Time-Reversed Dynamical Entropy and Irreversibility in Markovian Random Processes

A concept of time-reversed entropy per unit time is introduced in analogy with the entropy per unit time by Shannon, Kolmogorov, and Sinai. This time-reversed entropy per unit time characterizes the dynamical randomness of a stochastic process backward in time, while the standard entropy per unit time characterizes the dynamical randomness forward in time. The difference between the time-reversed and standard entropies per unit time is shown to give the entropy production of Markovian processes in nonequilibrium steady states.     

Photoinduced activation of molecular oxygen by various porphyrins,bis-porphyrins,phthalocyanines, pyridinoporphyrazins,and their metal derivatives

Aerobic irradiation of tetraphenylporphyrins, phthalocyanines, tetra-t-butylphthalocyanines, tetracarboxylphthalocyanines, tetrapyridinoporphyrazins and some of their metal derivatives with visible light (λ > 420 nm) give singlet oxygen by energy transfer and oxygen superoxide by electron transfer, but some of their Cu, Zn of Pt derivatives are efficient quenchers for superoxide formation.     

Bifurcation phenomena near homoclinic systems: A two-parameter analysis

The bifurcations of periodic orbits in a class of autonomous three-variable, nonlinear-differential-equation systems possessing a homoclinic orbit associated with a saddle focus with eigenvalues ( ±i,), where ¦/¦ < 1 (Sil'nikov's condition), are studied in a two-parameter space. The perturbed homoclinic systems undergo a countable set of tangent bifurcations followed by period-doubling bifurcations leading to periodic orbits which may be attractors if ¦/¦ < 1/2. The accumulation rate of the critical parameter values at the homoclinic system is exp(-2¦/¦). A global mechanism for the onset of homoclinicity in strongly contractive flows is analyzed. Cusp bifurcations with bistability and hysteresis phenomena exist locally near the onset of homoclinicity. A countable set of these cusp bifurcations with scaling properties related to the eigenvalues±i of the stationary state are shown to occur in infinitely contractive flows. In the two-parameter space, the periodic orbit attractor domain exhibits a spiral structure globally, around the set of homoclinic systems, in which all the different periodic orbits are continuously connected.     

A Practical General Method for the Preparation of Long Acenes

The field of long acenes, the narrowest of the zig-zag graphene nanoribbons, has been an area of significant interest in the past decade because of its potential applications in organic electronics, spintronics and plasmonics. However the low solubility and high reactivity of these compounds has so far hindered their preparation on large scales. We report here a concise strategy for the synthesis of higher acenes through Diels–Alder condensation of arynes with a protected tetraene ketone. After deprotection by cleavage of the ketal, the obtained monoketone precursors cleanly yield the corresponding acenes through quantitative cheletropic thermal decarbonylation in the solid state, at moderate temperatures of 155 to 205 °C. This approach allows the preparation of heptacene, benzo[a]hexacene, cis- and trans-dibenzopentacene and offers a valuable new method for the synthesis of even larger acenes.     

Supramolecular assembly of lysine-b-glycine block copolypeptides at different solution conditions

Here we report the supramolecular assembly of poly(l-lysine)-b-polyglycine diblock copolypeptides at different solution conditions. Light scattering and confocal microscopy indicate that the supramolecular aggregates initially formed in solution are vesicles with a broad size distribution, depending strongly on the initial processing conditions. The vesicles formed after multiple pH cycles appear independent of the initial processing conditions and are related to the thermodynamic nature of the assembled supramolecular aggregates. Circular dichroism results verify that this change in size observed over pH cyclings tracks with the conformation changes of the lysine block confined in the vesicle membranes. This appears interesting for peptosome-based materials, implying a high level of fluidity in the membrane that allows the supramolecular aggregates formed in solution to respond to changes in pH. The results also show that the external stimulus, which is the change of pH in this study, provides an additional means to regulate polypeptide vesicle size and size distribution.     

Nanofoaming in the surface of biopolymers by femtosecond pulsed laser irradiation

In this work, the nanostructuring induced in femtosecond (fs) laser irradiation of biopolymers is examined in self-standing films of collagen and gelatine. Irradiation by single 90 fs pulses at 800, 400 and 266 nm is shown to result in the formation of a modified layer with submicrometric size structures. The size and uniformity of the observed features are strongly dependent on irradiation wavelength and on the characteristics of the biopolymer (water content and mechanical strength). Examination of the films by laser induced fluorescence serves to assess the chemical modifications induced by laser irradiation, revealing changes in the emission bands assigned to the aromatic amino acid tyrosine and its degradation products. The results are discussed in the framework of a mechanism involving the generation of large free-electron densities, through multiphoton and avalanche ionization, which determine the temperature and stress distribution in the irradiated volume.     

IR laser ablation of doped poly(methyl methacrylate)

We investigate the TEA CO₂ laser ablation of films of poly(methyl methacrylate), PMMA, with average M_W 2.5, 120 and 996 kDa doped with photosensitive compounds iodo-naphthalene (NapI) and iodo-phenanthrene (PhenI) by examining the induced morphological and physicochemical modifications. The films casted on CaF₂ substrates were irradiated with a pulsed CO₂ laser (10P(20) line at 10.59 μm) in resonance with vibrational modes of PMMA and of the dopants at fluences up to 6 J/cm². Laser induced fluorescence probing of photoproducts in a pump and probe configuration is carried out at 266 nm. Formation of naphthalene (NapH) and phenanthrene (PhenH) is observed in NapI and PhenI doped PMMA, respectively, with relatively higher yields in high M_W polymer, in similarity with results obtained previously upon irradiation in the UV at 248 nm. Above threshold, formation of photoproducts is nearly complete after 200 ms. As established via optical microscopy, bubbles are formed in the irradiated areas with sizes that depend on polymer M_W and filaments are observed to be ejected out of the irradiated volume in the samples made with high M_W polymer. The implications of these results for the mechanisms of polymer IR laser ablation are discussed and compared with UV range studies.