Exciton Energy and Charge Transfer in Porphyrin Aggregate

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Exciton Energy and Charge Transfer in Porphyrin Aggregate/Semiconductor (TiO) composites 2

Sandeep Verma, and Hirendra Nath Ghosh J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/jz300639q • Publication Date (Web): 28 Jun 2012 Downloaded from http://pubs.acs.org on July 1, 2012

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The Journal of Physical Chemistry Letters

Exciton Energy and Charge Transfer in Porphyrin Aggregate/Semiconductor (TiO2) composites Sandeep Verma and Hirendra N. Ghosh* Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai, 400085, India. Authors Email: [email protected], [email protected] * To whom correspondence should be addressed. E-mail: [email protected], Fax: (+) 91-2225505331/25505151

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ABSTRACT: A porphyrin aggregate is reported that exhibits novel exciton state properties for light harvesting applications. This porphyrin aggregate enables control of energy dissipation of coherent excited states by changing the self-assembly pattern. New exciton spectral features create a new route of energy transfer in this porphyrin aggregate. The kinetic model of exciton state decay is addressed in this Perspective by reporting steady state and transient -emission and absorption- studies of porphyrin J- and H-aggregates. The porphyrin J-aggregate emerges with better spectral coverage and exciton dynamics which are suitable for light harvesting antenna function. This motif is explored in photo sensitization study of TiO2 semiconductor material. The transient absorption studies show that J-aggregate improves the photoinduced charge separation at porphyrin/TiO2 interface. The higher charge separation is attributed to exciton coupled charge transfer processes in porphyrin-J-aggregate/TiO2 hybrid material. It represents the potential of porphyrin aggregates in biomimetic artificial antenna activity.

TOC Graphic:

Keywords: “Porphyrin”, “Aggregate”, “exciton”, “Electron transfer”, “nanoparticle”

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An ordered self-assembly of porphyrin molecules is a novel proposition for lightharvesting and photovoltaic applications.1-5 The multilayer interfacing of porphyrin rings is very likely to mimic antenna function for its long range association of π-network. The localized π−π∗ transitions of monomer evolve as delocalized coherent excited states in aggregates of porphyrins and phthalocyanines.6,7 The excitons emerging from strong intermolecular dipoledipole interaction, are energetically different from localized excited states (π−π∗).6-8 The energy difference between exciton state and localized excited state, is conducive for intra-aggregate energy transfer. In a defect free aggregate, the exciton coherence can spread over full aggregate length and the exciton route of energy transfer can be very efficient. The energy relay concept of porphyrin aggregates is inspired from natural light harvesting complexes.9,10 In nature, the microorganizations of light harvesting complexes such as chlorophyll, phycocyanobilin, etc. are well supported by protein scaffold. It comprises an ideal packing environment which is required for strong excitonic interaction and long range dipole-dipole interaction. The energy transfer efficiency as high as 95% is the outcome of perfect stacking pattern of chromophores.11 The problem arises when natural antenna templates are mimicked by weak cohesion of selfassembled porphyrin. The porphyrin-aggregations are formed by many weak interactions viz. van der Waals, π−π, electrostatic and hydrogen bonding interactions. As a result, the stacking disorders are prone to occur in porphyrin-self-assembly which may lead to undesirable exciton trapping processes. This can be a major setback in employing exciton state to energy transfer process. Thus, the exciton temporal behavior is very critical factor for the performance of light harvesting porphyrin-aggregates. The porphyrin molecules tend to aggregate in solution phase with “face-to-face” or “edge 3



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-to-edge” assembly pattern of H- or J-aggregates, respectively. The advantage of using porphyrin in solution phase is the flexibility in making H- or J-aggregate by varying pH, counter ions and concentrations of the solution.12-14 In addition, a specific aggregate can be obtained by using different meso-substituent of porphyrin.15 However, the bath fluctuations can disrupt the long range order and introduces packing defects in aggregate structure. Formation of defects restrict the exciton coherence to size domain (Nc ~12-20) smaller than the actual size of aggregate (~ 100 nm).16 In the case of H-aggregates; the exciton state energy of smaller domain (part of ordered stacking) is lower than that of larger domain.17 It leads to energy localization on smaller domain. Furthermore, the radiative electronic transitions from the lowest exciton to the ground states are forbidden in H-aggregates.17 This presumably restricts long range energy transfer in H-aggregate. In this regard, the J-aggregate can be more useful as the largest domain consist of lowest exciton state and the relevant electronic transitions are optically allowed.18 The remarkable change in exciton state characteristics affects the exciton state decay differently in these two kinds of aggregates. A relative measure of exciton dynamics in H- and J-aggregate is crucial for photosensitization applications.

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Scheme 1. Schematic diagram of porphyrin-J-aggregate/TiO2 composite. Reproduced with permission from ref. 40, copyright Wiley-VCH Verlag GmbH & Co. KGaA As shown in scheme-1, the energy of exciton state of J-aggregate of TPPcat is lower than that of monomer porphyrin units. It is seen in the scheme that all the monomer units of an aggregate are not coupled with TiO2. Nonetheless, the interim monomers are able to transfer energy to existing aggregate. This directional energy transfer makes the aggregate more suitable for broader spectral sensitization. This can be very useful in sensitizing the wide band gap material. Earlier studies have shown feasibility of electron injection from surface attached porphyrin aggregates to semiconductor nanomaterials.19,20 The use of Frenkel exciton in interfacial charge transfer process is proposed in excitonic solar cell.21,22 The concept of exciton solar cell is based on exciton generation and diffusion to heterointerface where it dissociates into free charge carriers. In principle, the efficiency of excitonic solar cell is governed by exciton binding energy and its intrinsic relaxation dynamics. In general the exciton (Frenkel) binding energy is significantly high and this lies in the range of 0.8-1.0 eV.23 In DSSC, the electronic coupling strength of dye adsorbate to TiO2 electrode ( 1 ns(68 %)

200 fs (70 %) 1.5 ps (10%) 15 ps (10%) > 180 ps (10%)

100 fs (70 %) 1.5 ps (12%) 15 ps (12%) > 100 ps (6%)

internal conversion occurs in ns (-9.6%)

ns (-32%)

* contribution from localized charge transfer dynamics) The kinetics of delocalized electron in conduction band of TiO2 NP is monitored at 1000 nm.40,49 The decay of electron is probed simultaneously by bleach recovery kinetics at 670 nm. The fitting time constants are provided in table-2. A pulse width limited -single exponential rise (+100%; Table-2) in 1000 nm TA kinetics corresponds to electron injection from unthermalized S2 and S1 -states. Consequently, the bi-phasic vibrational relaxation of S1 state is absent in TA kinetics of monomer-TPPcat/TiO2 system. More importantly, the electron injection kinetics ( 1.0 eV)29 of porphyrin catechol-TiO2 pair. The resulting interfacial charge separation is stabilized via hole migration within aggregates. The immobilized hole is spatially less accessible to electron injected in TiO2 and hence leads to slower back electron transfer. Similar mechanism of hole migration is observed in natural light harvesting antenna complex- phycocyanin-allophycocyanin sensitized ZnO semiconductor system.51 Like porphyrin-aggregate, it comprises long range association of tetrapyrrole chromophores which improves the interfacial charge separation on ZnO surface. 18


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Essentially, it elucidates that large network of electron donor chromophores, screens the positive charge within light harvesting aggregate and enhances interfacial charge separation. In this perspective, the functioning of J-aggregate/TiO2 composite system enlighten the basic requirement of long range association of light absorbing chromophores to achieve energy-and charge transfer- antenna function. The self-assembled biomimetic chromophore rings addressed here, give insight into energy transfer, charge transport and charge separation at reaction centre. The critical step is to produce order specific assembly of interactive molecules for energyefficient-light-harvesting-devices.

AUTHOR INFORMATION Biographies Sandeep Verma received a Master of Science degree in Chemistry from University of Rajasthan in 2003 and joined at Bhabha Atomic Research Centre (BARC) in 2004. He started Ph.D. studies at Homi Bhabha National Institute in 2007 on photoinduced energy and electron transfer processes in light harvesting aggregates, organometallic complexes and semiconductor nanomaterials. His research work includes synthesis of aggregates, nanoparticles and ultrafast time resolved absorption and emission studies. His PhD thesis is accepted in 2012 and presently he is working as a scientist in BARC, Mumbai. Hirendra N. Ghosh obtained his Ph.D. degree in 1996 from University of Mumbai. He worked as a Post-doctoral fellow for the period of 1997-1998, at the Chemistry Department of Emory University, Atlanta, USA. Dr. Ghosh also visited Max-Born Institute, Berlin, Germany as a visiting scientist for one year (2007-2008). Currently Dr. Ghosh is working as senior scientist in 19



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Bhabha Atomic Research Centre, Mumbai, India. His current research interest includes ultrafast interfacial electron transfer dynamics in dye-sensitized semiconductor nanoparticles and charge carrier relaxation dynamics in quantum dot and quantum dot core-shell nano-structured materials and proton coupled electron transfer reaction (PCET) in solution phase using different ultrafast techniques like Femtosecond visible and Infrared Spectrometer.

Acknowledgement: We cordially thank our collaborator Dr. Amitava Das of Central Salt & Marine Chemicals Research Institute (CSMCRI) Bhavnagar, Gujarat (India) for scientific discussion and fruitful suggestions. We also thank Dr. D. K. Palit, Dr. S.K. Sarkar and Dr. T. Mukherjee for their encouragement.

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Quotes •

The exciton temporal behavior is very critical factor for the performance of light harvesting porphyrin-aggregates. (Page-3)

•

The strong electronic coupling helps to overcome thermodynamic and kinetic barrier in interfacial exciton dissociation. (Page-5)

•

The major concern is that whether the interfacial electron transfer takes place before onset of exciton decay or not. (Page-6)

•

The benefits of exciton electronic properties are subject to kinetic favour at the interface. (Page-11)

•

The functioning of J-aggregate/TiO2 composite system enlighten the basic requirement of long range association of light absorbing chromophores to achieve energy- and charge transfer- antenna function. (Page-18)

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Exciton Energy and Charge Transfer in Porphyrin Aggregate

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