Formation and Reactions of Brønsted and Lewis Acid Adducts with Electron-Rich Heteroaromatic Compounds.

Electron-rich heteroaromatics, such as furan, thiophene and pyrrole, as well as their benzo-condensed derivatives, are of great interest as components of natural products and as starting substances for various products including high-tech materials. Although their reactions with Brønsted and Lewis acids play important roles, in particular as the primary step of various transformations, they are often disregarded and mechanistically not understood. The present publication gives a first overview about this chemistry focusing on the parent compounds. It comprises reactions with strong Brønsted acids forming adducts that can undergo intramolecular proton and/or substituent transfer reactions, ring openings or ring transformations into other heterocycles, depending on their structure. Interactions with weak Brønsted acids usually initiate oligomerizations/polymerizations. A similar behaviour is observed in reactions of these heteroaromatics with Lewis acids. Special effects are achieved when the Lewis acids are activated through primary protonation. Deuterated Brønsted acids allow straight forward deuteration of electron-rich heteroaromatics. Mercury salts as extremely weak Lewis acids cause direct metalation in a straight forward way replacing ring H-atoms yielding organomercury heterocycles. This review will provide comprehensive information about the chemistry of adducts of such heterocycles with Brønsted and Lewis acids enabling chemists to understand the mechanisms and the potential of this field and to apply the findings in future syntheses.

Combined spectroelectrochemical and theoretical study of electron-rich dendritic 2,5-diaminothiophene derivatives: N,N,N',N'-tetrakis-(4-diphenylamino-phenyl)-thiophene-2,5-diamine.

The in situ spectroelectrochemical and electron spin resonance (ESR) behavior of the recently prepared N,N,N',N'-tetrakis-(4-diphenylamino-phenyl)-thiophene-2,5-diamine 11 is presented. The results are compared to the ones of the parent 2,5-bis-diphenylamino-thiophene 41 as well as to the corresponding high-molar third dendrimer generation 8 containing the same thiophene-2,5-diamine core. The dendritic compound 11 can be reversibly oxidized in three separated steps to yield the corresponding stable monocation 11(•+), dication 11(2+), and tetracation 11(4+). A well resolved ESR spectrum of the corresponding cation radical 11(•+) with dominating splittings from two nitrogen atoms and two hydrogen atoms was observed at the first oxidation peak similar to 41(•+). The shape of the SOMOs orbitals very well correlates with the proposed distribution of the unpaired electron mainly on the thiophene center and neighboring nitrogen atoms. The spin delocalization on the central thiophene moiety in the monocations for all three model compounds 41(•+), 11(•+), and 8(•+) was confirmed. The computed single occupied molecular orbital (SOMO) for trication 11(•3+) is completely different compared to the SOMO of the corresponding monocation 11(•+), and it confirms a largely delocalized unpaired spin density. Dominating diamagnetic product was determined at the third oxidation peak, confirming the formation of a tetracation by a two electron oxidation of ESR silent dication. The positive charge is fully delocalized over the lateral parts of the molecule leading to the high stability of tetracation 11(4+). The estimated theoretical limit energy of the lowest optical transition S0 → S1 is 2.90 eV, and it can be achieved for the 3D dendrimer generation.

Synthesis and characterization of near-infrared absorbing benzannulated aza-BODIPY dyes.

A series of novel aza-diisoindolmethine dyes 9 with six different aryl and heteroaryl groups at the indole moiety have been synthesized by the addition of aryl Grignard compounds to phthalodinitrile and subsequent reaction with formamide. A plausible reaction mechanism, through a Leuckart-Wallach-type reduction has been confirmed by means of DFT calculations of the related transition and intermediate states. The corresponding boron difluoride complexes (10) of 9 were prepared in a subsequent reaction step and the spectroscopic and electrochemical properties of 9 and 10 have been investigated both experimentally and theoretically. The aza-diisoindolmethines 9 exhibit an absorption maximum in the range from 615 to 720 nm, whereas the complexes 10 show a bathochromically shifted absorption maximum between 681 and 793 nm. Measurements of 9 and 10 by cyclic voltammetry display fully reversible redox waves for the reduction and oxidation with higher potentials for 10. From the measured redox potentials, the HOMO and LUMO energy levels were calculated for 9 and 10. The frontier orbital energies, the energies of the absorption bands, as well as the orbitals involved in the absorption process were calculated with DFT and compared to the measured results of 9 and 10. The absorption maximum can be related to an intense HOMO-LUMO transition and the more-pronounced stabilization of the LUMO upon complexation is the origin of the bathochromic shift of the absorption. Additionally, single-crystal structures for two species, 10 d and 10 f, are reported.

Structural changes in 2-diarylthiophene-substituted starburst compounds upon charging: a theoretical and spectroelectrochemical study.

The role of charging in structural changes of 2-diarylaminothiophene-substituted starburst compounds is clarified by combining theoretical and spectroelectrochemical studies. A systematic and comparative theoretical calculation based on density functional theory and semiempirical Austin Model 1 (AM1) calculations is performed on the neutral and charged states of four model tris(5-diarylamino-5-thienyl)-terminated starburst compounds with a central triphenylamine and 1,3,5-triphenylbenzene moiety. Our results indicate that the charging of molecules leads to structural changes by quinoid-type components mostly on the dendrimers terminated by phenothiazinyl fragments. Based on the optimal geometries, the spectroscopic properties were calculated using the semiempirical Zerner's intermediate neglect overlap method. The presented theoretical results and the spin electron distributions of charged states and their spectra are supported by the spectroelectrochemical observations caused by the different electron localization within the studied molecules after charging. The satisfactory agreement between theoretical electronic transitions and experimental values indicates that a rational design of tunable molecular layers in organic devices based on the starburst compounds described is possible.

Radical ions of alpha,alpha'-bis(diphenylamino)-capped oligothiophenes: a combined spectroelectrochemical and theoretical study.

A new homologous series of alpha,alpha'-bis(diphenylamino)-capped oligothiophenes, prepared by a palladium-catalyzed coupling reaction of stannylated 2-diphenylaminothiophenes with 2-mono- or 2,5-dibromothiophenes and their homologues, was studied by in situ ESR/UV-vis/NIR spectroelectrochemistry. In general, the oxidation potentials of these oligothiophenes were found to be proportional to the inverse number of thiophene units. However, the potential slope of the first oxidation is completely different from that of higher oxidation steps. Trication radicals were identified by electron spin resonance (ESR) for higher thiophene homologues in addition to monocation radicals (polarons). According to the ESR hyperfine structures, the unpaired electron is delocalized in the conjugated system. In contrast to the parent alpha,alpha'-bis(diphenylamino)-capped oligothiophenes, the UV-vis/NIR absorption maxima of the oxidized species strongly depend on the number of thiophene units. Spin-restricted and spin-unrestricted Kohn-Sham density functional calculations were used to explain and to understand these properties. Absorption wavelength and intensities were calculated by the time-dependent density functional response theory. Unrestricted density functional calculations of oligothiophene dications (bipolarons) with five or more thiophene rings result in spin-broken structures which may be considered as two-polaron biradicals (polaron pairs).

Preparation and characterization of alpha,alpha'-bisdiarylamino-capped oligothiophenes.

[reaction: see text] A series of alpha,alpha'-bisdiarylamino-capped oligothiophenes C(n) were prepared by the palladium-catalyzed reaction of the dibromo compounds A(i) with diarylamines, N,N-diarylamino-substituted thiophenes or 2,2'-bithiophenes BX(j). These easily oxidizable compounds exhibit a high tendency to form amorphous glasses and characteristic electrochemical and spectroscopic properties that depend significantly on the number of their thiophene moieties.

Preparation and characterization of 2-amino-5-thienyl-substituted multicharged methinium compounds.

[reaction: see text] A series of new 2-amino-5-thienyl-substituted multicharged methinium compounds have been prepared and characterized spectroscopically and electrochemically by reaction of lithiated species of N,N-disubstituted 2-aminothiophenes with alkyl derivatives of di- and tricarbonic acids and subsequent addition of perchloric acid.

Alkylene-bridged N,N,N'N'-Tetrasubstituted Bis(2-amino-5-thiazolyl)methinium Salts-A New Class of Strongly Fluorescent Dyes.

The fluorescence-enhancing effect of the oxygen bridge in pyronine and rhodamine di- or triphenylmethane dyes, is connected to a significant hypsochrome shift of the absorption bands. This shift is absent when the bridge is an alkylene unit. While the preparation of such compounds is difficult, that of the heterocyclic analogues 1 is relatively simple and gives rise to intensely fluorescent dyes with absorptions at long wavelength.

Homoleptic Co(II), Ni(II), Cu(II), Zn(II) and Hg(II) complexes of bis-(phenyl)-diisoindol-aza-methene.

The synthesis of five homoleptic transition metal complexes of bis-(phenyl)-diisoindol-aza-methene is described together with the optical, electrochemical and thermal properties of these compounds. Additionally, crystal structures for the Co and the Zn complex are reported.

Highly charged cations from N,N,N',N'-tetrakis(4-aminophenyl)benzidine and its N,N,N',N'-tetrakis(4-methoxyphenyl)-substituted homologue studied by thin-layer in situ electron spin resonance/UV-vis-NIR spectroelectrochemistry.

The redox behavior of N,N,N',N'-tetrakis(4-aminophenyl)benzidine (A) and its N,N,N',N'-tetrakis(4-methoxyphenyl)-substituted analogue (B) was studied in detail by a new designed in situ thin layer electron spin resonance (ESR)/UV-vis-NIR spectroelectrohemical cell. The spectroelectrochemical studies of cation radicals, dications, and tetracations indicate a strong difference in stability of higher ions of two model compounds with different phenyl substitution. In cyclovoltammetry the small peak separation of the first two oxidation peaks for both compounds points to a small energetic difference in the first two electron transfers, while the peak separation of the second and third peak is quite large. A well resolved ESR spectrum of the B(*+) cation radical with dominating splittings from two nitrogen atoms is observed and an ESR silent product was determined at the third oxidation peak for both compounds, confirming the formation of a tetracation by a two electron transfer at the third voltammetric peak. In contrast to structure A a more complex redox behavior was found for B under voltammetric cycling. The reaction mechanism indicates the transformation of B in a new product P with a carbazole moiety. This compound can form a highly stabilized cation radical P(*+). A theoretical study based on density functional theory calculations has clarified the role of charging in changes to the structures of both triarylamine derivatives A and B.

Thiophene-thiophene versus phenyl-phenyl coupling in 2-(Diphenylamino)-Thiophenes: an ESR-UV/Vis/NIR spectroelectrochemical study.

The oxidation of several 2-diphenylamino-substituted thiophenes and N,N'-bis(2-diphenylamino-5-thienyl)-substituted phenylene-1,4-diamines with different substitution patterns in the 5-position of their thiophene moieties was studied by cyclovoltammetric and spectroelectrochemical measurements (ESR, UV/Vis/NIR). These measurements revealed both the structure of the oxidation products obtained and that of their cationic intermediates, as well as the pathway of their formation and follow-up reactions. Thus, the formation of the radical cations in the first electrochemical oxidation step of the target molecules was demonstrated. Depending on the substitution pattern, these radical cations give rise to several consecutive processes to a different extent. Thus radical dimers, dehydrodimers with a 2,2-bithiophene, 3,3'-bithiophene, or benzidine structure, as well as radical monocationic and dicationic products can be formed. For N,N'-bis(2-diphenylamino-5-thienyl)-substituted phenylene-1,4-diamines rather stable radical cations and dications were primarily formed and unambiguously identified. These species were transformed into 2,2'-bithiophene dimers and oligomers when the 5-position of the thiophene moiety was unsubstituted. By in situ spectroelectrochemistry the influence of the substituents on the type of coupling reactions can be demonstrated and followed in detail.

Structure-property relationships in push-pull amino/cyanovinyl end-capped oligothiophenes: quantum chemical and experimental studies.

A series of push-pull chromophores built around thiophene-based pi-conjugating spacers and bearing various types of amino donors and cyanovinyl acceptors have been analyzed by means of UV-vis-NIR, IR, and Raman spectroscopic measurements in the solid state as well as in solution. The intramolecular charge transfer (ICT) of these pi-conjugated systems has also been tested by analyzing the ability of the solute molecules to undergo shifts in their fluorescence emission maxima with increasing solvent polarity. These push-pull oligomers also display an attractive electrochemical behavior since they generate stable species both upon oxidation and reduction. Oxidation mainly involves changes in the electron-rich aminooligothienyl half-part of the molecule and leads to the formation of stable cations. On the other hand, reduction to radical anions and dianions is mainly cyanovinyl-centered but also affects the pi-conjugated electron relay. Density functional theory (DFT) calculations have been carried out to help the assignment of the most relevant electronic and vibrational features and to derive useful information about the molecular structure of these NLO-phores.

The interplay of thio(seleno)amide/vinylogous thio(seleno)amide "resonance" and the anisotropic effect of thiocarbonyl and selenocarbonyl functional groups.

Amino-substituted thio(seleno)acrylamides 1-4 were synthesized and their 1H and 13C NMR spectra assigned. Both the NMR data and the results of theoretical calculations at the ab initio level of theory were employed to elucidate the adopted structures of the compounds in terms of E/Z isomerism and s-cis/s-trans configuration. In the case of the asymmetrically N(Me)Ph-substituted compounds, ab initio GIAO-calculated ring current effects of the N-phenyl group were applied to successfully determine the preferred conformer bias. The restricted rotations about the two C-N partial double bonds were studied by DNMR and the barriers to rotation (DeltaG(c)++) determined at the coalescence temperatures, and these were discussed with respect to the structural differences between the compounds. The barriers to rotation were also calculated at the ab initio level of theory where the best results (R(2) = 0.8746) were obtained only with inclusion of the solvent at the SCIPCM-HF/6-31G* level of theory. The calculations also provided means of assessing structural influences which were not available due to inaccessible rotation barriers. By means of natural bond orbital (NBO) analysis of 1-4, the occupation numbers of nitrogen lone pairs and bonding/antibonding pi/pi orbitals were shown to quantitatively describe thio(seleno)amide/vinylogous thio(seleno)amide "resonance". Finally, the thio(seleno)carbonyl anisotropic effect was quantitatively calculated by the GIAO method and visualized by isochemical shielding surfaces (ICSS). Only marginal differences between the two anisotropic effects were calculated and are therefore of questionable utility for previous and future applications with respect to stereochemical assignments.