Studies on Self-Assembly and Structure-Property Relationship of Functional Supramolecules: Unique Optoelectronic Functions of Dicyanodistyrylbenzene Derivative Crystal, and Their Charge-Transfer Complex

Abstract

establishing a unique way to develop stimuli-responsive fluorescence memory system. In this dissertation, I have focused on structure – property correlation in particular about the fluorescence and/or charge transport characteristics of the dicyanodistyrylbenzene based supramolecule and CT complexes, supported by precise quantum-chemical calculations based on well-defined structures. As in the case of single-component semiconductor solids, the molecular arrangements of this class of CT materials are revealed to derive great impact on their optoelectronic functions in CT complexes, combined with electronic properties of individual constituents. Through the conative regulations of such molecular/supramolecular parameters, in this respect, manifestation of unprecedented features or improvement of optoelectronic properties can successfully be achieved. The CT complex comprised of DCS and/or DSB molecules can thereby be understood as promising candidate for organic optoelectronics applications among the countless organic semiconducting materials.

the 2D assembly can be promoted by supramolecular synthons of DCS based D/A molecules (CT interaction and –CN induced H-bonding interaction). In virtue of ambipolar transporting nature (balanced p-/n-channel mobility of ca. 10-4 cm2 V-1 s-1) and unprecedentedly high fluorescence quantum yield, CT based light-emitting OFETs devices could successfully be demonstrated (EQE up to 1.5% in true ambipolar regime)

suggesting the great potential of mixed stack CT complexes for futuristic organic optoelectronics applications. In the meantime, effort has also been devoted for developing stimuli-responsive multi-color fluorescence switching systems comprised of mixed D-A CT complex. It was highly envisioned that high-contrast fluorescence switching with anticipatable emission energies (red ↔ blue) can be implemented by the distinctive luminescence properties of constituents (D: 4M-DSB

A: 3,3′-(1,4-phenylene)bis(2-(3,5-bis(trifluoromethyl)phenyl)acrylonitrile, Thio-Y) and their mixed CT phase, if the phase alternation can be promoted by strategic external stimulation. Due to the non-centrosymmetric design of the acceptor molecule, somewhat attenuated intermolecular interaction networks in CT phase can be attained

thus, lead to superior stimuli responsive features. The electrostatic supramolecular synthons, i.e., CT and H-bonding interactions, was found to be greatly affected by polar aprotic solvent vapor exposure which promoted D/A segregated structure formation. On the other hand, non-polar solvent and polar protic solvent induced mixed D-A CT phase formation by electrostatic and solvophobic effect governing procedure, respectively. Combined with peculiar biphasic solvent property dependence, thermal and mechanical stimuli can also be utilized to form CT phase for multi-stimuli responsive features due to the spontaneity of CT formation

Abstract Studies on Self-Assembly and Structure-Property Relationship of Functional Supramolecules: Unique Optoelectronic Functions of Dicyanodistyrylbenzene Derivative Crystal, and Their Charge-Transfer Complex

Sang Kyu Park Department of Materials Science and Engineering The Graduate School Seoul National University

Organic semiconductors based on π-conjugated molecules have emerged as one of the promising materials systems attributed to the advantageous features, such as chemical versatility, light-weight, transparency, low-temperature processability, and flexibility, compared to the inorganic counterparts. It has been revealed that the optoelectronic functionalities of organic semiconductor solids are governed not only by electronic features of constituents but also by their supramolecular arrangements. In this regard, rapid progresses in structure-property correlations have been achieved

in addition, strategies for controlling supramolecular assembly and thereby to modulate properties have also been regarded as an important research subject. Meanwhile, remarkable research interest has arouse on multi component donor-acceptor (D-A) complexes, heterojunctions, and interfaces

since wholly new physicochemical functionalities can be promoted – i.e., photovoltaic effect, charge-transfer induced transport enhancement, and excited complex luminescence. In particular, D-A charge-transfer (CT) complexes have long been regarded as one of the most pulsating research subjects by their prominent (metallic-)conductivities. However, studies on charge-transfer complexes with neutral characters are not much conducted by now, particularly with precise structure-property correlation. This class of CT complexes however exhibit great possibilities for their semiconducting properties and peculiar luminescence features by characteristic electronic and structural properties. Among the promising π-conjugated organic semiconductors, I have paid particular attention on multifunctional dicyanodistyrylbenzene (DCS) based torsion spring molecules which give rise to the unique optoelectronic characteristics – i.e., bright solid-state luminescence and favorable charge transport property. Among the DCS based semiconductors, elaborately designed (2Z,2′Z)-3,3′-(1,4-phenylene)bis(2-(3,5-bis(trifluoromethyl)phenyl)acrylonitrile), CN-TFPA, facilitated to form millimeter scaled two-dimensional (2D) n-type supramolecules driven from the panoply of well-balanced π-π and cyano-induced hydrogen bonding interactions, as two important supramolecular synthons. Attributed to the both electronic and structural factors, the 2D layered self-assembled structure provided distinguished aggregation induced enhanced emission (ΦF = ca. 80%) and n-type transporting nature (0.5 cm2 V-1 s-1) in OFET devices. The supramolecular features of the material rendered exfoliation and transfer capability similar to the graphene 2D layers case, affording novel way to constitute ultra-thin crystalline OFETs. Beside its unique supramolecular optoelectronic features, CN-TFPA was found to be an interesting A material to establish unique CT complexes for advanced optoelectronic functions. Considering favorable ambipolar charge-transporting nature combined with desired CT fluorescence, I rationally designed isometric D material (1,4-bis(3,5-dimethylstyryl)benzene, 4M-DSB) based on distyrylbenzene (DSB) molecular backbone structure

manifesting minimized structural mismatch with CN-TFPA, thus established regular quasi-1D mixed stack assembly with densely packed molecular arrangement. The indirect electronic coupling from superexchange mechanism indeed rendered peculiar ambipolar charge-transport (p-/n-channel mobility up to 6.7×10-3 cm2 V-1 s-1 and 6.7×10-2 cm2 V-1 s-1, respectively). Moreover, the electronic and structure character gave rise to high CT fluorescence quantum yield (ΦF = ca. 30-40%) by configuration interaction and minimized exciton trap density, which gave a great possibility for advanced CT optoelectronics by bridging charge transport and emission characteristics. As a next step toward developing mixed-stack CT based advanced optoelectronic devices, i.e. realization of electroluminescence (EL) during OFETs operation, further studies are greatly required to promote increment of fluorescence quantum yield of D-A CT systems. Introduction of DCS type D ((2Z,2′Z)-2,2′-(1,4-phenylene)bis(3-p-tolylacrylonitrile), 2MDCS) as an isometric donor counterpart for DCS based acceptor (CN-TFPA) facilitated great increase in CT emission quantum yield (ΦF = ca. 60%). The highly efficeint CT luminescence is attributed to the molecular orbital mixing character of the lowest singlet transition and reduced nonradiative decay constant by energy gap law. Furthermore, densely packed 2:1 (D:A) molecular arrangement was exhibited with anomalous two-dimensional supramolecular growth