Exciton Dynamics in Synthetic Multi-Chromophoric Model Systems

Investigating the excitonic properties of synthetic multichromophoric model systems can give insights into the behaviour of larger and more intricate structures, such as the photosynthetic complexes found in autotrophs or materials with applications in the area of organic photovoltaics (OPV). The unique properites of excitons depend critically on the electronic excited states of these systems, which present non-local character, and have short lifetimes. Hence, in order to characterise their dynamics it is helpful to employ laser spectroscopic techniques with ultrafast time resolution. Among these, the most widespread is broadband femtosecond transient absorption (fsTA), a two-pulse technique which has the drawback of being intrinsically ambiguous on the excitation frequency. A way to overcome this disadvantage is presented by two-dimensional electronic spectroscopy (2D-ES). In 2D-ES, the introduction of a third pulse allows the recovery of spectra in which excitation and detection frequencies are correlated on a two-dimensional surface. 2D-ES and fsTA have been used in a complementary fashion throughout this thesis in order to investigate photophysical processes in a range of different synthetic multichromophoric model systems. Experiments have been performed on a series of covalently-bound perylene bisimide (PBI) J-dimers. Here 2D-ES allowed us to identify a one- to two-exciton state transition in the strongly coupled dimer, which vanishes in the monomer or when the coupling is weakened. Such a transition is purely electronic in character, as confirmed by the calculated spectra, and its energy allowed us to estimate the excitonic coupling strength. We further report fsTA and 2D-ES studies of a subphthalocyanine-Zn porphyrin (SubPc-O-ZnTPP) heterodimer. fsTA allowed us to characterise the excitation energy transfer (EET) between the SubPc and ZnTPP moieties, which is well reproduced by the Forster model, while 2D-ES was used to observe sub-ps spectral diffusion, which is shown to be too fast to influence the incoherent EET. These studies were extended to larger systems. fsTA has been performed on a range of fully-conjugated porphyrin nanorings. fsTA and transient anisotropy on six-membered rings, with or without an inner template, revealed structural dynamics in the ground and in the excited states of the untemplated structure, which do not disrupt the exciton delocalisation. Finally, fsTA at increasing pump fluences allowed us to study exciton-exciton annihilation (EEA) dynamics in nanorings made up of 10, 20, 30 and 40 porphyrin units. Experiments confirmed that the exciton size is approximately 20 repeating units, and comparison with a one-dimensional diffusion model allowed estimation of the exciton diffusion coefficients, which decrease as the ring size increases, a result assigned to the increased static disorder experienced in larger structures

A Mass Balance-Based Method for the Anaerobic Digestion of Rice Straw

Current rice straw disposal practices have serious repercussions on the environment and, in addition, do not consider its energy potential. On the contrary, the anaerobic digestion of rice straw makes it possible to produce renewable energy and to reintroduce into the soil the nutrients present in the digestate, at the same time, reducing greenhouse gas emissions from paddies. For rice straw of different geographical origin, by applying a mass balance method to the digester, the minimum requirements in terms of conditioners (nitrogen, phosphorus and potassium) and water, which allow obtaining the maximum production of methane, were calculated. The results obtained show that after the first 30 days (hydraulic retention time) for each ton of rice straw digested, the daily water consumption varies considerably from one country to another, from a minimum value of 1.5 m3/d to a maximum of 4.3 m3/d. After the same time, the addition of nitrogen and phosphorus is only required for the optimal anaerobic digestion of Indian rice straw. The low presence of these elements in Indian straw requires an addition of 3 kg/d of urea and 1.5 kg/d of superphosphate to compensate for the lack of nitrogen and phosphorus, respectively. In all the examined cases, the concentration of potassium, even if higher than the optimal value, does not reach levels that can significantly affect the methane production

Rice Straw: A Waste with a Remarkable Green Energy Potential

With reference to the province of Novara in northwest Italy, this study aims to raise awareness about the environmental benefits that can derive from the use of alternative rice straw management practices to those currently in use, also highlighting how the use of these straws for energy purposes can be a valid alternative to the use of non-renewable resources. Using the LCA (Life Cycle Assessment) method, the two rice straw management practices currently in place (open field combustion and straw incorporation) were compared with an alternative strategy consisting in their collection and removal. The results show that removal of straw allows reducing the emissions of pollutants significantly: about one-hundredth of the PM (Particulate Matter) formation compared to the open-field burning and about one-tenth of the ozone depletion (CFCs, HCFCs, halons, etc.) compared to both the other two practices. Moreover, the LCA results show how the use of rice straw to produce energy as an alternative to conventional fuels helps to reduce the global warming potential of rice cultivatio

Excitonic coupling in covalently-bound Perylene Bisimide dimers revealed by two-dimensional electronic spectroscopy

Supramolecular structures based on Perylene Bisimides (PBIs) have been extensively studied because of their fundamental photophysical properties and for their application in a range of different optoelectronic devices. Two-dimensional electronic spectroscopy (2D-ES) is the most complete third order (χ(3)) technique, it has been shown that it is particularly useful to disentangle close-lying energy levels and to reveal dynamics in coupled molecular systems. Two different PBI covalently “head-to-tail” bound dimers (D0 and D1) with increasing interchromophoric separation, and a reference monomer (M) were synthesised and studied by means of 2D-ES in order to characterise the 1 to 2-exciton state transition and how its behaviour changes in relation to the PBI-PBI distance

Early-Type Galaxies in the Hubble Deep Field: The Star Formation History

We have investigated the properties of a complete K-band selected sample of 35 elliptical and S0 galaxies brighter than K=20.15 in the Hubble Deep Field, as representative of the field galaxy population. This sample has been derived from deep K-band image by the KPNO-IRIM camera, by applying a rigorous morphological classification scheme based on quantitative analyses of the surface brightness profiles. The broad-band spectra of the sample galaxies allow us to date their dominant stellar populations. The majority of bright early-types in this field are found at redshifts z<1.3 to display colors indicative of a fairly wide range of ages (typically 1.5 to 3 Gyrs). We find that the major episodes of star-formation building up typical M_star galaxies have taken place during a wide redshift interval 11.3, which should be detectable during the luminous star-formation phase expected to happen at these redshifts. Obvious solutions are a) that the merging events imply perturbed morphologies which prevent selecting them by our morphological classification filter, or b) that a dust-polluted ISM obscures the (either continuous or episodic) events of star-formation. We conclude that the likely solution is a combination thereof, i.e. a set of dust-enshrouded merging-driven starbursts occurring during the first few Gyrs of the galaxy's lifetime

Vibrational coherences in broadband 2D electronic spectroscopy: spectral filtering vs. excited state displacement

Coherences in ultrafast 2D electronic spectroscopy (2DES) reveal superpositions of quantum states corresponding to the motion of wavepackets within the potential energy surface of molecular systems. Whilst electronic coherences imply the transfer of energy between coupled chromophores, vibrational coherences track the motion of nuclear wavepackets, with their intensities governed by the displacement of the electronic excited states with respect to the ground state equilibrium geometry. Analysis of vibrational coherences thus provides valuable information on the ground and excited state structure of molecules, with ground state bleach (GSB) and stimulated emission (SE) pathways reporting on the S0 – S1 displacement and excited state absorption (ESA) pathways also involving an S1 – Sn displacement. Recent development of broadband 2DES experiments have enabled access to a greater range of coherences involving higher energy electronic states. However, a complete analysis must consider involvement of multiple vibrational modes, and any filtering of Liouville pathways due to the finite width of the excitation spectrum. Here, combining the equation of motion-phase matching approach for finite laser spectra with the hierarchical equation of motion to correctly account for dephasing and dissipation, we model half-broadband and broadband 2DES of cresyl violet to demonstrate the impact of spectral filtering vs. the relative displacement of two excited states (S1 and Sn) on the intensity distribution of peaks in the beating maps for two vibrational modes with frequencies 350 cm-1 and 585 cm-1. This study is motivated by recent experimental results from our group which interestingly show the greatest intensity of the beating maps for the 350 cm 1 mode localised in the excited state absorption region

One- to Two-Exciton Transitions in Perylene Bisimide Dimer Revealed by Two-Dimensional Electronic Spectroscopy

The excited-state energy levels of molecular dimers and aggregates play a critical role in their photophysical behavior and an understanding of the photodynamics in such structures is important for developing applications such as photovoltaics and optoelectronic devices. Here, exciton transitions in two different covalently bound PBI dimers are studied by two-dimensional electronic spectroscopy (2DES), a powerful spectroscopic method, providing the most complete picture of vibronic transitions in molecular systems. The data are accurately reproduced using the equation of motion-phase matching approach. The unambiguous presence of one-exciton to two-exciton transitions are captured in our results and described in terms of a molecular exciton energy level scheme based on the Kasha model. Furthermore, the results are supported by comparative measurements with the PBI monomer and another dimer in which the interchromophore distance is increased

Time-Resolved Structural Dynamics of Extended π-Electron Porphyrin Nanoring

Molecular structure design inspired by naturally occurring light harvesting systems has been intensely pursued over the last couple of decades. Interesting new structures include the π- conjugated porphyrin nanorings, which show promising features such as ultrafast excited-state delocalization, leading to suppressed radiative rates, superradiance with increasing temperature and energy transfer times comparable to their natural counterparts. An mportant question to be addressed in such systems is the role and time scale of structural motions and how they affect excited-state delocalization. Here it is shown that porphyrin nanorings which are not rigidified by a template are structurally heterogeneous in the ground state and evolve dynamically on a tens of picoseconds timescale. In the excited state a structural relaxation of the porphyrin nanorings is observed, on a picosecond timescale. Furthermore ultrafast excitation delocalization is observed, by anisotropy measurements, being insensitive to structural motions of the nanorings

Ultrafast excimer formation and solvent controlled symmetry breaking charge separation in the excitonically coupled subphthalocyanine dimer

Knowledge of the factors controlling excited state dynamics in excitonically coupled dimers and higher aggregates is critical for understanding natural and artificial solar energy conversion. In this work we report ultrafast solvent polarity dependent excited state dynamics of the structurally well‐defined subphthalocyanine dimer, μ‐OSubPc 2 . Stationary electronic spectra demonstrate strong exciton coupling in μ‐OSubPc 2 . Femtosecond transient absorption measurements reveal ultrafast excimer formation from the initially excited exciton, mediated by intramolecular structural evolution. In polar solvents the excimer state decays directly through symmetry breaking charge transfer to form a charge separated state. Charge separation occurs under control of solvent orientational relaxation