Supramolecular Self-assembly of Diketopyrrolopyrrole with Emergent Photophysics and Unprecedented Photoconductivity

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.

To address this problem, we have rationally designed a series of Hamilton-receptor-based supramolecules 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).

In the light of the foregoing, in this thesis, efforts are made to investigate new synthesized DPP-based supramolecular self-assembly and their emergent photophysics and unprecedented photoconductivity. Initially, the detailed mechanism of supramolecular polymers 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 (GIWAXS) and thin film X-ray diffraction measurements reveal that the packing order is enhanced for hexyl substituted DPP derivatives, resulting in high intrinsic charge carrier mobility of ∑μ=1.7 cm^2 V^(-1) s^(-1): unprecedented one as 1-dimensional supramolecular architecture with such small conjugated cores. 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 self-assembly is investigated by using transient absorption spectroscopy (TA). We obtain two markedly different aggregate coupling motifs (J 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. The resulting 1(TT) state is capable of symmetry-forbidden luminescence – a first among DPP materials. The low-lying excimer-like state below the exciton-coupled S1 acts as a trap that hinders 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 systems 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.