Representative Publications

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 2A_g-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.

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.

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. 

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.

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 (−C₆H₁₃) 3a, 2-ethylhexyl (-(2-C₂H₅)C₆H₁₂) 3b, triethylene glycol mono methyl ether (-(CH₂CH₂O)_3cH₃, TEG) 3c, and octadodecyl (-(8-C₈H₁₇)C₁₂H₂₂) 3d. N,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.