Ph.D. THESIS COLLOQUIUM
Name of the Student: Nilabja Maity
Ph.D. Supervisor: Prof. Satish Patil
Date & Time: 11th March, 2022 at 4:00 pm through Microsoft Teams
Title: Diketopyrrolopyrrole Based Supramolecular Polymers with Emergent Photophysics and Unprecedented Photoconductivity
Meeting Link:
Abstract:
Structural fluctuation in organic molecular semiconductors often plays a key role in fragmenting the conducting pathways even in their condensed phases due to the large fraction of free volumes, acting as trap sites for free charge carriers. Long-range ordering via non-covalent directional interactions between the monomeric unit in organic semiconductors is an excellent approach to reduce trap densities.1 To address this problem, we have rationally designed a series of Hamilton-receptor-based supramolecular polymers of DPP. The Hamilton receptor endows supramolecular polymerization via hydrogen bonding with enhanced structural ordering and excitonic couplings. Owing to their synthetic tunability, high stability, strong visible absorption, high fluorescent quantum yield, and reasonable charge carrier mobility, DPP based π-conjugated systems have potential applications in the field of molecular optoelectronic devices such as organic field-effect transistors (OFETs), Organic solar-cell devices (OPVs).2In the light of the foregoing, in this thesis, efforts are made to investigate new synthesized DPP-based supramolecular polymers and their emergent photophysics and unprecedented photoconductivity.
In this thesis, initially, the detailed mechanism of supramolecular polymerization via self-complementary intermolecular hydrogen bonding (-NH…O=C-) of Hamilton receptor is established by FT-IR, concentration-dependent, and diffusion order spectroscopy (DOSY) NMR studies. Further, the reversible nature of the self-assembly is established from the variable temperature-dependent NMR studies. The presence of a slipped stack arrangement between two DPP units and self-complementary intermolecular hydrogen bonding through amide moiety of Hamilton receptor is clearly elucidated from the single-crystal X-ray diffraction structure of HR-TDPP-C20.
Further, a flash photolysis time-resolved microwave conductivity study reveals unprecedented photoconductivity and charge carrier mobility in the thin film. Substituting different side-chains and introducing dihedral angle twists within the backbone observed a notable difference in solid-state packing, photoconductivity, and thin film morphology. Grazing incidence wide-angle X-ray scattering and thin film X-ray diffraction measurements display that the packing order is enhanced for hexyl side chain for the thiophene derivatives, resulting in a very high intrinsic charge carrier mobility in the interplaying regime of thermally activated site-by-site hopping and electronic band conduction: At the microscopic level, electron and atomic force microscopy show the unique self-assembly remarkably improves structural order via hydrogen bonding. These findings demonstrate that supramolecular self-assembly strategy via the present hydrogen bonding networks effectively reduces the structural defects in molecular semiconductors and further improves the performance of optoelectronic devices.
Subsequently, the excited state dynamics of DPP-based supramolecular polymers is investigated by using transient absorption spectroscopy (TA). We obtain two markedly different aggregate coupling motifs (J-like and H-like) for HR-TDPP-TEG in thin film, simply through the choice of solvent used in the deposition. Focusing on a characteristic 1(TT) photoinduced absorption band in the near-infrared, which is uncontaminated by thermal effects, we find that the J-like film exhibit singlet fission in the excited state. The resulting 1(TT) state is capable of symmetry-forbidden luminescence3 – a first among DPP materials. The low-lying excimer-like state below the exciton-coupled S1 acts as a trap that hinders promoting singlet fission in H-like film, highlighting the importance of intermolecular packing structures to manipulate the excited-state relaxation pathways.
Finally, we perceive that the DPP-based supramolecular polymers selectively recognize the barbituric acid (among Dopamine, Serotonin, and Uric acid) via six self-complementary hydrogen bonds. Upon mixing barbituric acid with DPP-based supramolecular polymers (HR-TDPP-C20 and HR-TDPP-HEX) form nano-rods microstructure, which might have a potential application in the field of biomolecular sensors.
References:
- Yamamoto, Y.; Zhang, G.; Jin, W.; Fukushima, T.; Ishii, N.; Saeki, A.; Seki, S.; Tagawa, S.; Minari, T.; Tsukagoshi, K.; Aida, T. Ambipolar-Transporting Coaxial Nanotubes with a Tailored Molecular Graphene – Fullerene Heterojunction. Proc. Natl. Acad. Sci. U. S. A.2009, 106, 21051–21056.
- Kanimozhi, C.; Yaacobi-Gross, N.; Chou, K. W.; Amassian, A.; Anthopoulos, T. D.; Patil, S. Diketopyrrolopyrrole-Diketopyrrolopyrrole-Based Conjugated Copolymer for High-Mobility Organic Field-Effect Transistors. J. Am. Chem. Soc.2012, 134, 16532–16535.
- Bossanyi, D. G.; Matthiesen, M.; Wang, S.; Smith, J. A.; Kilbride, R. C.; Shipp, J. D.; Chekulaev, D.; Holland, E.; Anthony, J. E.; Zaumseil, J.; Musser, A. J.; Clark, J. Emissive Spin-0 Triplet-Pairs Are a Direct Product of Triplet-Triplet Annihilation in Pentacene Single Crystals and Anthradithiophene Films. Nat. Chem. 2021, 13, 163-171.