Representative Publications

Is the Chemical Strategy for Imbuing “Polyene” Character in Diketopyrrolopyrrole-Based Chromophores Sufficient for Singlet Fission?

Tushita Mukhopadhyay, Andrew J. Musser, Boregowda Puttaraju, Joydeep Dhar, Richard H. Friend, and Satish Patil

J. Phys. Chem. Lett. 2017, 8 (5), 984–991.

DOI: 10.1021/acs.jpclett.6b02919

 In this work, we have rationally designed and synthesized a novel thiophene-diketopyrrolopyrrole (TDPP)-vinyl-based dimer. We have investigated the optical and electronic properties and have probed the photophysical dynamics using transient absorption to investigate the possibility of singlet exciton fission. These revealed extremely rapid decay to the ground state (<50 ps), which we confirm is due to intramolecular excitonic processes rather than large-scale conformational change enabled by the vinyl linker. In all cases, the main excited state appears to be “dark”, suggesting rapid internal conversion into a dark 2Ag-type singlet state. We found no evidence of triplet formation in TDPP-V-TDPP under direct photoexcitation. This may be a consequence of significant singlet stabilization in the dimer, bringing it below the energy needed to form two triplets. Our studies on this model compound set valuable lessons for design of novel triplet-forming materials and highlight the need for more broadly applicable design principles.

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Resonant Transport in Single Diketopyrrolopyrrole Junctions

Yaping Zang, Suman Ray, E-Dean Fung, Anders Borges, Marc H Garner, Michael L Steigerwald, Gemma C Solomon, Satish Patil, Latha Venkataraman

J. Am. Chem. Soc. 2018, 140 (41), 13167–13170.

DOI: 10.1021/jacs.8b06964.

We study the single-molecule transport properties of small bandgap diketopyrrolopyrrole oligomers (DPP n, n = 1-4) with lengths varying from 1 to 5 nm. At a low bias voltage, the conductance decays exponentially as a function of length indicative of nonresonant transport. However, at a high bias voltage, we observe a remarkably high conductance close to 10-2 G0 with currents reaching over 0.1 μA across all four oligomers. These unique transport properties, together with density functional theory-based transport calculations, suggest a mechanism of resonant transport across the highly delocalized DPP backbones in the high bias regime. This study thus demonstrates the unique properties of diketopyrrolopyrrole derivatives in achieving highly efficient long-range charge transport in single-molecule devices.

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Molecular Design Strategies for Efficient Intramolecular Singlet Exciton Fission

K. C. Krishnapriya, Andrew J. Musser, and Satish Patil

ACS Energy Lett. 2018, 4 (1), 192–202.

DOI: 10.1021/acsenergylett.8b01833.

The process of carrier multiplication via singlet fission can potentially exceed the Shockley–Queisser limit on the efficiency of single-junction photovoltaics. In the recent past, theoretical analysis provided the principal guidelines on molecular design strategies for singlet fission. In this Perspective, we focus instead on correlating experimental results for different classes of reported singlet fission materials to identify principles to aid in the design of new molecules for efficient intramolecular singlet fission. Building on an evaluation of several series of multichromophoric and polymeric singlet fission materials, we extract new suggested strategies for molecular design. 

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Spin Density Encodes Intramolecular Singlet Exciton Fission in Pentacene Dimers

K. C. Krishnapriya, Palas Roy, Boregowda Puttaraju, Ulrike Salzner, Andrew J. Musser, Manish Jain, Jyotishman Dasgupta & Satish Patil

NAT. COMMUN. 2019 101 2019, 10 (1), 1–8.

DOI: 10.1038/S41467-018-07736-3. 

The formation of two triplet excitons at the cost of one photon via singlet exciton fission in organic semiconductors can potentially enhance the photocurrent in photovoltaic devices. However, the role of spin density distribution in driving this photophysical process has been unclear until now. Here we present the significance of electronic spin density distribution in facilitating efficient intramolecular singlet exciton fission (iSEF) in π-bridged pentacene dimers. We synthetically modulate the spin density distribution in a series of pentacene dimers using phenyl-, thienyl- and selenyl- flanked diketopyrrolopyrrole (DPP) derivatives as π-bridges. Using femtosecond transient absorption spectroscopy, we find that efficient iSEF is only observed for the phenyl-derivative in ~2.4 ps while absent in the other two dimers. Electronic structure calculations reveal that phenyl-DPP bridge localizes α- and β-spin densities on distinct terminal pentacenes. Upon photoexcitation, a spin exchange mechanism enables iSEF from a singlet state which has an innate triplet pair character.

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Energy Transfer in Ternary Blend Organic Solar Cells: Recent Insights and Future Directions

Aiswarya Abhisek Mohapatra, Vivek Tiwari and Satish Patil

Energy Environ. Sci. 2021, 14 (1), 302–319

DOI: 10.1039/d0ee03170d

Resonance energy transfer (RET) can potentially improve the device efficiencies of ternary blend organic solar cells (TBSCs). However, several parameters, such as domain morphology, exciton lifetime, energy and charge transfer, influence the resulting photophysics. Owing to this, spectroscopic studies on TBSCs have not unambiguously deconvolved the role of RET in the observed enhancement of photocurrent densities, often downplaying the mechanistic aspects of the RET associated enhancement. In this perspective, we discuss and analyse the role of RET in enhancing the device efficiency by taking a few recent examples of TBSCs. Taking analogy from natural photosynthetic systems, we argue that deviations in the observed RET rates from a Förster type mechanism may be at play. We suggest new strategies to systematically correlate the Förster critical distance (R0) with increments in current density (ΔJSC) in order to gain mechanistic insights to optimize RET enhanced photocurrent for high efficiency organic solar cells.

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Room-Temperature Bandlike Transport and Hall Effect in a High-Mobility Ambipolar Polymer

PHYS. REV. B 2015, 91 (11), 115302 (1-16).

DOI: 10.1103/physrevb.91.115302.

The advent of a new class of high-mobility semiconducting polymers opens up a window to address fundamental issues in electrical transport mechanism such as transport between localized states versus extended state conduction. Here, we investigate the origin of the ultralow degree of disorder (Ea16meV) and the “bandlike” negative temperature (T) coefficient of the field effect electron mobility: μeFET(T) in a high performance (μeFET>2.5cm2V1s1) diketopyrrolopyrrole based semiconducting polymer. Models based on the framework of mobility edge with exponential density of states are invoked to explain the trends in transport. The temperature window over which the system demonstrates delocalized transport was tuned by a systematic introduction of disorder at the transport interface. Additionally, the Hall mobility (μeHall) extracted from Hall voltage measurements in these devices was found to be comparable to field effect mobility (μeFET) in the high T bandlike regime. Comprehensive studies with different combinations of dielectrics and semiconductors demonstrate the effectiveness of rationale molecular design, which emphasizes uniform-energetic landscape and low reorganization energy.

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Diketopyrrolopyrrole-Diketopyrrolopyrrole-Based Conjugated Copolymer for High- Mobility Organic Field-Effect Transistors
Catherine Kanimozhi, Nir Yaacobi-Gross, Kang Wei Chou§, Aram Amassian, Thomas D. Anthopoulos, and Satish Patil
Am. Chem. Soc. 2012, 134, 16532-16535.
DOI: 10.1021/ja308211n.

 In this communication, we report the synthesis of a novel diketopyrrolopyrrole–diketopyrrolopyrrole (DPP–DPP)-based conjugated copolymer and its application in high-mobility organic field-effect transistors. Copolymerization of DPP with DPP yields a copolymer with exceptional properties such as extended absorption characteristics (up to ∼1100 nm) and field-effect electron mobility values of >1 cm2 V–1 s–1. The synthesis of this novel DPP–DPP copolymer in combination with the demonstration of transistors with extremely high electron mobility makes this work an important step toward a new family of DPP–DPP copolymers for application in the general area of organic optoelectronics.

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Influence of Side-Chain on Structural Order and Photophysical Properties in Thiophene Based Diketopyrrolopyrroles: A Systematic Study
Mallari A. Naik, N. Venkatramaiah, Catherine Kanimozhi, and Satish Patil

Phys. Chem. C 2012, 116, 26128-26137.

DOI: 10.1021/jp306365q.

In this work, we have synthesized a series of TDPP derivatives with different alkyl groups such as n-hexyl (−C6H133a, 2-ethylhexyl (-(2-C2H5)C6H123b, triethylene glycol mono methyl ether (-(CH2CH2O)3cH3, TEG) 3c, and octadodecyl (-(8-C8H17)C12H223dN,N dialkylation of thiophene-diketopyrrolopyrrole (TDPP, 1) strongly influences its solubility, solid state packing, and structural order. These materials allow us to explicitly study the influence of alkyl chain on solid state packing and photophysical properties. TDPP moiety containing two different alkyl groups 3e (TEG and 2-ethylhexyl) and 3f (TEG and n-hexyl) were synthesized for the first time. The absorption spectra of all derivatives exhibited a red shift in solid state when compared to their solution spectra. The type of alkyl chains leads to change in the optical band gaps in solid state. The fluorescence study reveals that TDPP derivatives have strong π–π interaction in the solid state and the extent of bathochromic shift is due to combination of intramolecular interaction and formation of aggregates in solid state. This behavior strongly depends on the nature of alkyl chain. The presence of strong C–H···O inter chain interactions and CH−π interactions in solid state exhibits strong influence on the photophysical properties of TDPP chromophore.

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Palas Roy, Ajay Jha, Vineeth B. Yasarapudi, Thulasi Ram, Boregowda Puttaraju, Satish Patil & Jyotishman Dasgupta

Commun. 2017 81 2017, 8 (1), 1–10.

DOI: 10.1038/s41467-017-01928-z.

Donor-π-acceptor conjugated polymers form the material basis for high power conversion efficiencies in organic solar cells. Large dipole moment change upon photoexcitation via intramolecular charge transfer in donor-π-acceptor backbone is conjectured to facilitate efficient charge-carrier generation. However, the primary structural changes that drive ultrafast charge transfer step have remained elusive thereby limiting a rational structure-function correlation for such copolymers. Here we use structure-sensitive femtosecond stimulated Raman spectroscopy to demonstrate that π-bridge torsion forms the primary reaction coordinate for intramolecular charge transfer in donor-π-acceptor copolymers. Resonance-selective Raman snapshots of exciton relaxation reveal rich vibrational dynamics of the bridge modes associated with backbone planarization within 400 fs, leading to hot intramolecular charge transfer state formation while subsequent cooling dynamics of backbone-centric modes probe the charge transfer relaxation. Our work establishes a phenomenological gating role of bridge torsions in determining the fundamental timescale and energy of photogenerated carriers, and therefore opens up dynamics-based guidelines for fabricating energy-efficient organic photovoltaics.

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