X-ray crystal structures and conformational analysis of bicyclo[5.3.1]undec-1(11)-enes: twisting versus pyradidalization in anti-bredt olefins

The synthesis of several 9,11,11-trihalo[5.3.1]propellanes and their 4-dimethylsila analogues is described. They solvolyze under formation of the corresponding isomeric 7,9,11- trihalobicyclo[5.3.l]undec-1(11)-enes which are "anti-Bredt" olefins with a strained trans double bond in a bridged eight-membered ring; in the presence of nucleophiles such as water or ethanol, the corresponding 7-hydroxy or 7-ethoxy derivatives, respectively, are obtained. On the basis of the X-ray crystal structures of four of these compounds (1a, 9a, 15, 17b), the effect of strain and of the substitution pattern on the degree of twisting and pyramidalization of the double bond is discussed.     

Comparative studies of the trans-cis photoisomerizations of azobenzene and a bridged azobenzene

Using density-functional-based molecular dynamics simulations, we have performed comparative studies of the trans-cis isomerizations of azobenzene and bridged azobenzene (B-Ab) 5,6-dihydrodibenzo[c,g][1,2]diazocine induced by nπ* electronic excitation. The quantum yields found in our calculations, 45% for the bridged azobenzene versus 25% for azobenzene, are consistent with the experiment. Both isomerization processes involve two steps: (1) Starting from the trans structure, each molecule moves on its S(1) excited-state potential energy surface, via rotation around the NN bond, to an avoided crossing near the S(1)/S(0) conical intersection, where de-excitation occurs. (2) Subsequently, in the electronic ground state, there is further rotation around the NN bond, accompanied by twisting of the phenyl rings around their CN bonds, until the cis geometry is achieved. Because of its lower symmetry and smaller initial CNNC dihedral angle, the bridged azobenzene has a much shorter lifetime for the S(1) excited state, about 30 fs, as compared to about 400 fs for azobenzene. However, we find that the complete isomerizations have approximately the same time scales. Although the bridging feature in trans-B-Ab does not hinder rotation around the NN bond in step 1, it makes twisting of the two phenyl rings around the CN bonds much slower in step 2.     

Substituent-dependent photoinduced intramolecular charge transfer in N-aryl-substituted trans-4-aminostilbenes

The photochemical behavior of trans-4-(N-arylamino)stilbene (1, aryl = 4-substituted phenyl) in solvents more polar than THF is strongly dependent on the substituent in the N-aryl group. This is attributed to the formation of a twisted intramolecular charge transfer (TICT) state for those with a methoxy (1OM), methoxycarbonyl (1CO), or cyano (1CN) substituent but not for those with a methyl (1Me), hydrogen (1H), chloro (1Cl), or trifluoromethyl (1CF) substituent. On the basis of the ring-bridged model compounds 3-6, the TICT states for 1CN and 1CO result from the twisting of the anilino-benzonitrilo C-N bond, but for 1OM it is from the twisting of the stilbenyl-anilino C-N bond, both of which are distinct from the TICT states previously proposed for N,N-dimethylaminostilbenes.     

Synthesis,structural characterization and photoisomerization of cyclic stilbenes

Six cyclic stilbene derivatives with hindered free rotation around the C(vinyl)–C(phenyl) single bond were synthesized by McMurry coupling. The torsion angles around the double and the single bond, and the CC bond length were obtained for many of the compounds from their solid-state structures. The photochemical isomerization was subsequently investigated for all derivatives under various conditions. The parent 1-(1-tetralinylidene)tetralin underwent efficient oxidative electrocyclization. The 2,2,2′,2′-tetramethylated analogue was resistant towards photooxidation, however, its cis-isomer thermally reisomerized to the more stable trans-isomer.     

烯酮类化合物发光行为的研究

本工作合成了几种带桥键结构的烯酮类化合物, 研究了它们的光谱和光物理行为.结果表明那些分子内的双键因桥键化而成环的化合物具有很强的荧光量子产率,相反那些未桥键化或桥键化而双键未处于环内者则仅有较弱的荧光强度, 工作中还观察到该类化合物的"负溶致动力学效应"以及其荧光强度强烈地依赖于所用溶剂极性等现象,对所得结果进行了初步的讨论.     

Multicoordinational excited state twisting of indan-1,3-dione derivatives

Excited state relaxation of indan-1,3-dione derivatives with different substituents attached to the phenyl ring and with the bridged amino group was investigated by means of the steady-state fluorescence and femtosecond time-resolved absorption pump–probe spectroscopy. Bridging of the amino group increases the fluorescence quantum yield and the excited state lifetime. Analysis of the results indicates that the phenyl ring twisting around a single central bond leads to the nonradiative state formation and to subsequent fast relaxation to the ground state. Double bond twisting takes place in molecules with the bridged amino group and causes a large Stokes shift and slightly slower excited state relaxation.     

Electronic optical activity of (−)-α-phellandrene

Molecular orbital computations on the sign and magnitude of the Cotton effect of (?)-α-phellandrene (a conjugated diene) and the separate twisted butadiene chromophore were performed using configuration interaction (CI) and the random phase approximation (RPA) methods with a standard minimal basis set of STO/3G orbitals. The relative contributions to the rotatory strength of (?)-α-phellandrene which arise from a twist in the diene unit, the allylic axial bond, and nonplanarity of either one or both of the C?C double bonds were determined by examining various molecular geometries. This theoretical study confirms that the allylic axial substituent/bond provides the largest contribution to the longwavelength Cotton effect. It is found that the rotatory strength arising from distortion of the planar geometry of the double bonds tends to cancel the rotatory strength arising from the sense of twist of the diene unit. The computed energies suggest that molecular geometries where the trisubstituted bond is kept planar, and where twisting is allowed about the cis double bond, may be favored over geometries where torsion is allowed about both double bonds.     

Beiträge zur Chemie der Pyrrolpigmente, 22. Mitt.: Die Selektivität photochemischer Isomerisierungen bifunktioneller und bichromophorer Verbindungen mit geometrischer Isomerie — Studien an Gallenpigmentstrukturmodellsystemen

Bifunctional molecules of structural type5 and6 (resembling bilatrienes-abc) containing double bonds show a strong selectivity toward photochemical isomerization: Starting from the (Z,Z)-isomer the (Z,E)-isomer, is obtained but none of the (E,E)-isomer. The reason for this selectivity could be established by conformational analysis,PPP-calculations and the photochemical behaviour of N-methylated derivatives (8 and9). The configuration, (Z) at a particular double bond induces a certain amount of twisting at the attached, single bond on steric reasons. This twisting is smaller than the twisting observed for the double-single-bond system of configuration (E). It is this differrential twist which causes the observed selectivity.On the other hand bichromophoric systems like2 or4 give no selectivity on photochemical isomerization.Based on the experimental data an energy scheme for the singlet isomerization paths for bichromophoric and bifunctional systems was deduced.

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21. Mitt.:H. Falk, K. Grubmayr, K. Thirring undN. Gurker, Mh. Chem.109, 1183 (1978).     

ON THE MECHANISM OF WAVELENGTH REGULATION IN VISUAL PIGMENTS

Abstract— The contributions of different factors that might be responsible for the 500 nm absorption maximum of bovine rhodopsin are evaluated in detail. These include: (1) electrostatic interactions between the chromophore and a charged amino acid on the apoprotein; (2) exciton interactions with aromatic amino acids; (3) twisting about single bonds which have considerable double bond character; (4) weak interactions between the Schiff base and a putative counter-ion. Analysis of these mechanisms in terms of theoretical and experimental results suggests that(2–4) are all capable of contributing to the protein induced spectral shifts. However, the "external point charge" model proposed previously, i.e. mechanism (1), appears to provide the crucial interaction. In this paper, the theoretical basis for this model is discussed in detail. The model is briefly evaluated in light of the amino-acid sequence of bovine rhodopsin and possible implications for other visual pigments are considered.     

Second generation light-driven molecular motors. Unidirectional rotation controlled by a single stereogenic center with near-perfect photoequilibria and acceleration of the speed of rotation by structural modification

Nine new molecular motors, consisting of a 2,3-dihydro-2-methylnaphtho[2,1-b]thiopyran or 2,3-dihydro-3-methylphenanthrene upper part and a (thio)xanthene, 10,10-dimethylanthracene, or dibenzocycloheptene lower part, connected by a central double bond, were synthesized. A single stereogenic center, bearing a methyl substituent, is present in each of the motors. MOPAC93-AM1 calculations, NMR studies, and X-ray analysis revealed that these compounds have stable isomers with pseudoaxial orientation of the methyl substituent and less-stable isomers with pseudoequatorial orientation of the methyl substituent. The photochemical and thermal isomerization processes of the motors were studied by NMR and CD spectroscopy. The new molecular motors all show two cis-trans isomerizations upon irradiation, each followed by a thermal helix inversion, resulting in a 360 degrees rotation around the central double bond of the upper part with respect to the lower part. The direction of rotation is controlled by a single stereogenic center created by the methyl substituent at the upper part. The speed of rotation, governed by the two thermal steps, was adjusted to a great extent by structural modifications, with half-lives for the thermal isomerization steps ranging from t(1/2)(theta) 233-0.67 h. The photochemical conversions of two new motors proceeded with near-perfect photoequilibria of 1:99.     

New 6,6'-disubstituted-binaphthol derivatives as chiral dopants: Synthesis and temperature dependence of molecular conformations

A number of new chiral binaphthol (BN) derivatives with different substituents R,R' in the 6,6'-positions in open (BN-diethylethers) and bridged forms (BN-acetals) have been synthesized. The syntheses of the chiral 6,6'-disubstituted-2,2'-diethoxy-1,1'-binaphthyls (R,R' = -CH'CHCHO, -CH'CH-(p-BrPh), -CH'CH-(p-CHOPh)) and the chiral 9,14-disubstituted-dinaphtho [2,1-d:1',2'-f][1,3] dioxepins with R,R' = -CH'CH-(p-BrPh), -CH'CH-(p-CHOPh) are reported for the first time. The possible liquid crystalline properties and molar twisting powers (β_M) in three different nematic liquid crystals (LCs) of the BN derivatives were investigated. Derivatives with spatially extended substituents in the 6,6'-positions (e.g. styryl or vinyl) show unusually high molar twisting power (up to 124.5μm^-1). A direct correlation between the magnitude of β_M and the length of the substituents was found. Bridged forms, in which the dihedral angle θ between the naphthyl moieties is ≊ 54°, show higher twisting power than the corresponding open forms, where θ is allowed to vary around 90° resulting in an equilibrium between transoid and cisoid forms. From the different temperature dependencies of β_M of the open and bridged BN_s, a molecular model was developed relating the molecular conformation and twisting power. Although no mesophase was found in any of the compounds synthesized, they can be considered as important precursors for the synthesis of potential chiral BN-containing LCs.     

Conformations and reactions of bicyclo[3.2.1]oct-6-en-8-ylidene

Bicyclo[3.2.1]oct-6-en-8-ylidene (1) can assume either the conformation of "classical" carbene 1a or that of foiled carbene 1b in which the divalent carbon bends toward the double bond. Oxadiazoline precursors for the generation of 1 were prepared, followed by photochemical and thermal decomposition as well as flash vacuum pyrolysis (FVP) of a tosyl hydrazone sodium salt precursor, to give a number of rearrangement products. Matrix isolation experiments demonstrate the presence of a diazo intermediate and methyl acetate in all photochemical and thermal precursor reactions. The major product from rearrangements of "classical" bridged carbene 1a is bicyclo[3.3.0]octa-1,3-diene as a result of an alkyl shift, while dihydrosemibullvalene formed from a 1,3-C-H insertion. In contrast, thus far unknown strained bicyclo[4.2.0]octa-1,7-diene formed by a vinyl shift in foiled carbene 1b. The experimental results are corroborated by density functional theory (DFT), MP2, and G4 computations.     

Molecular mechanics of retinal: Twisting mechanism of chromophore in visual pigment

In a model calculation pulli 11-cis retinal (9-cis retinal) from both end sides, the 11–12 double bond (9–10 double bond) is found to be selectively twisted. This property is promising for the twisting mechanism of retinal chromophore in visual pigment as assumed by the Kakitani and Kakitani torsion model.     

Structure of the conical intersections driving the cis-trans photoisomerization of conjugated molecules

High-level ab initio calculations show that the singlet photochemical cis-trans isomerization of organic molecules under isolated conditions can occur according to two distinct mechanisms. These mechanisms are characterized by the different structures of the conical intersection funnels controlling photoproduct formation. In nonpolar (e.g. hydrocarbon) polyenes the lowest-lying funnel corresponds to a (CH)3 kink with both double and adjacent single bonds twisted, which may initiate hula-twist (HT) isomerization. On the other hand, in polar conjugated systems such as protonated Schiff bases (PSB) the funnel shows a structure with just one twisted double bond. The ground-state relaxation paths departing from the funnels indicate that the HT motion may take place in nonpolar conjugated systems but also that the single-bond twist may be turned back, whereas in free conjugated polar molecules such as PSB a one-bond flip mechanism dominates from the beginning. The available experimental evidence either supports these predictions or is at least consistent with them.     

Unsaturated binuclear cyclopentadienylrhenium carbonyl derivatives: metal-metal multiple bonds and agostic hydrogen atoms

The cyclopentadienylrhenium carbonyls Cp 2Re 2(CO) n (Cp = eta (5)-C 5H 5; n = 5, 4, 3, 2) have been studied by density functional theory. The global minima for the Cp 2Re 2(CO) n ( n = 5, 4, 3, 2) derivatives are predicted to be the singly bridged structure Cp 2Re 2(CO) 4(mu-CO) with a formal Re-Re single bond; the doubly semibridged structure Cp 2Re 2(CO) 4 with a formal ReRe double bond; the triply bridged structure Cp 2Re 2(mu-CO) 3 with a formal ReRe triple bond; and the doubly bridged structure Cp 2Re 2(mu-CO) 2, respectively. The first three of these predicted structures have been realized experimentally in the stable compounds (eta (5)-C 5H 5) 2Re 2(CO) 4(mu-CO), (eta (5)-Me 5C 5) 2Re 2(CO) 4 and (eta (5)-Me 5C 5) 2Re 2(mu-CO) 3. In addition, structures of the type Cp 2Re-Re(CO) n with both rings bonded only to one metal and unknown in manganese chemistry are also found for rhenium but at energies significantly above the global minima. The unsaturated Cp 2Re-Re(CO) n structures ( n = 4, 3, 2) have agostic Cp hydrogen atoms forming C-H-Re bridges to the unsaturated Re(CO) n group with a Re-H distance as short as 2.04 A.     

New one-dimensional azido-bridged manganese(II) coordination polymers exhibiting alternating ferromagnetic-antiferromagnetic interactions: structural and magnetic studies

Five Mn(II)[bond]azido coordination polymers of formula [Mn(L)(N(3))(2)](n) have been synthesized and crystallographically characterized, and their magnetic properties studied, where L's are the bidentate Schiff bases obtained from the condensation of pyridine-2-carbaldehyde with aniline (1) and its derivatives p-toluidine (2), m-toluidine (3), p-chloroaniline (4), and m-chloroaniline (5). All the complexes consist of the zigzag Mn(II)[bond]azido chains in which the Mn(II) ions are alternately bridged by two end-to-end (EE) and two end-on (EO) azido ligands, the cis-octahedral coordination being completed by the two nitrogen atoms of the Schiff base ligands. Compound 2 is unique in that the Mn[bond](EE-N(3))(2)[bond]Mn ring adopts an unusual twist conformation with the two linear azido bridges crossing each other. By contrast, the rings in the other compounds take the usual chair conformation with the two azido bridges parallel. The double EO bridging fragments in the complexes are similar with the bridging angles (Mn[bond]N[bond]Mn) ranging from 99.6 degrees to 104.0 degrees. Magnetic analyses reveal that alternating ferro- and antiferromagnetic interactions are mediated through the alternating EO and EE azido bridges with the J(F) and J(AF) parameters in the ranges of 4.1-8.0 and -11.8 to -15.4 cm(-1), respectively. Finally, the magnetostructural correlations are investigated. The present complexes follow the general trend that the ferromagnetic interaction through the double EO bridge increases with the Mn[bond]N[bond]Mn bridging angle, while the antiferromagnetic interaction through the double EE bridge is dependent on the distortion of the Mn[bond](N(3))(2)[bond]Mn ring from planarity toward the chair conformation and the Mn[bond]N[bond]N angle.     

Relationship between photoisomerization path and intersection space in a retinal chromophore model

A low-lying segment of the intersection space (IS) between the excited-state and the ground-state energy surfaces of a retinal chromophore model has been mapped using ab initio CASSCF computations. Analysis of the structural relationship between the computed IS cross-section and the excited state Z --> E isomerization path shows that these are remarkably close both in energy and in structure. Indeed, the IS segment and the Z --> E path remain roughly parallel and merge only when the double bond reaches a 70 degree twisting. This finding supports the idea that, in certain chromophores, a more extended segment of IS, and not a single conical intersection, contributes to the decay to the ground state.     

Primary photoprocess in vision: minimal motion to reach the photo- and bathorhodopsin intermediates

According to time-resolved spectroscopic measurements, the initial step of the photoreaction of rhodopsin occurs with a time constant of approximately 200 fs. The whole or a part of the retinal molecule cannot move any significant distance in such a short time. In this paper, we propose instead a minimal motion that accomplishes the important task of guiding the molecule to a configuration where it can decay to the ground-state surface, with a minimal loss of strain energy. This motion is proposed to involve a -90 degrees twisting of the C11=C12 double bond and a simultaneous twisting around two other double bonds in retinal to minimize the geometrical changes along the reaction path. The ONIOM method (complete active space self-consistent field for retinal and AMBER for the peptides) is used in a chromophore-cavity model to elucidate and confirm important features of the mechanism. The potential energy surface (PES) obtained according to the proposed mechanism show all of the characteristics of a fast photoreaction, meaning a downhill reaction path from the Franck-Condon point to an avoided crossing between S(1) and S(0). In this motion, only a few carbon and hydrogen atoms move more than 0.3 A, and the retinal structure is conserved in the protein cavity. We propose that the photorhodopsin intermediate is a retinal molecule formed on the excited-state PES. Bathorhodopsin, however, is a ground-state intermediate, still located inside the protein cavity.     

Photolyse von 2, 2, 4, 4-Tetramethylcyclobutanon-Derivaten

The photochemical reactions of the 2, 2, 4, 4-tetramethylcyclobutanones 1–6 carrying various substituents in 3-position were investigated. The major reaction in alcoholic solution or in the presence of other protic compounds was the formation of the semicyclic acetals 7–12 . Parallel to this reaction decarbonylation occurred, leading to stable cyclopropane derivatives in some cases, depending on the substituents present. Cyclopropanes with an exocyclic double bond underwent ring opening easily or, in case of an exocyclic carbon-nitrogen double bond, added alcohol, thus forming cyclopropane O, N-ketals. Alkyl-acyl biradicals are proposed as common intermediates for both photoreactions. Based on analogy to similar photoreactions reported in the literature, the formation of the semicyclic acetals is assumed to involve a carbene intermediate.     

Photochemistry of poly(vinyl cinnamylideneacetate) and related compounds

The photochemistry of the cinnamylideneacetyl group was investigated with respect to a photosensitive polymer, poly(vinyl cinnamylideneacetate). The photochemical reaction of 1,4-butanediol dicinnamylideneacetate was intramolecular cyclobutane formation. The photosensitive polymer underwent dimerization of the cinnamylideneacetyl moiety to form a cyclobutane ring. The reactivity of the double bond adjacent to the carbonyl group was larger than that of the double bond adjacent to the phenyl group in the chromophore. The quantum yield of the reaction was larger in the solid state than in solution: ? > 1.2 in crystalline state, ? = 0.5 in polymer film, ? = 0.1 in solution. The reaction was sensitized by triplet sensitizers (E_T > 42 kcal/mole). The thermal reaction of the polymer was completely different from the photochemical reaction. A radical initiator was very ineffective for reaction of the polymer.