A phenomenological approach to relaxation in disordered systems

We propose a phenomenological approach to relaxation in disordered systems which is modelled after the well-known percolation behaviour. The approach shows an interesting behaviour of the time correlation function of relevant variables which is characterized by three temporal regimes, an initial exponential decay, a power-law regime and a stretched exponential decay for long times. We give some relevant examples of this behaviour in a wide variety of systems such as gels, microemulsions and glasses.     

Intense non-photochemical hole-burning spectra with discrete phonon structure in an aromatic organic glass (diphenylanthracene)

Hole burning in a tetracene-doped 9,10-diphenylanthracene glass deposited at 4.2 K occurs at an initial efficiency of 0.2 ± 0.1. The hole depth in the saturation limit is 0.6. The spectra show sharp phonon structures. The role of the phenyl rings for non-photochemical hole burning (NPHB) is emphasized.     

Thermal broadening of optical homogeneous linewidths in organic glasses and polymers studied via photochemical hole-burning

Photochemical hole-burning was used to study optical dephasing in the S₁ ← S_o OO transitions of dimethyl-s-tetrazine, chlorin and free-base porphin in several organic glasses and polymers. The homogeneous width (Γ_hom) seems to follow a T^1.3±0.1 law between 0.4 and 20 K for all guest—host pairs, and extrapolates the lifetime-limited values of each guest for T→0. For a given temperature, Γ_hom, appears to be related to the number of OH groups in the glass, and to the size of side-groups on the polymer chain.     

Electronic structure and transport in disordered organic systems

The strongly-bound approximation is used to examine the effects of disorder on the electron energy spectrum. It is assumed that the exchange integrals in the nearest-neighbor approximation are random functions. The limiting cases of slight and pronounced disorder are discussed with allowance for correlation. The effects of some particular types of correlation are examined as regards type of spectrum and band broadening. It is found that the spectrum splits into two branches in the limiting case of extensive disorder. An expression for the electrical conductivity is derived for the case of slight disorder.     

Nuclear spin relaxation times dispersion in disordered polymer systems

Nuclear spin relaxation reveals a wealth of information concerning motional dynamics and morphology within a polymer system. The conventional breakdown of relaxation into discrete decay components is usually arbitrary and often ambiguous due to strong coupling and spin diffusion between domains. To avoid being limited by pre-determined models, we have explored the utility of maximum entropy regularization scheme to reconstruct the complete NMR spin relaxation distribution continuum. Case studies showed that the three characteristics of the distribution correlate with various aspects of polymer morphology and with physical properties. These positive results suggested that this approach has the potential of yielding profiles on polymer dynamics and information concerning phonon coupling between domains in complex polymer systems which are not previously accessible. Furthermore, the nondestructive nature of NMR gives this approach an extra advantage over conventional tests for “in-situ” studies of polymers.     

Reformulated space-charge-limited current model and its application to disordered organic systems

We have reformulated a traditional model used to describe the current-voltage dependence of low mobility materials sandwiched between planar electrodes by using the quasi-electrochemical potential as the fundamental variable instead of the local electric field or the local charge carrier density. This allows the material density-of-states to enter explicitly in the equations and dispenses with the need to assume a particular type of contact. The diffusion current is included and as a consequence the current-voltage dependence obtained covers, with increasing bias, the diffusion limited current, the space-charge limited current, and the injection limited current regimes. The generalized Einstein relation and the field and density dependent mobility are naturally incorporated into the formalism; these two points being of particular relevance for disordered organic semiconductors. The reformulated model can be applied to any material where the carrier density and the mobility may be written as a function of the quasi-electrochemical potential. We applied it to the textbook example of a nondegenerate, constant mobility material and showed how a single dimensionless parameter determines the form of the I(V) curve. We obtained integral expressions for the carrier density and for the mobility as a function of the quasi-electrochemical potential for a Gaussianly disordered organic material and found the general form of the I(V) curve for such materials over the full range of bias, showing how the energetic disorder alone can give rise, in the space-charge limited current regime, to an I∝V(n) dependence with an exponent n larger than 2.     

Influence of the host on vibronic relaxation: a study of free-base porphin in a series of n-alkanes by photochemical hole-burning

Photochemical hole-burning is used to determine the relaxation times of vibronic bands of the S₁ ← S₀ transition to free-base porphin in different substitutional sites of n-hexane, n-heptane, n-octane and n-decane at 1.6 K. The vibronic relaxation depends strongly on site and host. A correlation between the n-alkane chain length and the vibronic relaxation time is observed.     

Optical homogeneous linewidths in organic glasses below 1 K: a hole-burning study of free-base porphin in 2-methyltetrahydrofuran

Photochemical hole-burning has been used to determine the temperature dependence of the homogeneous linewidth Γ_hom of the 0-0 S₁ - S₀ transition of free-base porphin (H₂P) in 2-methyltetrahydrofuran (MTHF) from T ≈ 5 K down to 0.4 K. A _T^1.30±0.05 dependence for Γ_hom is observed which extrapolates to the lifetime-limited value (≈ 10 MHz) at T ≈ 0.05K.     

Probing the primary event in the photocycle of photoactive yellow protein using photochemical hole-burning technique

Photochemical hole-burning spectroscopy was used to study the excited-state electronic structure of the 4-hydroxycinnamyl chromophore in photoactive yellow protein (PYP). This system is known to undergo a trans-to-cis isomerization process on a femtosecond-to-picosecond time scale, similar to membrane-bound rhodopsins, and is characterized by a broad featureless absorbance at 446 nm. Resolved vibronic structure was observed for the hole-burned spectra obtained when PYP in phosphate buffer at pH 7 was frozen at low temperature and irradiated with narrow bandwidth laser light at 431 nm. The approximate homogeneous width of 752 cm-1 could be calculated from the deconvolution of the hole-burned spectra leading to an estimated dephasing time of approximately 14 fs for the PYP excited-state structure. The resolved vibronic structure also enabled us to obtain an estimated change in the C=C stretching frequency, from 1663 cm-1 in the ground state to approximately 1429 cm-1 upon photoexcitation. The results obtained allowed us to speculate about the excited-state structure of PYP. We discuss the data for PYP in relation to the excited-state model proposed for the photosynthetic membrane protein bacteriorhodopsin, and use it to explain the primary event in the function of photoactive biological protein systems. Photoexcitation was also carried out at 475 nm. The vibronic structure obtained was quite different both in terms of the frequencies and Franck-Condon envelope. The origin of this spectrum was tentatively assigned.     

Johari-Goldstein relaxation and crystallization of sorbitol to ordered and disordered phases

The equilibrium permittivity epsilon(s) and the dielectric relaxation spectra of supercooled liquid D-sorbitol were measured during its crystallization to orientationally disordered or ordered phases depending on the sample preparation procedure at several fixed temperatures up to a period of 6 days. The epsilon(s) measurements showed that when the sample was contaminated by a minute amount of crystals, it crystallized to an ordered phase. When the liquid was not contaminated, the sample crystallized to an orientationally disordered phase. When supercooled D-sorbitol was kept close to its T(g), its dielectric spectra did not change over a period of 138.5 h. It was found that the Johari-Goldstein (JG) relaxation rate of the orientationally disordered crystalline phase is higher in comparison with that of the supercooled liquid, the spectrum broader, and the relaxation strength lower. Its glasslike transition temperature is higher than T(g) of the liquid. The results on crystallization showed that the structural changes occurring at a temperature where the alpha relaxation emerges from the JG relaxation affects the crystallization kinetics of the liquid.     

Optical hole-burning in the ultraviolet spectrum of 3,4,9,10-dibenzpyrene at room temperature

Ultraviolet fluorescence spectra are presented for 3,4-benzpyrene, 1,12-benzeperylene and 3,4,9,10-dibenzpyrene, which were obtained by single-laser, consecutive two-proton excitation. Experiments using two-laser excitation have permitted the study of fluorescence line-narrowing in the spectrum of dibenzpyrene at 300 K in hexane solution. These unique observations are interpreted in terms of optical site-selection.     

Modeling disordered morphologies in organic semiconductors

Organic thin film devices are investigated for many diverse applications, including light emitting diodes, organic photovoltaic and organic field effect transistors. Modeling of their properties on the basis of their detailed molecular structure requires generation of representative morphologies, many of which are amorphous. Because time‐scales for the formation of the molecular structure are slow, we have developed a linear‐scaling single molecule deposition protocol which generates morphologies by simulation of vapor deposition of molecular films. We have applied this protocol to systems comprising argon, buckminsterfullerene, N,N‐Di(naphthalene‐1‐yl)‐N,N'‐diphenyl‐benzidine, mer‐tris(8‐hydroxy‐quinoline)aluminum(III), and phenyl‐C₆₁‐butyric acid methyl ester, with and without postdeposition relaxation of the individually deposited molecules. The proposed single molecule deposition protocol leads to formation of highly ordered morphologies in argon and buckminsterfullerene systems when postdeposition relaxation is used to locally anneal the configuration in the vicinity of the newly deposited molecule. The other systems formed disordered amorphous morphologies and the postdeposition local relaxation step has only a small effect on the characteristics of the disordered morphology in comparison to the materials forming crystals. © 2013 Wiley Periodicals, Inc.     

Optical and photochemical properties of N-benzalaminophenylpyridinium perchlorates

The electronic spectra of a number of benzylidene derivatives of N-aminoarylpyridinium perchlorates are associated with absorption localized on the individual pyridinium and azomethine fragments and with intramolecular charge transfer (ICT) from the azomethine fragment to the pyridinium ring. The luminescence is associated with the latter process. The photochromism is due to proton transfer in the excited state of the azomethine fragment, which competes with ICT, and is also due to structural factors.     

超分子体系中的光物理和光化学过程

总结了我们组近几年来对超分子体系中的光物理和光化学过程所做的工作,包括三个部分：(1)微反应器控制的有机光化学反应的选择性,(2)疏水、疏脂作用对光物理和光化学过程的影响,(3)超分子体系中的电子转移、能量传递和光化学转换。     

Visible light-driven organic photochemical synthesis in China

In recent years, visible light-driven organic photochemical synthesis has attracted wide research interest from academic and industrial communities due to its features of green and sustainable chemistry. In this flourishing area, Chinese chemists have devoted great efforts to different aspects of synthetic chemistry. This review will summarize their representative work according to the following categories: C–H functionalization, synthesis of aromatic aza-heterocycles, asymmetric organic photochemical synthesis, transformations of small molecules and biomolecule-compatible reactions.     

Phase stratification in disordered metallic systems

Phase stratification in a series of binary melts (AlBi, AlCd, AlIn, AlPb, CdGa, CrSn, and CuTl) and an amorphous NiZr alloy was studied. Equations describing the main stratification parameters in the liquid phase (the enthalpy ΔH, the relative volume change of melt formation ΔV/V ₀, and the highest temperature T _k at which phase stratification occurs) were obtained. An equation describing the concentration dependence of the relative integral Gibbs energy in the amorphous (liquid) phase was found. This equation took into account the influence of volume changes caused by melt formation on its entropy. The equations obtained were shown to qualitatively correctly describe phase stratification and the subsequent growth of crystalline phases in NiZr system alloys.     

Optical dynamics of the reaction center of photosystem II. A hole-burning and photon-echo study

Photon-echo and transient hole-burning experiments on the P-band of the reaction center of photosystem II are reported which show that the P-band exhibits a large homogeneous width. A photon-echo signal with a decay time of 500 ps is also observed in the same spectral region and assigned to extraneous chlorophyll. These observations are discussed with reference to the bacterial reaction center and photosystem I.     

Contributions in organic functional group transformations and photochemical and photophysical studies of selective organic substrates

A brief overview of our scientific contributions over the past few years and the results of some of our recent studies on fullerene clusters are presented Based on the CRSI Lifetime Achievement (Gold Medal 2001) lecture given at the Annual Meeting of the Chemical Research Society of India on 2 February 2002 in the National Chemical Laboratory, Pune     

On the random walk of energy to traps in disordered systems

Adams recently derived a theorem concerning solutions of the Schrödinger equation for an N-electron system. From this theorem he concludes that exact eigenvalues and (antisymmetric) eigenfunctions can be obtained by solving a different type of equation whose eigenfunctions are not antisymmetric. We critically discuss aspects of Adams' formalism.