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Chapter 4. Bridging Highly Luminescent – Ambipolar Transport

4.1. Introduction

Molecular heterosystems such as complexes and interfaces based on organic π- conjugated molecules have raised enormous research interest due to the great opportunities of novel optoelectronic functionalities beyond the individual characters of constituting materials.[1] Bestowing unconventional electronic features, such as (super-)conductivity,[2a-c] ferroelectricity,[2d,e] and magnetic properties,[2f] charge- transfer (CT) phenomenon based on electron donor (D) - acceptor (A) complexes or interfaces is highly attracting and paves the way toward developing next-generation organic devices applications. As theoretically and experimentally revealed, the characteristics of such complexes are strongly determined not only from their molecular arrangements (i.e., segregated and mixed stacks) but also from the energetic offset between ionization potential of D (IP(D)) and electron affinity of A (EA(A));[2g]

until now, however, much devote has been focused on unconventional metallic (super- )conductivity typically from segregately stacked systems with mixed valence

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characters. Only recently, mixed stacked semiconducting CT systems grasped further attention due to the intriguing behavior of such complex, for example ambipolar transport in organic field-effect transistor (OFET) devices, distinctive photoconductivity, and photoluminescent (PL) character.[3] These electrical and optical features of mixed stacked CT indeed offer elegant ways for realizing high performing balanced ambipolar transport, light propagation, and smart luminescence CT, of which the performances are highly controlled by chemical-/crystal-engineering strategies regulating electronic coupling between nearby molecules.

Indeed, it is currently well-established that mixed stack CT can transport both p- /n-type charge carriers by characteristic superexchange mechanism; i.e., despite absence of direct coupling between nearest donors or acceptors by intercalated counter constituent, frontier molecular orbital (MO) mixing between D-A can give rise to non- direct (effect) electronic coupling for hole and electron with highly balanced character in appropriate conditions.[4] First evaluation of CT complex as semiconducting active channel was conducted in charge density wave state of quasi-1D organic metal tetrathiafulvalene (TTF) - tetracyanoquinodimethane (TCNQ). Further evaluations of field-effect charge transport natures in band insulators (e.g., Mott insulator, and charge- ordered insulator) or ionic CT system have been successively carried out, some of which manifested characteristic ambipolar transport under metal-insulator transition conditions.[5] Recent advances in respect of developing materials have opened new horizon for achieving favorable ambipolar transport in ambient temperature condition

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using CT systems with neutral character.[3] Among these notable results, Qin et al. have successfully surmounted low field-effect mobility values (µh ~ 0.77, and µe ~ 0.24 cm2 V-1 s-1) utilizing appropriate D-A pair – i.e., meso-diphenyl tetrathia[22]annulene[2,1,2,1] (DPTTA) as a donor, and 4,8-bis(dicyanomethylene)- 4,8-dihydrobenzo[1,2-b:4,5-b’]-dithiophene (DTTCNQ) as an acceptor, offering unique quasi 2D electronic coupling comprised of both effective and direct coupling in two distinctive directions.[3d]

In addition to the ambipolarity, neutral CT complexes with adequate electronic as well as structural factors render great opportunities for realizing luminescent CT systems.[3h-k] In this class of materials, newly generated CT gap in a visible range can be manifested, dictated by IP(D) and EA(A) with electrostatic stabilization factor.[6]

The vertical transition from ground state S0 to the lowest excited singlet state S1 (i.e.,

1CT state) results in pronounce CT character due to the localized highest occupied MO (HOMO, H) and lowest unoccupied MO (LUMO, L) to D and A, respectively; which gives rise to bathochromic shift of CT fluorescence against those from individual D/A emission. The transition dipole moment (and oscillator strength f) of CT transition, however, is expected to show somewhat diminishing value, attributed to the negligible orbital overlap integral; thus, should lead to low radiative constant (kF). In this regard, realization of electroluminescence (EL) during OFETs operations (i.e., light-emitting OFET, LE-OFET) is greatly hampered by limited kF, despite notable optoelectronic functionalities (e.g., ambipolarity and fluorescence) of this class of CT systems.

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In this study, therefore, I gave a particular attention on possibility for bridging peculiar ambipolar transport with CT emission character to design unique CT semiconductor for LE-OFET application. For this, I emphasize the significances of both high luminescence quantum yield and balanced ambipolarity in CT cocrystal. To implement both important parameters, herein, further attention is addressed to molecularly tailored dicyanodistyrylbenzene (DCS) based CT system which manifested prominent CT luminescence with remarkable ambipolar transport.[7] In this study, a novel aggregation induced enhanced emission (AIEE) based CT system is reported, which comprised of DCS based donor (2MDCS, D1),[8] and acceptor (CN- TFPA, A1) with 2:1 stoichiometry;[9] manifesting unprecedentedly high CT emission PLQY (ΦF = ca. 60%). From in-depth explorations on structure-property relationship, not only tightly packed structure originated from minimized chemical structure mismatch (i.e., isometric approach) effectively reduced exciton trap density to lower the non-radiative decay rate (knr) but also higher energy transitions in S0 → S1 by configuration interaction with partial CT character promoted efficient radiative transition. In addition to the favorable CT emission, highly balanced ambipolar transport (µh and µe in the range of 10-4 cm2 V-1 s-1) is promoted in a programmed densely packed two-dimensional CT cocrystal sheet; which gives rise to notable EL emission during transistor action. The in-depth expedition of highly correlated structure, electronic, and optoelectronic properties of this 2D mixed stacked CT well rationalizes the potentialities of this class of CT materials for next-generation

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