Ultrafast excited state dynamics of fucoxanthin: excitation energy dependent intramolecular charge transfer dynamics

Carotenoids containing a carbonyl group in conjugation with their polyene backbone are naturally-occurring pigments in marine organisms and are essential to the photosynthetic light-harvesting function in aquatic algae. These carotenoids exhibit spectral characteristics attributed to an intramolecular charge transfer (ICT) state that arise in polar solvents due to the presence of the carbonyl group. Here, we report the spectroscopic properties of the carbonyl carotenoid fucoxanthin in polar (methanol) and nonpolar (cyclohexane) solvents studied by steady-state absorption and femtosecond pump-probe measurements. Transient absorption associated with the optically forbidden S(1) (2(1)A) state and/or the ICT state were observed following one-photon excitation to the optically allowed S(2) (1(1)B) state in methanol. The transient absorption measurements carried out in methanol showed that the ratio of the ICT-to-S(1) state formation increased with decreasing excitation energy. We also showed that the ICT character was clearly visible in the steady-state absorption in methanol based on a Franck-Condon analysis. The results suggest that two spectroscopic forms of fucoxanthin, blue and red, exist in the polar environment.     

Photoswitches and luminescent rigidity sensors based on fac-[Re(CO)3(Me4phen)(L)]+

The fac-[Re(CO)3(Me4phen)(trans-L)]+ complexes, Me4phen = 3,4,7,8-tetramethyl-1,10-phenanthroline and L = 4-styrylpyridine, stpy, or 1,2-bis(4-pyridyl)ethylene, bpe, were synthesized and characterized by their spectroscopic,photochemical, and photophysical properties. The complexes exhibit trans-to-cis isomerization upon 313, 334, 365,and 404 nm irradiation, and the true quantum yields can be efficiently determined by absorption changes combined with 1H NMR data. For fac-[Re(CO)3(Me4phen)(trans-bpe)]+ similar quantum yields were determined at all wavelengths investigated. However, a lower value (phitrue = 0.35) was determined for fac-[Re(CO)3(Me4phen)(trans-stpy)]+ at404 nm irradiation, which indicates different pathways for the photoisomerization process. The photoproducts, fac-[Re(CO)3(Me4phen)(cis-L)]+, exhibit luminescence at room temperature with two maxima ascribed to the 3ILMe4phen and 3MLCTRe-->Me4phen excited states. The luminescence properties were investigated in different media, and the behavior in glassy EPA at 77 K showed that the contribution of each emissive state is dependent on the excitation wavelength. The photochemical and photophysical behavior of the complexes were rationalized in terms of the energy gap of excited states and can be exploited in photoswitchable luminescent rigidity sensors.     

One-Step Synthesis of Cyclobutene-Annulated Fullerenes through a Cascade Reaction

The physicochemical properties of fullerene-based materials typically vary depending on the type and mode of addition of functional groups; therefore, developing fullerene derivatives with novel structures is imperative for further progress in materials science. In this study, we develop an efficient one-step strategy for synthesizing cyclobutene-annulated fullerene derivatives (cyclobutenofullerenes) and characterize their electronic properties. Despite the steric strain, cyclobutenofullerenes can be easily prepared via a one-step reaction of C₆₀ with a secondary propargylic phosphate. Structural analysis of the reaction intermediates suggests that the cascade reaction proceeds through a formal [2+2] cycloaddition of C₆₀ with an allene, caused by the 1,3-migration of the propargylic phosphate, followed by an additional 1,3-migration and the subsequent 1,2-elimination of the phosphodiester moiety.     

TG studies of a composite solid rocket propellant based on HTPB-binder

Thermal decomposition kinetics of solid rocket propellants based on hydroxyl-terminated polybutadiene-HTPB binder was studied by applying the Arrhenius and Flynn-Wall-Ozawa's methods. The thermal decomposition data of the propellant samples were analyzed by thermogravimetric analysis (TG/DTG) at different heating rates in the temperature range of 300-1200 K. TG curves showed that the thermal degradation occurred in three main stages regardless of the plasticizer (DOA) raw material, the partial HTPB/IPDI binder and the total ammonium perchlorate decompositions. The kinetic parameters E _a (activation energy) and A (pre-exponential factor) and the compensation parameter (S _p) were determined. The apparent activation energies obtained from different methods showed a very good agreement. This revised version was published online in August 2006 with corrections to the Cover Date.     

Analysis of application of Langmuir isotherm to heterogeneous systems: high-pressure conditions

In this paper, an adsorption model that is based on the statistical mechanics approach was applied to study the saturation phenomena in adsorption to calculate the minimum pressure needed to attain the complete surface coverage on a physical adsorption. The fundamental integral equation, Theta(T)(Q)=integral N(Q)Theta(Q)dQ, for the calculation of the coverage degree of the surface was developed for an exponential distribution function, N(Q)=(m/RT)exp(-mQ/RT), and the representation of local adsorption sites is given by the Langmuir expression, Theta(Q)=b(0)C exp(Q/RT)/(1+b(0)C exp(Q/RT)). At high values of the pressure C, a solution of the fundamental integral equation was obtained by imposing the condition b(0)C>1. The expression for the saturation condition, b(0)C>m/(m+1), was obtained; that is, the saturation phenomenon is dependent on two parameters, correlated with the energetic heterogeneity and adsorption energy of the system. The pressure in the analysis of the cited expression shows that, for low m values (more heterogeneous systems), saturation is attained for b(0)C>m, while for m congruent with 1 (more homogeneous systems), the saturation is attained for b(0)C>0.5.     

Potentiometrie

Ohne Zusammenfassung     

Excited-state dynamics in fac-[Re(CO)3(Me4phen)(L)]+

Excited-state dynamics in fac-[Re(CO)(3)(Me(4)phen)(cis-L)](+) (Me(4)phen = 3,4,7,8-tetramethyl-1,10-phenanthroline, L = 4-styrylpyridine (stpy) or 1,2-bis(4-pyridyl)ethylene (bpe)) were investigated by steady-state and time-resolved techniques. A complex equilibrium among three closely lying excited states, (3)IL(cis-L), (3)MLCT(Re→Me(4)phen), and (3)IL(Me(4)phen), has been established. Under UV irradiation, cis-to-trans isomerization of coordinated cis-L is observed with a quantum yield of 0.15 in acetonitrile solutions. This photoreaction competes with radiative decay from (3)MLCT(Re→Me(4)phen) and (3)IL(Me(4)phen) excited states, leading to a decrease in the emission quantum yield relative to the nonisomerizable complex fac-[Re(CO)(3)(Me(4)phen)(bpa)](+) (bpa = 1,2-bis(4-pyridyl)ethane). From temperature-dependent time-resolved emission measurements in solution and in poly(methyl methacrylate) (PMMA) films, energy barriers (ΔE(a)) for interconversion between (3)MLCT(Re→Me(4)phen) and (3)IL(Me(4)phen) emitting states were determined. For L = cis-stpy, ΔE(a) = 11 (920 cm(-1)) and 15 kJ mol(-1) (1254 cm(-1)) in 5:4 propionitrile/butyronitrile and PMMA, respectively. For L = cis-bpe, ΔE(a) = 13 kJ mol(-1) (1087 cm(-1)) in 5:4 propionitrile/butyronitrile. These energy barriers are sufficient to decrease the rate constant for internal conversion from higher-lying (3)IL(Me(4)phen) state to (3)MLCT(Re→Me(4)phen), k(i) ? 10(6) s(-1). The decrease in rate allows for the observation of intraligand phosphorescence, even in fluid medium at room temperature. Our results provide additional insight into the role of energy gap and excited-state dynamics on the photochemical and photophysical properties of Re(I) polypyridyl complexes.     

Complex,degradable polyester materials via ketoxime ether‐based functionalization: Amphiphilic,multifunctional graft copolymers and their resulting solution‐state aggregates

Degradable, amphiphilic graft copolymers of poly(ε‐caprolactone)‐graft‐poly(ethylene oxide), PCL‐g‐PEO, were synthesized via a grafting onto strategy taking advantage of the ketones presented along the backbone of the statistical copolymer poly(ε‐caprolactone)‐co‐(2‐oxepane‐1,5‐dione), (PCL‐co‐OPD). Through the formation of stable ketoxime ether linkages, 3 kDa PEO grafts and p‐methoxybenzyl side chains were incorporated onto the polyester backbone with a high degree of fidelity and efficiency, as verified by NMR spectroscopies and GPC analysis (90% grafting efficiency in some cases). The resulting block graft copolymers displayed significant thermal differences, specifically a depression in the observed melting transition temperature, T_m, in comparison with the parent PCL and PEO polymers. These amphiphilic block graft copolymers undergo self‐assembly in aqueous solution with the P(CL‐co‐OPD‐co‐(OPD‐g‐PEO)) polymer forming spherical micelles and a P(CL‐co‐OPD‐co‐(OPD‐g‐PEO)‐co‐(OPD‐g‐pMeOBn)) forming cylindrical or rod‐like micelles, as observed by transmission electron microscopy and atomic force microscopy. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3553–3563, 2010