one‐pot synthesis of 3‐amino‐1,2,4‐triazole developed via iron (III) catalyzed route is reported. The new method is more efficient, simple, and convenient and presents a concise new strategy for the synthesis of 3‐amino‐1,2,4‐triazole derivatives. The iron (III) complex intermediate assisted in the intramolecular bond cyclization owing to its Lewis acidity or oxidizing properties. A series of aromatic nitriles bearing different electron‐donating and electron‐withdrawing groups substituted at para and/or ortho positions were also investigated. The position of the substituents affected the yield of the final compound, with the para‐substituted substrates giving relatively higher yields.     

Synthesis,optical and photoconducting properties of platinum poly‐yne polymers functionalized with electron‐donating and electron‐withdrawing bithiazole units

By virtue of the electron‐donating and electron‐withdrawing properties of the thiazole ring, a new soluble rigid‐rod organometallic polymer containing electron‐donating and electron‐withdrawing trans‐[‐Pt‐(PBu₃)₂‐C≡C—R—C≡C—]_n (R = bithiazolediyl) groups is prepared by CuI‐catalyzed dehydrohalogenation. The thermal properties and the optical absorption and photoluminescence spectra of the polymer are reported. The polymer is luminescent with a singlet emission peak at 539 nm and photoconducting in a single‐layer sandwich structure photocell. The optical gap of the polymer is reduced compared to that for the oligothienyl analogue.     

Synthesis and Spectra Characteristics of Novel 3‐(para‐Bromophenyl)‐7‐(substituted vinyl) Coumarins

Five new coumarin derivatives ( 5a , 5b , 5c , 5d , 5e ) with extending para‐bromophenyl at the 3‐position and substituted vinyl at the 7‐position were synthesized and characterized by FT‐IR, ¹H NMR, and element analysis. The absorption and fluorescence characteristics of compounds 5a , 5b , 5c , 5d , 5e showed significant dependences on its molecular structure, which possessed large Stokes shifts (up to 8309 cm^?1) and high fluorescence quantum yield (up to 0.80) in CH₂Cl₂. These advantageous spectral properties should allow use in many areas.     

NLOphoric Triphenylamine Derived Donor‐π‐Acceptor‐π‐Donor Based Colorants: Synthesis,Spectroscopic, Density Functional Theory and Z‐scan Studies

Three Donor‐π‐Acceptor‐π‐Donor type styryl dyes ( 5a‐c ) with different secondary donors are synthesized and characterized to study their nonlinear and linear optical properties. The structure–property relationships of the dyes are described in the light of systematic photophysical and theoretical investigations. The photophysical characteristics of 5a‐c are influenced by the polarity of the medium, with an appreciable bathochromic shift in emission ( 5b = 81 nm) and large Stoke shifts ( 5b = 104–173 nm) in polar solvents. 5a‐c showed intramolecular charge transfer characteristics recognized with the help of emission solvatochromism, solvent polarity graphs, natural bond orbital analysis and HOMO–LUMO energy difference. The optimized geometry and frontier molecular orbitals reveal that the electron donation takes place from secondary donors and not from a fixed donor (triphenylamine) which is more twisted. The nonlinear optical properties obtained using solvent induced spectral shift and computational methods are found within the limiting values. Z‐scan results reveal saturable kind of behavior for 5a , 5b and 5c , whereas 5a and 5b show reverse saturable kind of behavior in acetone and ethanol and hence give optical limiting values. The two‐photon absorption cross section described by two‐level approximation is highest for 5b (251–300 GM).     

Large Two‐Photon Absorption of Terpyridine‐Based Quadrupolar Derivatives: Towards their Applications in Optical Limiting and Biological Imaging

Developing organic chromophores with large two‐photon absorption (TPA) in both organic solvents and aqueous media is crucial owing to their applications in solid‐state photonic devices and biological imaging. Herein, a series of novel terpyridine‐based quadrupolar derivatives have been synthesized. The influences of electron‐donating group, type of conjugated bridge, as well as solvent polarity on the molecular TPA properties have been investigated in detail. In contrast to the case in organic solvents, bis(thienyl)‐benzothiadiazole as a rigid conjugated bridge will completely quench molecular two‐photon emission in aqueous media. However, the combination of alkylcarbazole as the donor and bis(styryl)benzene as a conjugation bridge can enlarge molecular TPA cross‐sections in both organic solvent and aqueous media. The reasonable two‐photon emission brightness for the organic nanoparticles of chromophores 3 – 5 in the aqueous media, prepared by the reprecipitation method, enables them to be used as probes for in vivo biological imaging.     

Interaction of Crown Ether‐Annelated Styryl Dyes with Double‐Stranded DNA

DNA‐binding properties of 15‐crown‐5‐derived mono‐ and bis‐styryl dyes were investigated in the presence of calf thymus DNA. To access the factors that influence the DNA association in the series of these ligands, the structure of the molecules was varied by either changing size of the heterocyclic moiety or altering the position of the styryl substituents. The major binding mode for the monostyryl dyes is intercalation. Notably, binding of the dyes to the nucleic acids leads to a fluorescence enhancement by a factor of up to 54. Therefore, these cationic styryl derivatives may be applied as fluorescent “light‐up” probes for DNA detection.     

Theoretical investigation on the spectroscopic properties of furylfulgide with different substituents and design of novel bis‐furylfulgimide photochromes

Density functional theory and time‐dependent density functional theory were employed to theoretically analyze the effect of different substituents on the spectroscopic properties of furylfulgide. The result shows that the absorption spectra of ring‐closed isomer which substituted by an electron‐donating group (NH₂) at the R₃‐position of furylfulgide has an evident bathochromic shift compared with the others. Due to the steric hindrance effect, the difference of absorption wavelength was evidently enlarged by introducing several representative electron‐withdrawing groups at the R₆‐position of furylfulgide. In addition, we also designed a series of novel dimers which combined two furylfulgimide monomers into one new molecule. The relevant frontier molecular orbitals, energy levels and absorption properties were analyzed in detail by the calculation of low‐lying excited states. Finally, taking BFF‐6 (bis‐furylfulgimide) for an example, we discussed the transformation mechanism of four stable isomers in the toluene solution. Our conclusions manifest that the asymmetrical BFF‐6 can act as a potential multifunctional molecular switch in consideration of its distinguishable absorption bands and reversible conversion process. We hope that this research will be beneficial to design more practical and efficient molecular switch for further applications.     

Synthesis,Photophysical Properties,and Antimicrobial Activity of Novel Styryl Colorants Derived from 7‐Methoxy‐1,4‐diphenethyl‐1,2,3,4‐tetrahydroquinoxaline‐6‐carbaldehyde

The novel 1,4‐diphenethyl‐1,2,3,4‐tetrahydro‐7‐methoxyquinoxalin‐6‐carbaldehyde was synthesized by reductive alkylation of 6‐methoxy quinoxaline with phenyl acetic acid and was further subjected to Knoevenagel condensation with various active methylene compounds to synthesize novel styryl colorants. Photophysical properties of styryl colorants were studied using UV–visible and fluorescence spectroscopy. These colorants displayed orange to violet hue and showed fluorescence emission maxima in the region of 560–640 nm, and displayed a large Stokes shift (85–104 nm). Compounds were subjected to thermogravimetric analysis which showed excellent stability up to 310°C. These styryl compounds were evaluated for their antimicrobial study as antifungal against Candida albicans C. albicans and Aspergillus niger and antibacterial against Escherichia coli and Staphylococcus aureus. The results revealed good antimicrobial activity against tested organisms. The synthesized chromophores were characterized using elemental analysis, FTIR, ¹³C‐NMR and ¹H‐NMR spectroscopy and mass spectrometry.     

Modulating the Properties of Azulene‐containing Polymers Through Functionalization at the 2‐Position of Azulene

Poly(2‐arylazulene‐alt‐fluorene) and poly(2‐arylazulene‐alt‐thiophene) are synthesized via Suzuki and Stille cross‐coupling polymerization, respectively, using 1,3‐dibromo‐2‐arylazulenes as monomers, which are prepared by a novel directed C?H activation method of 2‐carboxylic azulene and subsequent bromination reaction. Our study shows that functionalization at the 2‐position of azulene monomers influences polymer properties. For instance, different from electron‐withdrawing groups that discourage the protonation of azulene, electron‐donating aryl groups, however, enhances the sensitivity of response to acid. Protonation of the polymers leads to significant shifts in absorption spectra accompanying with obvious color changes from green to brown in majority cases because of the formation of poly(azulenium cation). The electrochromic properties of polymers are examined, exhibiting that nature of aryl group at the 2‐position of azulene influences the stability of their electrochromic devices.     

Efficient Two‐Photon Excited Amplified Spontaneous Emission from Organic Single Crystals

E, E‐1, 4‐bis[4′‐(N,N‐dibutylamino)styryl]‐2,5‐dimethoxy‐benzene (DBASDMB) organic crystals with high crystalline quality, large size and excellent optical properties are prepared. The linear and nonlinear properties in the crystal are comparatively studied. The relaxation dynamics pumped by two‐photon are very similar with that pumped by one‐photon. The crystal exhibits very strong two‐photon excited fluorescence and amplified spontaneous emission. Efficient two‐photon absorption, reasonably high fluorescent quantum efficiency, and high crystal quality together with stimulated emission make organic crystals ideal for the application in frequency upconversion and other optoelectronic fields.     

Synthesis,Photophysical Properties of Tribranched Chromophores Based on 1,3,5‐Triazine Core and Different Electro‐donating End‐groups

Three two‐photon absorption (TPA) tribranched chromophores were successfully prepared, in which 1,3,5‐triazine is been as electron deficient core, 1,4‐phenylenedivinylene as conjugated bridge, 3,4‐ethylenedioxythiophene (EDOT) ( T1 ), N‐methylpyrrole ( T2 ) or triphenylamine ( T3 ) as electron‐donating end‐groups. Their photophysical properties were studied by absorption, one‐ and two‐photon fluorescence and TPA cross‐section determination. The nonlinear transmission (NLT) measurement in femtoseconds (fs) regime at 800 nm indicates that TPA cross‐section (₂ values of T1 , T2 and T3 with extended Π‐conjugated bridge are much larger than the corresponding chromophore T4 with a short length bridge, and TPA cross‐section of T1 with end‐groups EDOT exhibits a remarkable enhancement compared with T2 and T3 having the same length Π‐system. The chromophores T1 , T2 and T3 show also remarkable up‐converted luminescence and optical limiting activity.     

Rational Design,Synthesis, and Optical Properties of Film‐Forming,Near‐Infrared Absorbing,and Fluorescent Chromophores with Multidonors and Large Heterocyclic Acceptors

A new series of film‐forming, low‐bandgap chromophores ( 1 a,b and 2 a,b ) were rationally designed with aid of a computational study, and then synthesized and characterized. To realize absorption and emission above the 1000 nm wavelength, the molecular design focuses on lowering the LUMO level by fusing common heterocyclic units into a large conjugated core that acts an electron acceptor and increasing the charge transfer by attaching the multiple electron‐donating groups at the appropriate positions of the acceptor core. The chromophores have bandgap levels of 1.27–0.71 eV, and accordingly absorb at 746–1003 nm and emit at 1035–1290 nm in solution. By design, the relatively high molecular weight (up to 2400 g mol^?1) and non‐coplanar structure allow these near‐infrared (NIR) chromophores to be readily spin‐coated as uniform thin films and doped with other organic semiconductors for potential device applications. Doping with [6,6]‐phenyl‐C₆₁ butyric acid methyl ester leads to a red shift in the absorption only for 1 a and 2 a . An interesting NIR electrochromism was found for 2 a , with absorption being turned on at 1034 nm when electrochemically switched (at 1000 mV) from its neutral state to a radical cation state. Furthermore, a large Stokes shift (256–318 nm) is also unique for this multidonor–acceptor type of chromophore, indicating a significant structural difference between the ground state and the excited state. Photoluminescence of the film of 2 a was further probed at variable temperatures and the results strongly suggest that the restriction of bond rotations certainly helps to diminish non‐radiative decay and thus enhance the luminescence of these large chromophores.