Electronic absorption and emission spectroscopic investigations of the interaction of the fullerenes, C60 and C70, with amines and aromatic molecules

Electronic absorption spectroscopy and fluorescence spectroscopy have been used to investigate the interaction of the fullerenes C60 and C70 with diethylaniline, and with aromatic solvents such as benzene. C60 interacts weakly with aromatic amines in the ground state while C70 does not interact at all. Steady state fluorescence emission and lifetime measurements show that both C60 and C70 form excited state complexes (exciplexes) with the amines in non-aromatic solvents such as methylcyclohexane, but not in benzene. In benzene, only fluorescence quenching is observed due to the interaction between the π systems of the aromatic solvent and the fullerene in the ground state. This is also borne out by the systematic study of solvent effects on the absorption and emission spectra of the fullerenes.

Exact eigenstates of the Pariser-Parr-Pople model for anthracene

The pi-electronic structure of anthracene is discussed by combining exact solutions of the Pariser-Parr-Pople (PPP) model and semiempirical PM3 calculations. Symmetry adaptation of the 2.8 million singlet valence-bond (VB) diagrams is explicitly demonstrated for D2h and electron-hole symmetry. Standard PPP parameters provide a comprehensive fit to one- and two-photon anthracene spectra and intensities up to the strong 1 B-1(3u)-absorption at 5.24 eV, the 10th excited state in the dense correlated spectrum, and indicate a reassignment of two-photon absorptions. The singlet-triplet gap and fine-structure constants also agree with experiment. Fully-relaxed PM3 geometries are obtained for the anthracene ground state and for singlet, triplet, and charged bipolarons. The PM3 bond lengths correlate well with PPP bond orders for the idealized structure. Single-determinantal PM3 excitation and relaxation energies for bipolarons are consistent with exact PPP results and contrast all-valence electron with pi-electron calculations. Several correlation effects are noted in the rich pi-spectra of anthracene in connection with improved PPP modeling of conjugated molecules and polymers.

Excitation and relaxation energies of trans-stilbene: Confined singlet, triplet, and charged bipolarons

The π-electronic excitations and excited-state geometries of trans-stilbene (tS) are found by combining exact solutions of the Pariser-Parr-Pople (PPP) model and semiempirical Parametric Method 3 (PM3) calculations. Comprehensive comparisons with tS spectra are obtained and related to the fluorescence and topological alternation of poly(paraphenylenevinylene) (PPV). The one-photon absorption and triplet of tS correspond, respectively, to singlet and triplet bipolarons confined to two phenyls, while the tS2- ground state is a confined charged bipolaron. Independent estimates of the relaxation energy between vertical and adiabatic excitation show the bipolaron binding energy to depend on both charge and spin, as expected for interacting π electrons in correlated or molecular states. Complete configuration interaction within the PPP model of tS accounts for the singlet-triplet gap, for the fine-structure constants and triplet-triplet spectra, for two-photon transitions and intensities, and for one-photon spectra and the radiative lifetime, although the relative position of nearly degenerate covalent and ionic singlets is not resolved. The planar PM3 geometry and low rotational barrier of tS agree with resolved rotational and vibrational spectra in molecular beams. PM3 excitation and relaxation energies for tS bipolarons are consistent with experiment and with PPP results. Instead of the exciton model, we interpret tS excitations in terms of states that are localized on each ring or extended over an alternating chain, as found exactly in Hückel theory, and find nearly degenerate transitions between extended and localized states in the singlet, triplet, and dianion manifolds. The large topological alternation of the extended system increases the ionicity and interchanges the order of the lowest one- and two-photon absorption of PPV relative to polyenes.

Coordination-Driven Self-Assembly of M3L2 Trigonal Cages from Preorganized Metalloligands Incorporating Octahedral Metal Centers and Fluorescent Detection of Nitroaromatics

The design and preparation of novel M3L2 trigonal cages via the coordination-driven self-assembly of preorganized metalloligands containing octahedral aluminum(III), gallium(III), or ruthenium(II) centers is described. When tritopic or dinuclear linear metalloligands and appropriate complementary subunits are employed, M3L2 trigonal-bipyramidal and trigonal-prismatic cages are self-assembled under mild conditions. These three-dimensional cages were characterized with multinuclear NMR spectroscopy (H-1 and P-31) and high-resolution electrospray ionization mass spectrometry. The structure of one such trigonal-prismatic cage, self-assembled from an arene ruthenium metalloligand, was confirmed via single-crystal X-ray crystallography. The fluorescent nature of these prisms, due to the presence of their electron-rich ethynyl functionalities, prompted photophysical studies, which revealed that electron-deficient nitroaromatics are effective quenchers of the cages' emission. Excited-state charge transfer from the prisms to the nitroaromatic substrates can be used as the basis for the development of selective and discriminatory turn-off fluorescent sensors for nitroaromatics.

Solvent effect on the spectroscopic properties of 6MAMC and 7MAMC

The absorption and emission spectra of two dyes namely 6MAMC and 7MAMC have been recorded at room temperature in solvents of different polarities. The ground-state dipole moments (mu(g)) of these two were determined experimentally by Guggenheim method and were compared with theoretical values obtained using quantum chemical method. The exited state (mu(e))dipole moments were estimated from Lippert's, Bakhshiev's and Chamma-Viallet's equations by using the variation of the Stokes shift with the solvent dielectric constant and refractive index. The ground and excited-state dipole moments were calculated by means of the solvatochromic shift method and also the excited-state dipole moments are determined in combination with ground-state dipole moments. It was observed that dipole moments of excited state were higher than those of the ground state, indicating a substantial redistribution of the pi-electron densities in a more polar excited state for these two dyes. (C) 2010 Elsevier B.V. All rights reserved.

Why the Definition of Eusociality Is Not Helpful to Understand Its Evolution and What Should We Do about It

The evolution of altruism is the central problem of the evolution of eusociality. The evolution of altruism is most likely to be understood by studying species that show altruism in spite of being capable of ''selfish'' individual reproduction. But the definition of eusociality groups together primitively eusocial species where workers retain the ability to reproduce on their own and highly eusocial species where workers have lost reproductive options. At the same time it separates the primitively eusocial species from semisocial species, species that lack life-time sterility and cooperatively breeding birds and mammals, in most of which, altruism and the associated social life are facultative. The definition of eusociality is also such that it is sometimes difficult to decide,what is eusocial and what is not. I therefore suggest that, (1) we expand the scope of eusociality to include semisocial species, primitively eusocial species, highly eusocial species as well as those cooperatively breeding birds and mammals where individuals give up substantial or all personal reproduction for aiding conspecifics, (2) there should be no requirement of overlap of generations or of life-time sterility and (3) the distinction between primitively and highly eusocial should continue, based on the presence or absence of morphological caste differentiation.

Impact of metal binding on the antitumor activity and cellular imaging of a metal chelator cationic imidazopyridine derivative

A new water soluble cationic imidazopyridine species, viz. (1E)-1-((pyridin-2-yl)methyleneamino)-3-(3(pyridin-2-yl) imidazo1,5-a]pyridin-2(3H)-yl)propan-2-ol (1), as a metal chelator is prepared as its PF6 salt and characterized. Compound 1 shows fluorescence at 438 nm on excitation at 342 nm in Tris-HCl buffer giving a fluorescence quantum yield (phi) of 0.105 and a life-time of 5.4 ns. Compound 1, as an avid DNA minor groove binder, shows pUC19 DNA cleavage activity in UV-A light of 365 nm forming singlet oxygen species in a type-II pathway. The photonuclease potential of 1 gets enhanced in the presence of Fe2+, Cu2+ or Zn2+. Compound 1 itself displays anticancer activity in HeLa, HepG2 and Jurkat cells with an enhancement on addition of the metal ions. Photodynamic effect of 1 at 365 nm also gets enhanced in the presence of Fe2+ and Zn2+. Fluorescence-based cell cycle analysis shows a significant dead cell population in the sub-G1 phase of the cell cycle suggesting apoptosis via ROS generation. A significant change in the nuclear morphology is observed from Hoechst 33258 and an acridine orange/ethidium bromide (AO/EB) dual nuclear staining suggesting apoptosis in cells when treated with 1 alone or in the presence of the metal ions. Apoptosis is found to be caspase-dependent. Fluorescence imaging to monitor the distribution of 1 in cells shows that 1 in the presence of metal ions accumulates predominantly in the cytoplasm. Enhanced uptake of 1 into the cells within 12 h is observed in the presence of Fe2+ and Zn2+.

Smart applications for energy harvested WSNs

A Wireless Sensor Network (WSN) powered using harvested energies is limited in its operation by instantaneous power. Since energy availability can be different across nodes in the network, network setup and collaboration is a non trivial task. At the same time, in the event of excess energy, exciting node collaboration possibilities exist; often not feasible with battery driven sensor networks. Operations such as sensing, computation, storage and communication are required to achieve the common goal for any sensor network. In this paper, we design and implement a smart application that uses a Decision Engine, and morphs itself into an energy matched application. The results are based on measurements using IRIS motes running on solar energy. We have done away with batteries; instead used low leakage super capacitors to store harvested energy. The Decision Engine utilizes two pieces of data to provide its recommendations. Firstly, a history based energy prediction model assists the engine with information about in-coming energy. The second input is the energy cost database for operations. The energy driven Decision Engine calculates the energy budgets and recommends the best possible set of operations. Under excess energy condition, the Decision Engine, promiscuously sniffs the neighborhood looking for all possible data from neighbors. This data includes neighbor's energy level and sensor data. Equipped with this data, nodes establish detailed data correlation and thus enhance collaboration such as filling up data gaps on behalf of nodes hibernating under low energy conditions. The results are encouraging. Node and network life time of the sensor nodes running the smart application is found to be significantly higher compared to the base application.

Density-matrix renormalization-group study of low-lying excitations of polyacene within a Pariser-Parr-Pople model

We have carried out symmetrized density-matrix renormalization-group calculations to study the nature of excited states of long polyacene oligomers within a Pariser-Parr-Pople Hamiltonian. We have used the C-2 symmetry, the electron-hole symmetry, and the spin parity of the system in our calculations. We find that there is a crossover in the lowest dipole forbidden two-photon state and the lowest dipole allowed excited state with size of the oligomer. In the long system limit, the two-photon state lies below the lowest dipole allowed excited state. The triplet state lies well below the two-photon state and energetically does not correspond to its description as being made up of two triplets. These results are in agreement with the general trends in linear conjugated polymers. However, unlike in linear polyenes wherein the two-photon state is a localized excitation, we find that in polyacenes, the two-photon excitation is spread out over the system. We have doped the systems with a hole and an electron and have calculated the charge excitation gap. Using the charge gap and the optical gap, we estimate the binding energy of the 1(1)B(-) exciton to be 2.09 eV. We have also studied doubly doped polyacenes and find that the bipolaron in these systems, to be composed of two separated polarons, as indicated by the calculated charge-density profile and charge-charge correlation function. We have studied bond orders in various states in order to get an idea of the excited state geometry of the system. We find that the ground state, the triplet state, the dipole allowed state, and the polaron excitations correspond to lengthening of the rung bonds in the interior of the oligomer while the two-photon excitation corresponds to the rung bond lengths having two maxima in the system.

Direct and sensitized (energy and electron transfer) geometric isomerization of stilbene within zeolites: a comparison between solution and zeolite as reaction media

The trans- and cis-stilbenes upon inclusion in NaY zeolite are thermally stable. Direct excitation and triplet sensitization results in geometric isomerization and the excited state behavior under these conditions are similar to that in solution. Upon direct excitation, a photostationary state consisting of 65% cis and 35% trans isomers is established. Triplet sensitization with 2-acetonaphthone gave a photostationary state consisting of 63% cis and 37% trans isomers. These numbers are similar to the ones obtained in solution. Thus, the presence of cations and the confined space within the zeolite have very little influence on the overall chemistry during direct and triplet sensitization. However, upon electron transfer sensitization with N-methylacridinium (NMA) as the sensitizer within NaY, isomerization from cis-stilbene radical cation to trans-stilbene occurs and the recombination of radical ions results in triplet stilbene. Prolonged irradiation gave a photostationary state (65% cis and 35% trans) similar to triplet sensitization. This behavior is unique to the zeolite and does not take place in solution. Steady state fluorescence measurements showed that the majority of stilbene molecules are close to the N-methylacridinium sensitizer. Diffuse reflectance flash photolysis studies established that independent of the isomer being sensitized only trans radical cation is formed. Triplet stilbene is believed to be generated via recombination of stilbene radical cation and sensitizer radical anion. One should be careful in using acidic HY zeolite as a medium for photoisomerization of stilbenes. In our hands, in these acidic zeolites isomerization dominated the photoisomerization. (C) 2002 Elsevier Science B.V. All rights reserved.

I*(P-2(1/2)) production from CH3I photodissociation at 236 nm

The quantum yield of I*((2)p(1/2)) production from CH3I photolysis at 236 nm in the gas phase has been measured as 0.69 +/- 0.03. The implication is that direct excitation to the (1)Q(1) excited state is significant at this wavelength. The dynamics of I* formation at other excitation energies covering the entire A-band of absorption of CH3I has been discussed in the light of this measurement.

Light-induced geometric isomerization of 1,2-diphenylcyclopropanes included within Y zeolites: Role of cation-guest binding

Through a systematic study of several diphenylcyclopropane derivatives, we have inferred that the cations present within a zeolite control the excited-state chemistry of these systems. In the parent 1,2-diphenylcylopropane, the cation binds to the two phenyl rings in a sandwich-type arrangement, and such a mode of binding prevents cis-to-trans isomerization. Once an ester or amide group is introduced into the system (derivatives of 2beta,3beta-diphenylcyclopropane-1alpha-carboxylic acid), the cation binds to the carbonyl group present in these chromophores and such a binding has no influence on the cis-trans isomerization process. Cation-reactant structures computed at density functional theory level have been very valuable in rationalizing the observed photochemical behavior of diphenylcyclopropane derivatives included in zeolites. While the parent system, 1,2-diphenyleylopropane, has been extensively investigated in the context of chiral induction in solution, owing to its failure to isomerize from cis to trans, the same could not be investigated in zeolites. However, esters of 2beta,3beta-diphenylcyclopropane-1alpha-carboxylic acid could be studied within zeolites in the context of chiral induction. Chiral induction as high 20% ee and 55% de has been obtained with selected systems. These numbers, although low, are much higher than what has been obtained in solution with the same system or with the parent system by other investigators (maximum similar to10% ee).

density matrix renormalization group study of polyacene

We study the nature of excited states of long polyacene oligomers within a Pariser-Parr-Pople (PPP) Hamiltonian using the Symmetrized Density Matrix Renormalization Group (SDMRG) technique. We find a crossover between the two-photon state and the lowest dipole allowed excited state as the system size is increased from tetracene to pentacene. The spin-gap is the smallest gap. We also study the equilibrium geome tries in the ground and excited states from bond orders and bond-bond correlation functions. We find that the Peierls instability in the ground state of polyacene is conditional both from energetics and structure factors computed froth correlation functions.

Coalescence of magic sized CdSe into rods and wires and subsequent energy transfer

We report on the synthesis of CdSe magic-sized clusters (MSCs) and their evolution into 1D rod and wires retaining the diameter of the order of MSCs. At the beginning of the reaction, different classes of stable MSCs with band gaps of 3.02 eV and 2.57 eV are formed, which exhibit sharp band edge photoluminescence features with FWHM in the order of similar to 13 nm. Reaction annealing time was carried out in order to monitor the shape evolution of the MSCs. We find that magic sized CdSe evolve into 1D rod and wires retaining the same diameter upon increasing annealing time. We observed the gradual emergence of new red shifted emission peaks during this shape evolution process, which emerge as a result of one dimensional energy transfer within the magic sized clusters during their subsequent transformation into rods and wires. The smallest, the second smallest sized MSC and the wires sequentially act as donors and acceptors during the size evolution from small MSCs to larger ones, and then eventually to wires. Steady-state and time-resolved luminescent spectroscopy revealed Forster resonance energy transfer (FRET) between the MSCs to the rods and wires.

Optimal Sleep-Wake Scheduling for Quickest Intrusion Detection using wireless sensor Networks

We consider the problem of quickest detection of an intrusion using a sensor network, keeping only a minimal number of sensors active. By using a minimal number of sensor devices,we ensure that the energy expenditure for sensing, computation and communication is minimized (and the lifetime of the network is maximized). We model the intrusion detection (or change detection) problem as a Markov decision process (MDP). Based on the theory of MDP, we develop the following closed loop sleep/wake scheduling algorithms: 1) optimal control of Mk+1, the number of sensors in the wake state in time slot k + 1, 2) optimal control of qk+1, the probability of a sensor in the wake state in time slot k + 1, and an open loop sleep/wake scheduling algorithm which 3) computes q, the optimal probability of a sensor in the wake state (which does not vary with time),based on the sensor observations obtained until time slot k.Our results show that an optimum closed loop control onMk+1 significantly decreases the cost compared to keeping any number of sensors active all the time. Also, among the three algorithms described, we observe that the total cost is minimum for the optimum control on Mk+1 and is maximum for the optimum open loop control on q.