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Three S = 1 bis(semiquinone) complexes have been prepared. To ensure ferromagnetic intramolecular exchange coupling, the two semiquinones are attached 1,3 to a 5-substituted phenylene ring. The biradical complexes differ in their meta-substituents: 1-NMe(2)(), X = N,N-dimethylamino; 1-t-Bu, X = tert-butyl; 1-NO(2)(), X = nitro. All three structures have been determined by X-ray crystallography. Results of structural studies indicate that the biradical ligands of all three complexes have nearly identical conformations with average semiquinone ring torsions of 32 degrees +/- 2 degrees relative to the 5-substituted phenylene ring. The exchange parameter, J (Eta = -2JS(1).S(2)), ranges from +31.0 +/- 0.6 cm(-)(1) for 1-NO(2)() to +59.3 +/- 1.2 cm(-)(1) for 1-t-Bu, with J = +34.9 +/- 0.7 cm(-)(1) for 1-NMe(2)(). Since the conformations are nearly identical, the differences in exchange coupling parameter J are due to substituent effects. The experimental results are supported by Hückel theory arguments and previous computational work.
The excited-state dynamics of two conjugated bis[(porphinato)zinc(II)] (bis[PZn]) species, bis[(5,5'-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]ethyne (DD) and [(5,-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]-[(5',-15'-ethynyl-10',20'-bis(heptafluoropropyl)porphinato)zinc(II)]ethyne (DA), were studied by pump-probe transient absorption spectroscopy and hole burning techniques. Both of these meso-to-meso ethyne-bridged bis[PZn] compounds display intense near-infrared (NIR) transient S(1)-->S(n) absorptions and fast relaxation of their initially prepared, electronically excited Q states. Solvational and conformational relaxation play key roles in both DD and DA ground- and excited-state dynamics; in addition to these processes that drive spectral diffusion, electronically excited DA manifests a 3-fold diminution of S(1)-->S(0) oscillator strength on a 2-20 ps time scale. Both DD and DA display ground-state and time-dependent excited-state conformational heterogeneity; hole burning experiments show that this conformational heterogeneity is reflected largely by the extent of porphyrin-porphyrin conjugation, which varies as a function of the pigment-pigment dihedral angle distribution. While spectral diffusion can be seen for both compounds, rotational dynamics driving configurational averaging (tau approximately 30 ps), along with a small solvational contribution, account for essentially all of the spectral changes observed for electronically excited DD. For DA, supplementary relaxation processes play key roles in the excited-state dynamics. Two fast solvational components (0.27 and 1.7 ps) increase the DA excited-state dipole moment and reduce concomitantly the corresponding S(1)-->S(0) transition oscillator strength; these data show that these effects derive from a time-dependent change of the degree of DA S(1)-state polarization, which is stimulated by solvation and enhanced excited-state inner-sphere structural relaxation.
The photooxygenation of oxazolidinone-substituted enecarbamates leads to diastereomerically pure dioxetanes. The high diastereoselectivity is rationalized in terms of effective pi-facial control achieved by shielding one side of the double bond with the chiral auxiliary. The absolute configuration of the dioxetanes is assigned by derivatization to diols.
4-(N,N-Dimethyl-amino)benzonitrile (DMABN) is a prototype molecule for dual fluorescence. The anomalous emission has been attributed to an intramolecular charge-transfer (ICT) state, and the structure of the latter is still subject to some controversy. We applied a recently developed analytical gradient code for the approximate coupled-cluster singles-and-doubles method CC2 in combination with accurate basis sets to address this problem. Fully optimized excited state structures are presented for the ICT state and the so-called locally excited state, and recent transient IR and Raman measurements on the excited states are interpreted by means of calculated harmonic frequencies. Strong evidence is found for an electronic decoupling of the phenyl and the dimethyl-amino moiety, resulting in a minimum structure for the ICT state with a twisted geometry. In contrast to previous findings, the structure of this state is, at least in the gas phase, not C(2v) symmetric but distorted towards C(s) symmetry. The distortion coordinate is a pyramidalization of the phenyl carbon atom carrying the dimethyl-amino group. The results from the CC2 model are supported by single-point calculations using more elaborate coupled-cluster models (CCSD, CCSDR(3)) and by CASSCF calculations.
Water-soluble poly(alkyl aryl ether) dendrimers have been explored for their use as hosts of organic substrates in aqueous media. Prototypical photoreactions, namely, photo-Fries reaction of (a) 1-naphthyl benzoate and (b) 1-naphthyl phenyl ester and alpha-cleavage reaction of (a) dibenzyl ketones and (b) benzoin alkyl ethers, have been examined. We find that a dendritic microenvironment not only restricts the mobility of radical intermediates but also rigidly encapsulates the substrate, intermediates, and products from "leaking" to the bulk environment. Comparative studies of the same photoreactions in micellar media demonstrate that dendritic media offer much better constrainment than the micelles.
The excited-state properties of uracil, thymine, and nine other derivatives of uracil have been studied by steady-state and time-resolved spectroscopy. The excited-state lifetimes were measured using femtosecond fluorescence upconversion in the UV. The absorption and emission spectra of five representative compounds have been computed at the TD-DFT level, using the PBE0 exchange-correlation functional for ground- and excited-state geometry optimization and the Polarizable Continuum Model (PCM) to simulate the aqueous solution. The calculated spectra are in good agreement with the experimental ones. Experiments show that the excited-state lifetimes of all the compounds examined are dominated by an ultrafast (<100 fs) component. Only 5-substituted compounds show more complex behavior than uracil, exhibiting longer excited-state lifetimes and biexponential fluorescence decays. The S(0)/S(1) conical intersection, located at CASSCF (8/8) level, is indeed characterized by pyramidalization and out of plane motion of the substituents on the C5 atom. A thorough analysis of the excited-state Potential Energy Surfaces, performed at the PCM/TD-DFT(PBE0) level in aqueous solution, shows that the energy barrier separating the local S(1) minimum from the conical intersection increases going from uracil through thymine to 5-fluorouracil, in agreement with the ordering of the experimental excited-state lifetime.
The first comprehensive quantum mechanical study of solvent effects on the behavior of the two lowest energy excited states of uracil derivatives is presented. The absorption and emission spectra of uracil and 5-fluorouracil in acetonitrile and aqueous solution have been computed at the time-dependent density-functional theory level, using the polarizable continuum model (PCM) to take into account bulk solvent effects. The computed spectra and the solvent shifts provided by our method are close to their experimental counterpart. The S0/S1 conical intersection, located in the presence of hydrogen-bonded solvent molecules by CASSCF (8/8) calculations, indicates that the mechanism of ground-state recovery, involving out-of-plane motion of the 5 substituent, does not depend on the nature of the solvent. Extensive explorations of the excited-state surfaces in the Franck-Condon (FC) region show that solvent can modulate the accessibility of an additional decay channel, involving a dark n/pi* excited state. This finding provides the first unifying explanation for the experimental trend of 5-fluorouracil excited-state lifetime in different solvents. The microscopic mechanisms underlying solvent effects on the excited-state behavior of nucleobases are discussed.
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The first example of the formation of a sulfone concomitant with double bond migration during photooxidation of a sulfide is reported. Evidence is presented which demonstrates that the double bond migration is not a result of a prior acid-catalyzed rearrangement of an unrearranged sulfone precursor. This unusual observation is used to argue that the sulfone is formed via rearrangement of a hydroperoxy sulfonium ylide intermediate.
The mechanism and the origins of the stereocontrol observed in the reaction between differently substituted alkenes and stable (phosphino)(silyl)carbenes giving cyclopropanes have been studied computationally. These cyclopropanation reactions proceed via asynchronous concerted mechanisms involving early transition structures with a significant charge transfer from the carbene to the alkene moiety. The geometric features of these transition structures preclude a significant overlap between the orbitals required for secondary orbital interactions between the reactants. The stereoselectivity observed experimentally stems from favorable electrostatic and steric interactions between the reactants leading to the stereoisomers in which the phosphanyl and carbonyl or aryl groups are cis to each other.
Photooxygenation of 1,4-cyclohexadiene afforded hydroperoxy endoperoxides 3 and 4 in a ratio of 88:12. Reduction of 3 with LiAlH(4) or thiourea followed by acetylation of the hydroxyl group and KMnO(4) oxidation of the double bond gave proto-quercitol 10b. Application of the same reaction sequences to 4 resulted in the formation of gala-quercitol 14. Quercitols were easily obtained by ammonolysis of acetate derivatives in MeOH. The outcome of dihydroxylation reactions were supported by conformational analysis.
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The energetics of the (1)CH(2) + C(2)H(2) --> H + C(3)H(3) reaction are accurately calculated using an extrapolated coupled-cluster/complete basis set (CBS) method based on the cc-pVDZ, cc-pVTZ, and cc-pVQZ basis sets. The reaction enthalpy (0 K) is predicted to be -20.33 kcal/mol. This reaction has no classical barrier in either the entrance or exit channel. However, there are several stable intermediates-cyclopropene (c-C(3)H(4)), allene (CH(2)CCH(2)), and propyne (CH(3)CCH)-along the minimum energy path. These intermediates with zero-point energy corrections lie below the reactants by 87.11 (c-C(3)H(4)), 109.69 (CH(2)CCH(2)), and 110.78 kcal/mol (CH(3)CCH). The vibrationally adiabatic ground-state (VAG) barrier height for c-C(3)H(4) isomerization to allene is obtained as 45.2 kcal/mol, and to propyne as 37.2 kcal/mol. In addition, the (1)CH(2) + C(2)H(2) reaction is investigated utilizing the dual-level "scaling all correlation" (SAC) ab initio method of Truhlar et al., i.e., the UCCSD(SAC)/cc-pVDZ theory. Results show that the reaction occurs via long-lived complexes. The lifetime of the cyclopropene intermediate is obtained as 3.2 +/- 0.4 ps. It is found that the intermediate propyne can be formed directly from reactants through the insertion of (1)CH(2) into a C-H bond of C(2)H(2). However, compared to the major mechanism in which the propyne is produced through a ring-opening of the cyclopropene complex, this reaction pathway is much less favorable. Finally, the theoretical thermal rate constant exhibits a negative temperature dependence, which is in excellent agreement with the previous results. The temperature dependence is consistent with the earlier RRKM results but weaker than the experimental observations at high temperatures.