Ultrafast excited state dynamics of provitamin D3 and analogs in solution and in lipid bilayers.

The photochemical ring-opening reaction of 7-dehydrocholesterol (DHC, provitamin D3) is responsible for the light-initiated formation of vitamin D3 in mammalian skin membranes. Visible transient absorption spectroscopy was used to explore the excited state dynamics of DHC and two analogs: ergosterol (provitamin D2) and DHC acetate free in solution and confined to lipid bilayers chosen to model the biological cell membrane. In solution, the excited state dynamics of the three compounds are nearly identical. However, when confined to lipid bilayers, the heterogeneity of the lipid membrane and packing forces imposed on the molecule by the lipid alter the excited state dynamics of these compounds. When confined to lipid bilayers in liposomes formed using DPPC, two solvation environments are identified. The excited state dynamics for DHC and analogs in fluid-like regions of the liposome membrane undergo internal conversion and ring-opening on 1 ps-2 ps time scales, similar to those observed in isotropic solution. In contrast, the excited state lifetime of a subpopulation in regions of lower fluidity is 7 ps-12 ps. The long decay component is unique to these liposomes and results from the structural properties of the lipid bilayer. Additional measurements in liposomes prepared with lipids having slightly longer or shorter alkane tails support this conclusion. In the lipid environments studied, the longest lifetimes are observed for DHC. The unsaturated sterol tail of ergosterol and the acetate group of DHC acetate disrupt the packing around the molecule and permit faster internal conversion and relaxation back to the ground state.

Exceptional Photochemical Stability of the Co-C Bond of Alkynyl Cobalamins, Potential Antivitamins B12 and Core Elements of B12-Based Biological Vectors.

Alkynylcorrinoids are a class of organometallic B12 derivatives, recently rediscovered for use as antivitamins B12 and as core components of B12-based biological vectors. They feature exceptional photochemical and thermal stability of their characteristic extra-short Co-C bond. We describe here the synthesis and structure of 3-hydroxypropynylcobalamin (HOPryCbl) and photochemical experiments with HOPryCbl, as well as of the related alkynylcobalamins: phenylethynylcobalamin and difluoro-phenylethynylcobalamin. Ultrafast spectroscopic studies of the excited state dynamics and mechanism for ground state recovery demonstrate that the Co-C bond of alkynylcobalamins is stable, with the Co-N bond and ring deformations mediating internal conversion and ground state recovery within 100 ps. These studies provide insights required for the rational design of photostable or photolabile B12-based cellular vectors.

Primed for Efficient Motion: Ultrafast Excited State Dynamics and Optical Manipulation of a Four Stage Rotary Molecular Motor.

All isomers of a four stage rotary molecular motor, dimethyl-tetrahydro-bi(cyclopenta[α]napthal-enylidene), are studied with ultrafast transient absorption spectroscopy. Single and two pulse excitations (pump and delayed repump with a different wavelength) are used to optically probe the excited state dynamics. These measurements demonstrate that this motor is not only designed for unidirectional isomerization, but is also "primed" for efficient rotary motion. The yield for photoisomerization from the stable P-cis isomer to the metastable M-trans isomer is 85% ± 10%, while the yield for the undesired back reaction is ca. 0.08 (+0.02, -0.05). The yield for photoisomerization from stable P-trans to the metastable M-cis isomer is ca. 85% ± 3% and the yield for the back reaction is 15% ± 3%. Excitation of P-trans in the lowest singlet state results in formation of a dark state on a 3.6 ps time scale and formation of the M-cis isomer on a ca. 12 ps time scale. Excitation of P-cis in the lowest singlet state results in formation of a dark state on ca. 13 ps time scale and formation of the M-trans isomer on a 71 ps time scale. Excitation of either isomer at 269 nm, higher in the excited state manifold, accesses additional excited state pathways, but does not change the ultimate product formation. This result suggests that pulse sequences accessing higher excited states may provide a tool to manipulate the molecular motor. Pulse sequences using a 269 nm pump pulse and a 404 nm repump pulse are able to increase the yield of the P-cis to M-trans reaction but only decrease the yield of the P-trans to M-cis reaction. These pulse sequences are unable to access reaction pathways that bypass the helix inversion step, although other wavelengths and time delays might yet provide optical control of the entire reaction cycle. We propose intermediates and candidate conical intersections between all four isomers.

Polarized XANES Monitors Femtosecond Structural Evolution of Photoexcited Vitamin B12.

Ultrafast, polarization-selective time-resolved X-ray absorption near-edge structure (XANES) was used to characterize the photochemistry of vitamin B12, cyanocobalamin (CNCbl), in solution. Cobalamins are important biological cofactors involved in methyl transfer, radical rearrangement, and light-activated gene regulation, while also holding promise as light-activated agents for spatiotemporal controlled delivery of therapeutics. We introduce polarized femtosecond XANES, combined with UV-visible spectroscopy, to reveal sequential structural evolution of CNCbl in the excited electronic state. Femtosecond polarized XANES provides the crucial structural dynamics link between computed potential energy surfaces and optical transient absorption spectroscopy. Polarization selectivity can be used to uniquely identify electronic contributions and structural changes, even in isotropic samples when well-defined electronic transitions are excited. Our XANES measurements reveal that the structural changes upon photoexcitation occur mainly in the axial direction, where elongation of the axial Co-CN bond and Co-NIm bond on a 110 fs time scale is followed by corrin ring relaxation on a 260 fs time scale. These observations expose features of the potential energy surfaces controlling cobalamin reactivity and deactivation.

Toward the Design of Photoresponsive Conditional Antivitamins B12: A Transient Absorption Study of an Arylcobalamin and an Alkynylcobalamin.

Cobalamins are of widespread importance in biology. Both of the cofactors essential for human metabolism, the organocobalamins coenzyme B12 and methylcobalamin, are highly photolabile, as are other alkylcobalamins. The alkynylcobalamin phenylethynylcobalamin (PhEtyCbl) and the arylcobalamin 4-ethylphenylcobalamin (EtPhCbl) with "atypical" Co-C-bonds to unsaturated carbons, were recently designed as metabolically inert cobalamins, classified as "antivitamins B12". The further development of an ideal light-activated or "conditional" antivitamin B12 would require it to be readily converted by light into an active B12 vitamin form. Very photolabile "antivitamins B12" would also represent particularly useful scaffolds for therapeutic light-activated reagents. Here, the photoactive arylcobalamin EtPhCbl and the remarkably photostable alkynylcobalamin PhEtyCbl are examined using femtosecond to picosecond UV-visible transient absorption spectroscopy. PhEtyCbl undergoes internal conversion to the ground state with near unit quantum yield on a time scale < 100 ps and an activation energy of 12.6 ± 1.4 kJ/mol. The arylcobalamin EtPhCbl forms an excited state with a ca. 247 ps lifetime. This excited state branches between internal conversion to the ground state and formation of a long-lived base-off species with a quantum yield of ∼9%. Anaerobic steady state photolysis of "light-sensitive" EtPhCbl results in the formation of cob(II)alamin, but only with quantum yield <1%. Hence, our studies suggest that suitably modified arylcobalamins may be a rational basis for the design of photoresponsive "antivitamins B12".

Probing the Biexponential Dynamics of Ring-Opening in 7-Dehydrocholesterol.

Our prior discovery of a novel biexponential photochemical ring-opening in 7-dehydrocholesterol (DHC) to previtamin D3 [ Tang J. Chem. Phys. 2011 , 134 , 104503 ] is further explored with ultrafast transient absorption spectroscopy, and the results are compared with recently reported high-level theoretical calculations. Three types of experiments are reported. First, variation of the excitation wavelength from 297 to 266 nm leaves the excited state dynamics unaffected. The biexponential decay of the excited state absorption is independent of excitation wavelength with time constants of 0.57 ± 0.06 and 1.88 ± 0.09 ps, in excellent agreement with the results reported earlier (0.56 ± 0.06 and 1.81 ± 0.15 ps) following excitation at 266 nm. Second, variation of the chirp of the excitation pulse influences the relative amplitude of the fast and slow decay components but has no influence on the photoproduct yield. Third, a 545 nm pulse delayed by 0.64 ps with respect to the initial 266 nm pulse was used to perturb the "slow" population and probe the influence of additional electronic or vibrational energy on the reaction process. The results show ultrafast internal conversion Sn → S1 on a ca. 150 fs time scale but no subsequent effect on the reaction dynamics. The experiments reported here are consistent with the recent state averaged complete active space self-consistent field ab initio multiple spawning (SA-CASSCF-AIMS) calculations of Snyder et al. [ J. Phys. Chem. Lett. 2016 , 7 , 2444 ] that assign the biexponential decay to nonequilibrium dynamics related to the opening and closing motion of the cyclohexadiene ring moiety on the excited state surface. These new experiments support the model prediction that the biexponential dynamics does not involve multiple minima and demonstrate the direction for new experimental designs to manipulate the product yields and pathways.

Solvent dependent branching between C-I and C-Br bond cleavage following 266 nm excitation of CH2BrI.

It is well known that ultraviolet photoexcitation of halomethanes results in halogen-carbon bond cleavage. Each halogen-carbon bond has a dominant ultraviolet (UV) absorption that promotes an electron from a nonbonding halogen orbital (nX) to a carbon-halogen antibonding orbital (σ*C-X). UV absorption into specific transitions in the gas phase results primarily in selective cleavage of the corresponding carbon-halogen bond. In the present work, broadband ultrafast UV-visible transient absorption studies of CH2BrI reveal a more complex photochemistry in solution. Transient absorption spectra are reported spanning the range from 275 nm to 750 nm and 300 fs to 3 ns following excitation of CH2BrI at 266 nm in acetonitrile, 2-butanol, and cyclohexane. Channels involving formation of CH2Br + I radical pairs, iso-CH2Br-I, and iso-CH2I-Br are identified. The solvent environment has a significant influence on the branching ratios, and on the formation and stability of iso-CH2Br-I. Both iso-CH2Br-I and iso-CH2I-Br are observed in cyclohexane with a ratio of ~2.8:1. In acetonitrile this ratio is 7:1 or larger. The observation of formation of iso-CH2I-Br photoproduct as well as iso-CH2Br-I following 266 nm excitation is a novel result that suggests complexity in the dissociation mechanism. We also report a solvent and concentration dependent lifetime of iso-CH2Br-I. At low concentrations the lifetime is >4 ns in acetonitrile, 1.9 ns in 2-butanol and ~1.4 ns in cyclohexane. These lifetimes decrease with higher initial concentrations of CH2BrI. The concentration dependence highlights the role that intermolecular interactions can play in the quenching of unstable isomers of dihalomethanes.

Ultrafast electrocyclic ring opening of 7-dehydrocholesterol in solution: the influence of solvent on excited state dynamics.

Broadband UV-visible femtosecond transient absorption spectroscopy and steady-state integrated fluorescence were used to study the excited state dynamics of 7-dehydrocholesterol (provitamin D(3), DHC) in solution following excitation at 266 nm. The major results from these experiments are: (1) The excited state absorption spectrum is broad and structureless spanning the visible from 400 to 800 nm. (2) The state responsible for the excited state absorption is the initially excited state. Fluorescence from this state has a quantum yield of ∼2.5 × 10(-4) in room temperature solution. (3) The decay of the excited state absorption is biexponential, with a fast component of ∼0.4-0.65 ps and a slow component 1.0-1.8 ps depending on the solvent. The spectral profiles of the two components are similar, with the fast component redshifted with respect to the slow component. The relative amplitudes of the fast and slow components are influenced by the solvent. These data are discussed in the context of sequential and parallel models for the excited state internal conversion from the optically excited 1(1)B state. Although both models are possible, the more likely explanation is fast bifurcation between two excited state geometries leading to parallel decay channels. The relative yield of each conformation is dependent on details of the potential energy surface. Models for the temperature dependence of the excited state decay yield an intrinsic activation barrier of ∼2 kJ/mol for internal conversion and ring opening. This model for the excited state behavior of DHC suggests new experiments to further understand the photochemistry and perhaps control the excited state pathways with optical pulse shaping.

Probing the Formation and Conformational Relaxation of Previtamin D3 and Analogues in Solution and in Lipid Bilayers.

The photosynthesis of vitamin D3 in mammalian skin results from UV-B irradiation of provitamin D3 (7-dehydrocholesterol, DHC) at ca. 290 nm. Upon return to the ground state, the hexatriene product, previtamin D3, undergoes a conformational equilibration between helical gZg and more planar tZg and tZt forms. The helical gZg forms provide a pathway for the formation of vitamin D3 via a [1,7]-sigmatropic hydrogen shift. Steady state photolysis and UV transient absorption spectroscopy are combined to explore the conformational relaxation of previtamin D3 formed from DHC in isotropic solution and confined to lipid bilayers chosen to model the biological cell membrane. The results are compared with measurements for two analogues: previtamin D2 formed from ergosterol (provitamin D2) and previtamin D3 acetate formed from DHC acetate. The resulting spectral dynamics are interpreted in the context of simulations of optical excitation energy and oscillator strength as a function of conformation. In solution, the relaxation dynamics and steady state product distributions of the three compounds are nearly identical, favoring tZg forms. When confined to lipid bilayers, the heterogeneity and packing forces alter the conformational distributions and enhance the population of a gZg conformer capable of vitamin D formation.

Communications: Photoinitiated bond dissociation of bromoiodomethane in solution: Comparison of one-photon and two-photon excitations and the formation of iso-CH(2)Br-I and iso-CH(2)I-Br.

Broadband UV-visible femtosecond transient absorption spectroscopy was used to monitor the excited state photochemistry of CH(2)BrI following one-photon excitation at 266 or 271 nm and two-photon excitation at 395 or 405 nm in 2-butanol. The results for one-photon excitation agree with earlier studies in acetonitrile, showing clear formation of iso-CH(2)Br-I following cleavage of the C-I bond. In contrast, two-photon excitation at 395 nm results in the appearance of a blueshifted photoproduct absorption band assigned to formation of iso-CH(2)I-Br following cleavage of the C-Br bond. The results are discussed in the context of prior experimental and theoretical work and the prospects for optical control of bond cleavage.

Second harmonic excitation spectroscopy of silver nanoparticle arrays.

Frequency-scanned excitation profiles of coherent second harmonic generation (SHG) were measured for silver nanoparticle arrays prepared by nanosphere lithography. The frequency of the fundamental beam did not coincide with the localized surface plasmon resonance (LSPR) of the nanoparticles and was tuned so that the coherent second harmonic (SH) emission was in the region of the LSPR at 720-750 nm. The SH emission from the arrays was compared with a smooth silver film to identify an enhancement of SH emission efficiency that peaks near approximately 650 nm for nanoparticles 50 nm in height. The polarization and orientation dependence of this enhancement suggests that it is related to a dipolar LSPR mode polarized normal to the plane of the substrate. Linear extinction spectra are dominated by in-plane dipoles and do not show this weak out-of-plane LSPR mode. The nanoparticle arrays are truncated tetrahedrons symmetrically oriented by nanosphere lithography to cancel SH from in-plane dipoles which allows observation of the weak out-of-plane component.

Plasmonic properties of film over nanowell surfaces fabricated by nanosphere lithography.

In this work, a detailed and systematic study of the plasmonic properties of a novel film over nanowell surface is investigated. These nanostructures are fabricated using nanosphere lithography and reactive ion etching and structurally characterized by AFM and SEM. The resulting structures show remarkably narrow plasmon bands in reflectance spectra (as little as 0.10 eV) and greater sensitivity to external dielectric environment than has been seen in other nanoparticle systems, resulting in an improvement in the figure of merit (FOM = refractive index sensitivity (eV.RIU(-1))/full width at half-maximum (eV)) for refractive index sensing. Theoretical modeling for the plasmon spectra of these nanostructures is done using discrete dipole approximation code under periodic boundary conditions. The modeling results match the measurements accurately in aspects of the variation of the plasmon line shape with altering internanowell distance and dielectric environment.

Photochemical ring-opening and ground state relaxation in α-terpinene with comparison to provitamin D3.

Ultrafast broadband UV-visible transient absorption spectroscopy is used to characterize the photochemistry of α-terpinene, a 1,4-disubstituted-1,3-cyclohexadiene natural product. These results are compared with experiments probing the analogous ring-opening reaction of 7-dehydrocholesterol (DHC, provitamin D3) and the subsequent relaxation of previtamin D3. The major experimental results are as follows: (1) Like DHC, but unlike 1,3-cyclohexadiene, α-terpinene exhibits a broad excited state absorption (ESA) spectrum in the visible. The lifetime of the excited state is ca. 0.16 ps in 1-butanol and 0.12 ps in hexane. (2) The state responsible for the ESA is the initially excited state. Fluorescence from this state has a quantum yield of ~2 × 10(-5). The fluorescence quantum yield is an order of magnitude smaller, and the excited state lifetime is an order of magnitude shorter than that observed for DHC. (3) The initial gZg-triene photoproduct absorbs to the red, and the relaxed tZg-triene product absorbs to the blue of α-terpinene. The gZg→tZg reaction of the vibrationally hot photoproduct requires ca. 6.5 ps with no significant dependence on solvent polarity or viscosity. Thermalization occurs on a time scale of 2-4 ps depending on solvent, but shows no particular trends within the solvent series. (4) The conformational relaxation of previtamin D3 occurs on a similar time scale of ca. 5-8 ps with a modest dependence on the solvent viscosity.

Ultrafast ring-opening reactions: a comparison of alpha-terpinene, alpha-phellandrene, and 7-dehydrocholesterol with 1,3-cyclohexadiene.

The excited and ground state dynamics of a series of 1,3-cyclohexadiene derivatives and their hexatriene photoproducts are studied using ultrafast broadband UV-visible transient absorption spectroscopy. The substitution pattern around the cyclohexadiene backbone alters the excited state potential energy surface in the Franck-Condon region as evidenced by changes in the excited state absorption and fluorescence properties of the systems. Unsubstituted 1,3-cyclohexadiene and alpha-phellandrene exhibit no excited state absorption while a strong excited state absorption in the visible spectral region is observed for both alpha-terpinene and Provitamin D3. Steric factors introduced by the ring substitutions determine the dominant rotational isomer at equilibrium for the hexatriene photoproducts. Coupling to the solvent during the conformational relaxation from the initial helical all-cis hexatriene is unique to each photoproduct, but the relaxation process occurs on a 6-10 ps timescale regardless of the size or substitution pattern on the triene.

Ultrafast electron-transfer and solvent adiabaticity effects in viologen charge-transfer complexes.

We report ultrafast electron transfer (ET) in charge-transfer complexes that shows solvent relaxation effects consistent with adiabatic crossover models of nonadiabatic ET. The complexes of either dimethyl viologen (MV) or diheptyl viologen (HV) with 4,4'-biphenol (BP) (MVBP or HVBP complexes) have identical charge-transfer spectra and kinetics in ethylene glycol with approximately 900 fs ET decay. We assign this decay time as largely due to adiabatic control of a predicted approximately 40 fs nonadiabatic ET. The MVBP decay in methanol of 470 fs is reduced in mixtures having low (2-20%) concentrations of acetonitrile to as short as 330 fs; these effects are associated with faster relaxation time in methanol and its mixtures. In contrast, HVBP has much longer ET decay in methanol (730 fs) and mixture effects that only reduce its decay to 550 fs. We identify the heptyl substituent as creating major perturbations to solvent relaxation times in the methanol solvation shell of HVBP. These charge-transfer systems have reasonably well-defined geometry with weak electronic coupling where the electronic transitions are not dependent on intramolecular motions. We used a nonadiabatic ET model with several models for adiabatic crossover predictions to discuss the small variation of energy gap with solvent and the ET rates derived from adiabatic solvent control. A time correlation model of solvent relaxation was used to define the solvent relaxation times for this case of approximately zero-barrier ET.

Solvent adiabaticity effects on ultrafast electron transfer in viologen charge transfer complexes.

Charge recombination (CR) kinetics following photoinduced charge transfer are measured by optical transient absorption for complexes of dimethyl viologen and diheptyl viologen with 4,4'-biphenol (MVBP and HVBP) in methanol. Exponential time constants for MVBP and HVBP are 480 and 790 fs, respectively. Kinetic differences cannot be rationalized with a standard equilibrium nonadiabatic rate formula using parameters obtained from linear absorption and resonance Raman measurements, which give nearly indistinguishable results for the two complexes. Solvent relaxation times and adiabaticities of MVBP are calculated using a full solvation correlation function approach. This analysis suggests that the smaller CR rate of HVBP is due to solvent reorganization differences, and is consistent with a greater adiabatic contribution for HVBP than MVBP. We conjecture that interactions between the diheptyl aliphatic groups of HVBP and the local solvent structure are responsible for the CR differences.

Controlling plasmon line shapes through diffractive coupling in linear arrays of cylindrical nanoparticles fabricated by electron beam lithography.

The effect of diffractive coupling on the collective plasmon line shape of linear arrays of Ag nanoparticles fabricated by electron beam lithography has been investigated using Rayleigh scattering spectroscopy. The array spectra exhibit an intricate multi-peak structure, including a narrow mode that gains strength for interparticle distances that are close to the single particle resonance wavelength. A version of the discrete dipole approximation method provides an excellent qualitative description of the observed behavior.