Exciton Energy and Charge Transfer in Porphyrin Aggregate

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Exciton Energy and Charge Transfer in Porphyrin Aggregate/ Semiconductor (TiO2) Composites Sandeep Verma and Hirendra N. Ghosh*

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Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Mumbai, 400085, India 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 Haggregates. The porphyrin J-aggregate emerges with better spectral coverage and exciton dynamics, which are suitable for light-harvesting antenna functions. This motif is explored in a photosensitization study of TiO2 semiconductor materials. The transient absorption studies show that the J-aggregate improves the photoinduced charge separation at the porphyrin/TiO2 interface. The higher charge separation is attributed to exciton-coupled charge-transfer processes in porphyrin J-aggregate/TiO2 hybrid materials. It represents the potential of porphyrin aggregates in biomimetic artificial antenna activity.

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n ordered self-assembly of porphyrin molecules is a novel proposition for light-harvesting and photovoltaic applications.1−5 The multilayer interfacing of porphyrin rings is very likely to mimic the antenna function for its long-range association of π-network. The localized π−π* transitions of the monomer evolve as delocalized coherent excited states in aggregates of porphyrins and phthalocyanines.6,7 The excitons emerging from strong intermolecular dipole−dipole interactions are energetically different from those in localized excited states (π−π*).6−8 The energy difference between the exciton state and localized excited state is conducive for intra-aggregate energy transfer. In a defect-free aggregate, the exciton coherence can spread over the 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, and so forth are well-supported by a protein scaffold. It is comprised of 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 a perfect stacking pattern of chromophores.11 The problem arises when natural antenna templates are mimicked by weak cohesion of self-assembled porphyrin. The porphyrin aggregations are formed by many weak interactions, namely, 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 an exciton state to energy transfer process. Thus, the exciton temporal behavior is a very critical factor for the performance of light-harvesting porphyrin aggregates. © 2012 American Chemical Society

The exciton temporal behavior is a very critical factor for the performance of light-harvesting porphyrin aggregates. The porphyrin molecules tend to aggregate in the solution phase with a “face-to-face” or “edge-to-edge” assembly pattern of H- or J-aggregates, respectively. The advantage of using porphyrin in the solution phase is the flexibility in making an H- or J-aggregate by varying the pH, counterions, and concentrations of the solution.12−14 In addition, a specific aggregate can be obtained by using a different meso-substituent of porphyrin.15 However, the bath fluctuations can disrupt the long-range order and introduce packing defects in the aggregate structure. Formation of defects restrict the exciton coherence to the size domain (Nc ≈ 12−20) to be smaller than the actual size of the aggregate (∼100 nm).16 In the case of H-aggregates, the exciton state energy of the smaller domain (part of the ordered stacking) is lower than that of the larger domain.17 It leads to energy localization on the smaller domain. Furthermore, the radiative electronic transitions from the lowest exciton to the ground state are forbidden in Haggregates.17 This presumably restricts long-range energy transfer in the H-aggregate. In this regard, the J-aggregate can be more useful as the largest domain consists of the 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 Received: May 19, 2012 Accepted: June 28, 2012 Published: June 28, 2012 1877

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Scheme 1. Schematic Diagram of the Porphyrin J-Aggregate/TiO2 Compositea

a

Reproduced with permission from ref 39, copyright Wiley-VCH Verlag GmbH & Co. KGaA.

thermodynamic and kinetic barriers in interfacial exciton dissociation. The strong electronic interaction between dye and TiO2 NPs facilitates formation of a CT complex. The adiabatic electron transfer is a main feature of CT complexes.29 The CT complex in the porphyrin J-aggregate/TiO2 system facilitates adiabatic electron transfer, which dominates over exciton decay in the aggregated moiety. Femtosecond time-resolved spectroscopic investigations of catechol/TiO2 systems have revealed the direct pathways of electron transfer from the dye-HOMO to TiO2 conduction band.30 Therefore, it is logical to use catechol functionality in porphyrin aggregates because it also offers significantly large electronic coupling on the TiO2 surface (∼1 eV range) in comparison to that of the carboxylate group.31 Besides energetic and kinetic aspects, this moiety also offers a hydrogen-bonding network for structural integrity of aggregates.14 In this scenario, the exciton-coupled CT phenomenon seems feasible in ordered porphyrin aggregates/TiO2 composites. The major concern is whether the interfacial electron transfer takes place before the onset of exciton decay or not. The knowledge about kinetic competition is pivotal to lightharvesting aggregates for their transformation into an energy funnel in the photosensitization process. Porphyrin Aggregates. The exciton coupling of transition dipole moments of Soret and Q-bands of porphyrin monomers results in exciton bands in the aggregate.6−8,18 The energetic shift of exciton bands is different for J- and H-aggregates with respect to monomer bands. Therefore, the blue or red shift in electronic transitions is used as an marker of H- or Jaggregation, respectively. The optical absorption spectra of two types of aggregates are shown in Figure 1. The two distinct aggregates are further established in circular dichroism spectra and AFM images. The H-aggregate forms due to hydrogen bonding in neutral aqueous solution (∼6−8 pH). In acidic solution (1.8 pH), the repulsive forces between the protonated porphyrin and HTPPcat+ causes a slipped plane configuration of the J-aggregates. These results suggest that the hydrogenbonding network is beneficial in the formation of two different H- and J-aggregates under different pH conditions. It is remarkable that J-aggregates are formed in the presence of the NO3− counterion but not with Cl− ions. The helical tubular structure of J-aggregates is pointed out by bisignate CD (∼490 and ∼740 nm).7,32 Recent studies show that rolling of the aggregate’s sheet to form tubular structures is stabilized by hydrogen bonding.33 Presumably, the NO3− counterions

kinds of aggregates. A relative measure of exciton dynamics in H- and J-aggregates is crucial for photosensitization applications. As shown in Scheme 1, the energy of the exciton state of the J-aggregate of TPPcat is lower than that of monomer porphyrin units. It is seen in the scheme that all of the monomer units of an aggregate are not coupled with TiO2. Nonetheless, the interim monomers are able to transfer energy to an 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 the feasibility of electron injection from surface-attached porphyrin aggregates to semiconductor nanomaterials.17,19 The use of a Frenkel exciton in the interfacial charge-transfer (CT) process is proposed in the excitonic solar cell.20,21

The strong electronic coupling helps to overcome thermodynamic and kinetic barriers in interfacial exciton dissociation. The concept of the exciton solar cell is based on exciton generation and diffusion to a heterointerface where it dissociates into free charge carriers. In principle, the efficiency of the excitonic solar cell is governed by the 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.22 In DSSC, the electronic coupling strength of the dye adsorbate to the TiO2 electrode (180 ps (10%)

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

Adapted from ref 24, copyright American Chemical Society.

between the Q-band and Soret band. Thus, the decay of the S2 exciton is reflected in the S1 exciton dynamics. Two distinct exciton relaxation processes of the H- and J-aggregates are depicted in Scheme 2. Scheme 2. In-Phase and Out-of-Phase Dipole−Dipole Interactions Shown with Parallel and Opposite Arrow Signsa

Figure 3. (Left) Transient absorption spectra of (A) the monomerTPPcat (5.8 pH), (B) H-aggregates of TPPcat, (C) monomer-TPPcat (1.8 pH), and (D) J-aggregates of TPPcat at 200 and 400 fs and 1, 5, and 20 ps delay times after 400 nm photoexcitation. (Right) Transient absorption kinetics of S2 states of the monomer and aggregates of TPPcat as monitored at 490 nm. Reproduced with permission from ref 24, copyright American Chemical Society. a

monomer porphyrins.24 The internal conversion is followed by vibrational redistribution in the hot S1 state (3 ± 1 and 15 ± 5 ps; Table 1). In the literature, this biphasic relaxation of the S1 state is attributed to intramolecular (solute) and intermolecular (solute−solvent) vibrational cooling processes.40 Thermally relaxed S1 states of porphyrin are found to be emissive with a 2−9 ns lifetime. The aggregation introduces an additional ultrafast decay process of the S2 exciton that occurs on the subpicosecond time scale. The S2 exciton state of the Haggregate (100 fs (70%)) decays more rapidly than vibrational redistribution of the monomer porphyrin (3 ± 1 and 15 ± 5 ps). The intra-aggregate vibronic coupling of the H-aggregate produces nonradiative exciton decay on an ultrashort (100 fs) time scale. Unlike the H-aggregate, the decay of the S2 exciton of the Jaggregate is expected to be a radiative process. The decay time constant for the S2 exciton of the J-aggregate is observed to be 200 fs by monitoring the bleach recovery kinetics at 490 nm. Interestingly, the ultrafast decay component (200 fs) also appears in the bleach recovery kinetics for the Q-band (750 nm). This observation indicates the intensity borrowing effects

The low-energy exciton states are forbidden in H-aggregates. The high-energy exciton state decays nonradiatively on a 100 fs time scale. The low-energy S2 and S1 exciton states of the J-aggregates are optically allowed and undergo radiative decay with 200 fs and 180 ps time constants, respectively. Reproduced with permission from ref 24, copyright American Chemical Society.

As shown in Scheme 2, the temporal responses of Jaggregates are more suitable for photosensitization applications as compared to those of H-aggregates. Having the low-energy exciton states, the J-aggregate receives energy from the porphyrin monomer. Therefore, the J-aggregate acts as an energy funnel, which can improve the interface activity in the aggregate/inorganic matrix. Recent mesoscale integration of the porphyrin J-aggregate/titania (TiO2 nanofiber) has shown an increased photocatalytic activity of such a novel hybrid material.41 The benefits of exciton electronic properties are subject to kinetic favor at the interface. The kinetic model is facile if an ultrafast quenching mechanism is operative at the Jaggregate/TiO2 heterojunction. It is then possible that the exciton dissociation is followed by interfacial charge separation. 1880

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The benefits of exciton electronic properties are subject to kinetic favor at the interface. The exciton-coupled CT at the J-aggregate/TiO2 system is illustrated in the next section. Exciton-Coupled CT Interaction: Porphyrin/TiO2 System. The state-of-the-art porphyrin DSSC has shown record 12.3% light-to-current conversion efficiency.42 However, the efficiencies of most porphyrin DSSCs are reported to be less than that of N3dye-DSSC.43 Despite the high-energy excited states of the porphyrin, the interfacial charge separation is low due to inefficient electron injection and lesser charge stabilization in the porphyrin/TiO2 system.44 This is partly due to poor electronic coupling and nonspecific aggregate formation of the porphyrin on the TiO2 surface.45,46 The electronic coupling at the dye/TiO2 NP interface can be controlled by using suitable binding groups. The catechol linkage leads to strong binding to the TiO2 surface.47 The strong coupling is evidenced in the absorption spectrum of the catechol-functionalized porphyrin TPPcat/TiO2 NP system. Figure 4 (left) shows two new absorption bands appearing in the 450−600 and 675−825 nm regions. These two bands

Figure 5. Transient absorption spectrum of (A) the monomerTPPcat/TiO2 NP and (B) J-aggregate-TPPcat/TiO2 NP in aqueous solution at 200 and 500 fs and 1, 5, and 20 ps delay times. (C) 1000 nm TA kinetics of (a) monomer-TPPcat/TiO2 and (b) J-aggregateTPPcat/TiO2. (D) 670 nm TA kinetics of (c) monomer-TPPcat/TiO2 and (d) J-aggregate-TPPcat/TiO2. Reproduced with permission from ref 39, copyright Wiley-VCH Verlag GmbH & Co. KGaA.

absence of an exciton decay component in TA kinetics indicates that electron injection is accomplished before the onset of exciton decay processes. This is the advantage of using catechol functionality as it ensures efficient electron injection even from short-lived exciton states. The regeneration of the ground state via the BET process is monitored by 670 or 1000 nm decay kinetics. As shown in Figure 5, the BET dynamics is slower in the J-aggregate-TPPcat/TiO2 as compared to that in the monomer-TPPcat/TiO2 system. The faster BET dynamics in the monomer-TPPcat/TiO2 system is due to the close proximity of charge-separated species (electron and cation). In the J-aggregate, the exciton coherence delocalizes the hole over many porphyrin molecules (NC ≈ 14). Thus, the hole produced after electron injection is no longer bound to the terminal porphyrin but migrates away from the J-aggregateTPPcat/TiO2 interface. The large spatial charge separation leads to slow charge recombination in the J-aggregate-TPPcat/ TiO2 system. Apart from this, J-aggregate formation effectively reduces the localized electron contribution in interfacial charge separation. The surface-localized electrons are main features of the strongly coupled catechol/TiO2 system. The recombination of the localized electron with the dye cation is known to be on an ultrafast 100−400 fs time scale.30 In the TPPcat/TiO2 system, a near-perpendicular geometry of the meso-substituted catechol ring with respect to the porphyrin ring49 reduces the porphyrin/TiO245 coupling. Therefore, the contribution of pure catechol/TiO2 CT is expected to be high in the overall charge recombination process. The CT recombination of the monomer-TPPcat/TiO2 and J-aggregate-TPPcat/TiO2 are monitored by bleach recovery at 730 nm and excited-state absorption at 570 nm, simultaneously, shown in Figure 6. The bleach recovery of monomer-TPPcat/TiO2 shows an additional ultrafast component of ∼130 fs (54%), which is attributed to CT recombination dynamics. Interestingly, this process is suppressed in the J-aggregate-TPPcat/TiO2 system. In J-aggregate-TPPcat/TiO2, only the terminal TPPcat porphyrin is directly bound to the TiO2 surface, whereas the rest of porphyrin’s stack participates in electron injection through exciton coherence. This effectively minimizes the pure catechol/TiO2 CT interaction without affecting the electrondonating strength of the porphyrin aggregate. The 730 nm bleach recovery kinetics of the J-aggregate-TPPcat/TiO2 system

Figure 4. Optical absorption spectra of TPPcat under different conditions: (a) monomer, (b) monomer/TiO2, (c) J-aggregate, (d) Jaggregate/TiO2, and (e) bare TiO2. (Inset) PL spectra of (A) monomer, (B) monomer/TiO2, (C) J-aggregate, and (D) J-aggregate/ TiO2 systems. Reproduced with permission from ref 39, copyright Wiley-VCH Verlag GmbH & Co. KGaA.

correspond to strong CT interactions in the catechol/TiO2 and porphyrin/TiO2 systems. The CT interaction facilitates adiabatic electron transfer from the porphyrin to TiO2 NP. It improves the electron injection efficiency, which is noticed in emission quenching (∼94%) measurements (Figure 4 (inset)). Nonetheless, the back electron transfer (BET) also increases due to strong electronic coupling (CT) of the catechol/TiO2 linkage. The dynamics of the electron injection and BET processes are shown in Figure 5. The kinetics of delocalized electron in the conduction band of the TiO2 NP is monitored at 1000 nm.39,48 The decay of the 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 biphasic vibrational relaxation of the S1 state is absent in TA kinetics of the monomer-TPPcat/TiO2 system. More importantly, the electron injection kinetics (1.0 eV)28 of the porphyrin catechol/TiO2 pair. The resulting interfacial charge separation is stabilized via hole migration within the aggregates. The immobilized hole is spatially less accessible to the electron injected in TiO2 and hence leads to slower BET. A similar mechanism of hole migration is observed in the natural light-harvesting antenna complex phycocyanin− allophycocyanin-sensitized ZnO semiconductor system.50 Like the porphyrin aggregate, it is comprised of long-range association of tetrapyrrole chromophores, which improves the interfacial charge separation on the ZnO surface. Essentially, it elucidates that the large network of electron-donor chromophores screens the positive charge within the light-harvesting aggregate and enhances the interfacial charge separation. In this Perspective, the functioning of the J-aggregate/TiO2 composite system enlightens the basic requirement of long-range association of light-absorbing chromophores to achieve energy and CT antenna function. The self-assembled biomimetic chromophore rings addressed here give insight into energy transfer, charge transport, and charge separation at the reaction center. The critical step is to produce order-specific assembly of interactive molecules for energy-efficient light-harvesting devices.

also exhibits an additional 500 fs (40%) component that is not observed in 1000 nm (delocalized electron) TA kinetics. The 570 nm decay kinetics reaffirms the presence of an ultrafast component (500 fs) in the J-aggregate-TPPcat/TiO2 system. It resembles the exciton-coupled CT transition in the J-aggregateTPPcat/TiO2 system.

The functioning of the J-aggregate/TiO2 composite system enlightens the basic requirement of long-range association of lightabsorbing chromophores to achieve energy- and chargetransfer antenna functions. The exciton-coupled CT interaction allows the J-aggregate to sensitize the TiO2 NP as a single entity. It substantially compensates the weak coupling arising from the nearperpendicular orientation of the porphyrin ring and the meso-substituted phenyl linkage to the TiO2 surface. This system comprises the advantage of antenna functionality of porphyrin aggregates and efficient electron injection due to catecholate binding. The mechanism of interfacial CT is shown in Scheme 3, where the strong coupling between the terminal porphyrin catechol and TiO2 facilitates an ultrafast electron injection from exciton states, well before the onset of the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+) 91-22-25505331/ 25505151. Notes

The authors declare no competing financial interest. 1882

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Biographies

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Sandeep Verma received a Master of Science degree in Chemistry from the University of Rajasthan in 2003 and joined at the Bhabha Atomic Research Centre (BARC) in 2004. He started Ph.D. studies at the Homi Bhabha National Institute in 2007 on photoinduced energyand electron-transfer processes in light-harvesting aggregates, organometallic complexes, and semiconductor nanomaterials. His research work includes synthesis of aggregates, nanoparticles, and ultrafast timeresolved absorption and emission studies. His Ph.D. thesis was 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 the University of Mumbai. He worked as a Postdoctoral fellow for the period of 1997−1998 at the Chemistry Department of Emory University, Atlanta, U.S.A. Dr. Ghosh also visited Max-Born Institute, Berlin, Germany as a visiting scientist for 1 year (2007−2008). Currently, Dr. Ghosh is working as a senior scientist at the Bhabha Atomic Research Centre, Mumbai, India. His current research interest includes ultrafast interfacial electron-transfer dynamics in dyesensitized semiconductor nanoparticles and charge carrier relaxation dynamics in quantum dot and quantum dot core−shell nanostructured materials and proton-coupled electron-transfer reactions (PCET) in the solution phase using different ultrafast techniques like femtosecond visible and infrared spectrometers.

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ACKNOWLEDGMENTS 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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NOTE ADDED AFTER ASAP PUBLICATION This Perspective was published on the Web on June 28, 2012. In the concluding paragraph, minor wording changes were made. The corrected version was reposted on July 5, 2012.

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