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Page updated at 10:18:14 AM on Tuesday, May 22nd, 2012

Study of energy and charge flow in novel ad-hoc synthesized systems: towards intelligent systems for organic photovoltaics

Funded with €768,800.00 (on €1,034,000.00) for 36 months (from March, 2012 to March, 2015) by Italian MIUR within FIRB 2010 Edition

Person in charge: Mariangela Di Donato
LENS members: Paolo Foggi, Andrea Lapini, Manuela Lima, Agnese Marcelli, Roberto Righini

Revealing the structure-function relationships in artificial photosynthetic devices for synthesis and design of next generation of organic photovoltaic cells.

Figure 1 - An example of light-harvesting antenna, a special membrane-associated pigment-protein complex able to absorb sun photons.

The increasing demand for clean and renewable energy sources has promoted many attempts at mimicking natural photosynthesis through the development of artificial systems able to efficiently absorb solar light and transform it into useful forms of energy.

In the primary steps of photosynthesis, sun photons are absorbed by special membrane-associated pigment-protein complexes, the light-harvesting antennas. These are organized multi-chromophoric systems, able to absorb the incident light and funnel the excitation energy to an acceptor component (the reaction center). In the course of evolution, nature has developed antenna systems able to collect solar energy in a very efficient way, and to channel it to the reaction center, where its conversion into (electro-)chemical energy takes place.

Within this research line our aim is to elucidate the structure-function relationships in artificial photosynthetic devices and to understand the mechanisms influencing the efficiency of energy transfer and charge separation.

Figure 2 - Three different generations of multichromophoric systems of increasing complexity under investigation.

We aim at synthetizing and studying new multichromophore units functioning as artificial photosynthetic devices. In collaboration with the organic chemistry group from the University of Parma we are identifying a series of suitable chromophores, responding to specific energetic requirements. These molecules are then linked to calix[n]arenes, a family of versatile organic scaffolds that can be easily functionalized with multiple photoactive units at fixed distances and orientation (Fig. 1).

We will synthesize a small library of multichromophoric derivatives having different number and type of chromophores, variable size and flexibility, and both covalent and non covalent linkages. Overall we aim at adopting a systematic approach, based on the investigation of different generations of multichromophoric systems of increasing complexity: starting from simple bi-chromophore assemblies (first generation), we will then investigate systems with an increased number of chromophores of the same kind (second generation) or of different kind (third generation), i.e see Fig. 2.

As a result of the study of the minimal-unit systems, we expect to obtain a direct understanding of the basic photophysical and photochemical mechanisms underlying the behavior of the more complex systems, furnishing us strategic guidelines for engineering the molecular assemblies of the next generation. The knowledge acquired with this systematic approach will finally result in the acquisition of knowledge to be exploited for the design of high-performing artificial photosynthetic devices. These systems will be studied using a combination of state-of-the-art nonlinear spectroscopies and theoretical tools, in order to identify the structural factors that are crucial for efficient energy and electron transfer.

To this aim, the LENS unit will develop and apply a new kind of nonlinear technique, the two-dimensional (2D) visible spectroscopy, which has the ability not only of spectrally resolving the excitation and emission energies of multiple pigments with femtosecond time resolution, but also of determining the exciton interactions between them. Furthermore, techniques based on the analysis of the Mid-infrared (MIR) spectral region, already available, will be applied. In particular, we will use time-resolved visible-pump/midIR-probe spectroscopy and 2D-IR spectroscopy, which combine the time resolution provided by ultrashort laser pulses (<50 fs) and the sensitivity of the infrared spectral region to small perturbations of the molecular environment, thus allowing to investigate structural changes occurring in complex multichromophoric systems on the sub-picosecond timescale.

In order to fully exploit the potentiality of two-dimensional spectroscopies, the use of suitable theoretical models is almost compulsory to give the correct interpretation of the complex experimental outcomes. For this reason, the present project also relies on the well-documented expertise of the Pisa and Parma research units in the development of theoretical and computational models for the description of complex spectroscopic properties. A tight interplay between theory and experiment will be the core of this research, whose final outcome is that of identifying the structure-function relationships needed to obtain very efficient and specifically designed multichromophoric systems for energy and charge transfer, to be employed in organic photovoltaic devices.



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