Reversible trans-cis photoisomerization of new pyrrolidene heterocyclic imines

A series of novel pyrrolidene imines bearing functionalized aryl or naphthyl moieties was synthesized and their photochromic properties studied by UV spectroscopy. UV irradiation of these heterocyclic Schiff bases at room temperature promotes the trans-cis photoisomerization of the C=N double bond with formation of a variable amount of the cis-isomer that, in the absence of light, returns thermally to the original form in few seconds. The thermal cis-trans re-isomerization of these molecular switches is much more slower than for the common benzylidene aniline-type imines, allowing the observation of the photochromic phenomena at room temperature. Strong electron-donor substituents in the para-position of the aniline part of the molecule decreases even more the kinetics of the thermal cis-trans re-isomerization leading to a manifest change in the UV spectrum.Fundação para a Ciência e a Tecnologia (FCT

Reactivity studies of thienyl and (oligo)thienylpyrroles

Currently, a variety of synthetic approaches to substituted pyrroles exist, although their synthesis, in general, remains challenging. Often, the yields are rather low and a significant number of by-products, such as undesired regioisomers, are obtained. Furthermore, pyrroles are susceptible to chemical degradation as they are rather easily oxidized; this further hampers their synthesis and especially their isolation and purification.1 The chemistry of thienylpyrroles is a very recent field in the chemistry of heterocyclic compounds. In the last few years, synthetic 2-thienylpyrrole derivatives have come in focus. However, even more than 60 years since the first 2-thienylpyrrole: bis-2-[5-(2-thienyl)pyrrole]azametine dihydrochloride) has been reported by Edward Knott2 at Kodak, the synthesis of functionalized thienyl and (oligo)thienylpyrroles remains challenging. Often, the yields are low and the regioselectivity is only modest.3 Before our recent work,3-4 only a few papers were published concerning the regioselectivity studies of 1-(alkyl)aryl-2-thienylpyrrole systems. Following our interest in the chemistry and optical applications of new functionalized systems bearing thiophene and pyrrole rings we have used heterocyclic systems 1 bearing pyrrole and thiophene rings (1, 2 or 3) as precursors for the synthesis of functionalyzed pyrrole derivatives. Compounds 1 have proved to be versatile substrates in several reactions (aromatic electrophilic substitutions: azo coupling, direct tricyanovinylation reaction, Vilsmeier-Haack formylation) and metalation followed by reaction with DMF, allowing the preparation of interesting new donor-acceptor substituted heterocyclic systems, selectively functionalized on the pyrrole 2 or on the thiophene 3 rings. Precursors 1 were prepared through a palladium-catalyzed decarboxylative cross-coupling reaction. The structures of the new compounds were unambiguously confirmed by their analytical and spectral data. Acknowledgments Thanks are due to the Fundação para a Ciência e Tecnologia (Portugal) and FEDER-COMPETE for financial support through the Centro de Química - Universidade do Minho, Project PTDC/QUI/66251/2006 (FCOMP-01-0124-FEDER-007429), Project PEst-C/QUI/UI0686/2011 (F-COMP-01-0124-FEDER-022716) and a PhD grant to M. C. R. Castro (SFRH/BD/78037/2011). The NMR spectrometer Bruker Avance III 400 is part of the National NMR Network and was purchased within the framework of the National Program for Scientific Re-equipment, contract REDE/1517/RMN/2005 with funds from POCI 2010 (FEDER) and FCT. References 1. Schmuch, C.; Rupprecht, D. Synthesis 2007, 3095. 2. Knott, E. B. J. Chem. Soc. 1947, 1196. 3. (a) Raposo, M. M. M. in Targets in Heterocyclic Systems: Chemistry and Properties, "Recent developments in the chemistry of 2-thienylpyrroles: synthesis, reactivity and applications", Attanasi, O. A.; Spinelli, D. Eds.; Royal Society of Chemistry: London, 2008, Vol. 11, pp 122-154. 4. (a) Raposo, M. M. M.; Sousa, A. M. R. C.; Fonseca, A. M. C.; Kirsch, G. Tetrahedron 2005, 61, 8249. (b) Raposo, M. M. M.; Sousa, A. M. R. C.; Kirsch, G.; Ferreira, F.; Belsley, M.; Matos Gomes, E.; Fonseca, A. M. C. Tetrahedron 2005, 61, 11991. (c) Raposo, M. M. M.; Sousa, A. M. R. C.; Fonseca, A. M. C.; Kirsch G. Tetrahedron 2006, 62, 3493. (d) Raposo, M. M. M.; Sousa, A. M. R. C.; Kirsch, G.; Cardoso, P.; Belsley, M.; Matos Gomes, E.; Fonseca, A. M. C. Org. Lett. 2006, 8, 3681. (e) Raposo, M. M. M.; Castro, M. C. R.; Schellenberg, P.; Fonseca, A. M. C.; Belsley, M. Tetrahedron 2011, 67, 5189

Synthesis of π-conjugated systems bearing thiophene and pyrrole heterocycles through palladium-catalyzed decarboxylative cross-coupling reaction

Thiophene and pyrrole moieties play important roles in synthetic and medicinal chemistry, as they are present in a large number of natural products and biologically active compounds. For this reason, amongst all five-membered aromatic heterocycles, molecules containing the pyrrole nucleus have attracted the greatest attention of researchers and have been studied in most detail. Especially, 2-aryl- and 2-heteroaryl-substituted pyrroles are of great interest to the pharmaceutical industry, for instance, as precursors in the synthesis of chemotherapeutics.1 Furthermore, synthetic pyrrole -containing derivatives (e.g. functionalized thienylpyrroles and 2,5-dithienylpyrroles) and -conjugated oligo- and polypyrrolic systems are of growing relevance in materials science, supramolecular chemistry, and nanotechnology. For example, they have found application in anion binding and cation coordination, conducting organic polymers, liquid crystals, and nonlinear optics.2 The wide array of interesting properties has inspired the development of several procedures for the preparation of differently substituted pyrroles. On the other hand, methods for the construction of 2-(2´-thienyl)pyrroles and (oligo)thienylpyrroles remains limited. Therefore the development of efficient methods for the synthesis of these compounds has become an important and topical area of heterocyclic chemistry. In our laboratories we have developed a convenient method for the syntheis of 1-alkyl(aryl)-2-thienylpyrroles, through the combination of the Friedel-Crafts and Lawesson reactions.3 The transition-metal-catalyzed decarboxylative cross-coupling has been recently utilized as a new synthetically useful tool for the preparation of heterocyclic systems.4 Having in mind our recent work and also our interest in the development of synthetic methodologies for the synthesis of novel heterocyclic systems we report in this communication the synthesis and the characterization of new -conjugated systems 1 bearing thiophene and pyrrole rings through palladium-catalyzed decarboxylative cross-coupling reaction between 1-methyl-1H-pyrrole-2-carboxylic acid 1 and thienyl bromides 2. Several reaction conditions (catalyst, solvent, additive, base, stoichiometry and temperature) were studied in order to obtain better yields for compounds 3. Some of the precursor thienyl bromides 2 were prepared through Suzuki coupling reaction. The new compounds were completely characterized by the usual spectroscopic techniques. Acknowledgments Thanks are due to the Fundação para a Ciência e Tecnologia (Portugal) and FEDER-COMPETE for financial support through the Centro de Química - Universidade do Minho, Project PTDC/QUI/66251/2006 (FCOMP-01-0124-FEDER-007429), Project PEst-C/QUI/UI0686/2011 (F-COMP-01-0124-FEDER-022716) and a PhD grant to M. C. R. Castro (SFRH/BD/78037/2011). The NMR spectrometer Bruker Avance III 400 is part of the National NMR Network and was purchased within the framework of the National Program for Scientific Re-equipment, contract REDE/1517/RMN/2005 with funds from POCI 2010 (FEDER) and FCT. References 1. Bellina, F.; Rossi, R. Tetrahedron 2006, 62, 7213. 2. (a) Raposo, M. M. M. in Targets in Heterocyclic Systems: Chemistry and Properties, "Recent developments in the chemistry of 2-thienylpyrroles: synthesis, reactivity and applications", Attanasi, O. A.; Spinelli, D. Eds.; Royal Society of Chemistry: London, 2008, Vol. 11, pp 122-154. (b) Raposo, M. M. M.; Sousa, A. M. R. C.; Kirsch, G.; Cardoso, P.; Belsley, M.; Matos Gomes, E.; Fonseca, A. M. C. Org. Lett. 2006, 8, 3681. (c) Raposo, M. M. M.; Castro, M. C. R.; Schellenberg, P.; Fonseca, A. M. C.; Belsley, M. Tetrahedron 2011, 67, 5189. 3. (a) Raposo, M. M. M.; Sampaio, A. M. B. A.; Kirsch, G. Synthesis 2005, 2, 199. (b) Raposo, M. M. M.; Sousa, A. M. R. C.; Fonseca, A. M. C.; Kirsch G. Tetrahedron 2006, 62, 3493. 4. Zhao, D.; You, J.; Hu, C. Chem. Eur. J. 2011, 17, 5466

Photochromic heteroarylethenes with fast thermal isomerization kinetics

Supplementary data to this article can be found online at https://doi.org/10.1016/j.dyepig.2022.111000Stilbenes (diphenylethenes) are fully examined organic chromophores for very diverse applications. However, these molecules might show poor stability under irradiation because several simultaneous reactions (oxidative photocyclization and dimerization) can take place during the photochemical isomerization of their C=C bond. In this context, heteroarylethenes emerge as powerful counterparts thanks to their improved stability. In addition, the chosen heterocycles and their functionalization allows to fine tune and increase the speed at which the thermal back reaction occurs, broadening the scope of application of the resulting chromophores. Here we report on the thermal Z-to-E isomerization kinetics of a series of rationally designed heteroarylethenes and how the chemical architecture of the chromophore modulates the speed of the process. Specifically, the metastable Z isomers display relaxation times covering a wide time window, from a few hours to hundreds of microseconds. In fact, one of the studied dyes is the fastest heteroarylethene known to date. It should be also noticed that the described heteroarylethenes exhibit great photochemical stability, it being possible to switch them back and forth many times without degradation.This research was funded by the Ministerio de Ciencia, Innovacion y Universidades (Spain)/Agencia Estatal de Investigacion (Spain)/Fondo Europeo de Dearrollo Regional (FEDER, European Union), grant number PGC2018-095477-B-I00. Thanks are also due to Fundação para a Ciência e Tecnologia (FCT) and to FEDER-COMPETE for financial support through the research center CQUM (UID/QUI/0686/2016) and (UID/ QUI/0686/2020). The NMR spectrometers are part of the National NMR Network (PTNMR) and are partially supported by Infrastructure Project Nº 022161 (co-financed by FEDER through COMPETE 2020, POCI and PORL and FCT through PIDDAC). Authors also thank Prof. Santi Nonell for granting access to the nanosecond laser flash photolysis setup

Highly efficient and thermally stable NLO organic materials based on pyrrole and thiophene heterocycles

A series of novel donor-acceptor chromophores designed to have good second order nonlinear optical responses has been synthesized and characterized. This series of compounds was designed to explore the consequence of using different electron accepting moieties which were linked through an arylthiophene bridge to a pyrrole heterocycle that plays the role of an auxiliary donor group. These new push-pull chromophores have been extensively characterized using cyclic voltammetry, thermogravimetric analysis and hyper-Rayleigh scattering (HRS) in solution. The measured molecular first hyperpolarizabilities and the observed electrochemical behavior showed that they were very sensitive to the acceptor strength of the acceptor moieties. Moreover, the combination of their good nonlinearity and high thermal stability make them good candidates for potential device applicationsThanks are due to the Fundacao para a Ciencia e Tecnologia (Portugal) and FEDER for financial support through the Centro de Quimica and Centro de Fisica-Universidade do Minho, Project PTDC/QUI/66251/2006 (FCOMP-01-0124-FEDER-007429), Project PTDC/CTM/105597/2008 with funding from COMPETE/FEDER and a research grant to M.C. R. Castro (UMINHO/BI/142/2009). The NMR spectrometer Bruker Avance III 400 is part of the National NMR Network and was purchased within the framework of the National Program for Scientific Re-equipment, contract REDE/1517/RMN/2005 with funds from POCI 2010 (FEDER) and FCT

Synthesis and evaluation of thermal, electrochemical and nonlinear optical properties of dicyano- and tricyanovinyl-bithienylpyrroles

Materials exhibiting large optical nonlinearities continue to be the subject of intense studies because of their potential applications in modern communication technologies involving optical data processing, transmission, or storage, where they are gradually replacing classical electronic devices.[1] Earlier we reported on the synthesis and evaluation of thermal, redox and NLO properties of donor-acceptor substituted thienylpyrrole derivatives. Our studies have shown that replacing the benzene ring of a chromophore bridge with heterocycles (thiophene, pyrrole, (benzo)thiazole), results in an enhanced molecular hyperpolarizability of conjugated donor–acceptor compounds.[2,3,4,5,6] As part of an on-going program to develop strong charge transfer donor-acceptor pyrrole systems with enhanced nonlinear optical (NLO) properties we have explored the potential of bithienyl-pyrrole derivatives 1-4 as efficient π-conjugated systems functionalized with dicyanovinyl- or tricyanovinyl- acceptor groups substituted at thiophene or pyrrole rings. Compounds 3a-b were prepared, in good yields, through Knoevenagel condensation of the precursor aldehydes 2a-b with malononitrile, and the tricyano derivative 4 were synthesized through direct tricyanovinylation reaction of 1 with tetracyanoethylene. On the other hand, bithienyl-pyrroles 1 and 2b were prepared through palladium-catalyzed decarboxylative cross-coupling reaction between 1-methyl-1H-pyrrole-2-carboxylic acid and the appropriate bithienyl bromides, whereas formyl-precursor 2a were prepared through Vilsmeier-Haack formylation from the corresponding bithienyl-pyrrole 1. The molecular structure, especially the degree of planarity and the relative orientations of the auxiliary donor and acceptor entities, is found to strongly influence their electrochemical and optical (linear and nonlinear) properties. Here we report on the synthesis of compounds 1-4 and evaluate their potential as efficient and thermally stable NLO materials by experimentally characterizing their redox properties, thermal stability (TGA) and second order nonlinear optical response (HRS). These results indicate that these compounds have good potential for a variety of NLO applications. Acknowledgments Thanks are due to the Fundação para a Ciência e Tecnologia (Portugal) and FEDER-COMPETE for financial support through the Centro de Química - Universidade do Minho, Projects PTDC/QUI/66251/2006 (FCOMP-01-0124-FEDER-007429), PTDC/CTM/105597/2008 (FCOMP-01-0124-FEDER-009457), PEst-C/QUI/UI0686/2011 (F-COMP-01-0124-FEDER-022716) and a PhD grant to M. C. R. Castro (SFRH/BD/78037/2011). The NMR spectrometer Bruker Avance III 400 is part of the National NMR Network and was purchased within the framework of the National Program for Scientific Re-equipment, contract REDE/1517/RMN/2005 with funds from POCI 2010 (FEDER) and FCT. [1] G.S. He, L.-S. Tan, Q. Zheng, P. N. Prasad Chem. Rev. 2008, 108, 1245-1330. [2] M. M. M. Raposo, A. M. R. C. Sousa, G. Kirsch, F. Ferreira, M. Belsley, E. Matos Gomes, A. M. C. Fonseca Tetrahedron 2005, 61, 11991-11998. [3] M. M. M. Raposo, A. M. R. C. Sousa, G. Kirsch, P. Cardoso, M. Belsley, E. Matos Gomes, A. M. C. Fonseca Org. Lett. 2006, 8, 3681-3684. [4] R. M. F. Batista, S. P. G. Costa, E. L. Malheiro, M. Belsley, M. M. M. Raposo Tetrahedron 2007, 63, 4258-4265. [5] M. M. M. Raposo, M. C. R. Castro, P. Schellenberg, A. M. C. Fonseca, M. Belsley Tetrahedron 2011, 67, 5189-5198. [6] M. M. M. Raposo, A. M. C. Fonseca, M. C. R. Castro, M. Belsley, M. F. S. Cardoso, L. M. Carvalho, P. J. Coelho Dyes Pigments 2011, 91, 62-73

Synthesis and evaluation of NLO properties of π-conjugated donor‐acceptor systems bearing pyrrole and thiophene heterocycles

Two series of novel push-pull heterocyclic azo dyes have been synthesized and characterized. The two series of compounds were based on different combinations of π-conjugated bridges (bithiophene and thienylpyrrole) which also act simultaneously as donor groups, together with diazo(benzo)thiazolyl as acceptor moieties. Their thermal stability and electrochemical behavior were characterized, while hyper-Rayleigh scattering (HRS) was employed to evaluate their second-order nonlinear optical properties. The results of these studies have been critically analyzed together with several thienylpyrrole azo dyes reported earlier from our laboratories in which the thienylpyrrole system was used as the donor group functionalized with aryl and (benzo)thiazolyldiazene as acceptor moiety. The measured molecular first hyperpolarizabilities and the observed linear optical and redox behavior showed strong variations in function of the heterocyclic spacers used (bithiophene or thienylpyrrole) and were also sensitive to the acceptor strength of the diazenehetero(aryl) moiety.Thanks are due to the Fundacao para a Ciencia e Tecnologia (Portugal) and FEDER for financial support through the Centro de Quimica and Centro de Fisica-Universidade do Minho, Project PTDC/QUI/66251/2006 (FCOMP-01-0124-FEDER-007429), Project PTDC/CTM/105597/2008 with funding from COMPETE/FEDER and a research grant to M. C. R. Castro (UMINHO/BI/142/2009). The NMR spectrometer Bruker Avance III 400 is part of the National NMR Network and was purchased within the framework of the National Program for Scientific Re equipment, contract REDE/1517/RMN/2005 with funds from POCI 2010 (FEDER) and FCT

Push-pull bithiophene azo-chromophores bearing thiazole and benzothiazole acceptor moieties: synthesis and evaluation of their redox and nonlinear optical properties

Three series of bithiophene azo dyes functionalized with thiazol-2-yl or benzothiazol-2-yl-diazene acceptor moieties were synthesized through azo coupling reaction using 2,2´-bithiophene, 5-alkoxy-2,2´-bithiophenes, 5-N,N-dialkylamino-2,2´-bithiophenes and thiazolyl and benzothiazolyl diazonium salts as coupling components. The 5-alkoxy-2,2´-bithiophene precursors yielded the 5-thiazolylazo-5´-alkoxy-2,2´-bithiophenes, while the azo coupling reaction of 5-N,N-dialkylamino-2,2´-bithiophenes with the thiazolyldiazonium salt gave 4-thiazolyl-azo-5-N,N-dialkylamino-2,2´-bithiophenes. A different reactivity behavior was observed for 2,2-bithiophene with thiazolyl diazonium salts which gave rise to the expected azo dyes together with several arylation products. The redox behavior, thermal stability, and the first hyperpolarizability of the novel chromophores were evaluated through cyclic voltammetry, thermogravimetric analysis (TGA) and hyper-Rayleight scattering (HRS) respectively. By varying the position of the thiazolyldiazene acceptor group on the bithiophene system, the electrochemical behavior as well as the optical (linear and nonlinear) properties of the donor-acceptor -conjugated systems can readily be tuned. Push-pull 5-thiazolylazo-5´-alkoxy-2,2´-bithiophenes exhibit the most promising redox and the solvatochromic properties and second order nonlinear optical response. The redox and the optical properties also show notable variations for the different heterocyclic spacers and were also sensitive to the electronic acceptor strength of the (benzo)thiazolyldiazene moieties.Thanks are due to the Fundacao para a Ciencia e Tecnologia (Portugal) and FEDER for financial support through the Centro de Quimica and Centro de Fisica-Universidade do Minho, Project PTDC/QUI/66251/2006 (FCOMP-01-0124-FEDER-007429), Project PTDC/CTM/105597/2008 with funding from COMPETE/FEDER and a research grant to M. C. R. Castro (UMINHO/BI/142/2009). The NMR spectrometer Bruker Avance III 400 is part of the National NMR Network and was purchased within the framework of the National Program for Scientific Re-equipment, contract REDE/1517/RMN/2005 with funds from POCI 2010 (FEDER) and FCT

Synthesis, redox and quadratic nonlinear optical properties of novel thienylpyrrole derivatives containing pyridinium electron-acceptor group

Quaternized pyridine derivatives have attracted interest for many years from various perspectives, especially for potential applications in electronic/optical devices.1 Pyridinium salts are readily synthesized, highly stable, and often combine intense visible absorptions with reversible electrochemistry. Notable recently proposed applications of these compounds range from advanced telecommunications to fluorescent probes for bioimaging.2 As part of an ongoing program to develop strong charge transfer donor-acceptor thienylpyrrole systems with enhanced nonlinear optical (NLO) properties3 we have explored the potential of pyrrole 4 and thienylpyrrole-based chromophores 5-6 as efficient π-electron donor moieties functionalized with N-methylpyridinium acceptor group substituted at thiophene or pyrrole rings (Figure). Compounds 4-6 were prepared, in good yields, through Knoevenagel condensation of the precursor pyrrole 1 or thienylpyrrole 2-34 aldehydes with 1-methylpyridinium salt. On the other hand, formyl-pyrroles 1 were prepared through Paal-Knorr synthesis followed by Vilsmeier formylation. The molecular structure, especially the degree of planarity and the relative orientations of the auxiliary donor and acceptor entities, is found to strongly influence their electrochemical and NLO properties. In this communication we report on the synthesis, redox properties, thermal stability and first order nonlinear optical hyperpolarizabilities for the novel cationic chomophores 4-6. These results indicate that these compounds have goo