From charge‐transfer excitations to charge‐transport phenomena in organic molecular crystals

Abstract

Functional devices based on properties inherent in single organic molecules offer promise for use in technological applications, particularly building blocks that can take on diverse electronic functions with tuning through chemical design and synthesis. Morphological features of curved aromatic structures offer exploration into a wealth of phenomenology as a function of environment, such as exemplified for super atomic molecular orbitals. This review discusses current stateof-the-art electronic structure approaches for prediction of structural, electronic, optical, and transport properties of planar and curved designed components. Important principle for design lies in understanding and control of the solid-state packing and intermolecular interactions of individual building block units, for which many body perturbation techniques are essential. Molecular and solid state engineering is shown to be effective toward tailoring new materials with optimal transport properties, with valuable insight provided by high level computational prediction.charge transfer, charge transport, curved aromatics, GW-BSE theory, SAMO | I N TR ODU C TI ONSome of the most relevant problems in the field of condensed matter, nanoscience, and renewable energy applications are related to the discovery and understanding of complex materials engineered to meet a specific functionality. [1][2][3] Renewed interest in the use of conjugated molecules with delocalized p electrons as key components in nanoscale electronics and optoelectronic devices has motivated recent abilities to construct singlemolecule junctions and measure their transport properties. [4][5][6] As conductor dimensions approach the nanoscale, design principles turn toward creation of molecules with tunable functionality to enable control of charge transport on the molecular scale. [5,7] Organic-organic (hybrid) interfaces are primary components in emerging technologies, such as organic and molecular electronics [8] and photovoltaic cells.[9] These complex systems can often strongly influence the electrical and optical characteristics of a specific structure or device.[10] Key material properties, such as the level alignment at functional interfaces, [11,12] the ability of charge carriers to be injected into a conductive substrate, the efficiency of optical excitations to split into free carriers, [13] and so on, poorly described within standard density functional theory (DFT) methodologies, [14] are ultimately factors in determining unique electronic properties of a given complex. As such, pressing needs have emerged for improved understanding of the details involved in optimization and control of electronic transport phenomena in aromatic systems, two key representative components of which constitutes the focus of the present work.I. The conventional mechanism for charge transport in conjugated materials involves the overlap of delocalized p molecular orbitals, which extend through a plane and can facilitate transport. Directed confinement of electronic states in one...

Mapping Charge-Transfer Excitations in Bacteriochlorophyll Dimers from First Principles

“…Photoinduced charge-transfer excitations are of central importance to the primary processes of natural photosynthesis and for photovoltaic and photocatalytic applications [1,2]. In organic semiconductors, charge-transfer excitations are believed to be important intermediates between excited states localized on donor molecules and charge-separated electron-hole states on acceptor and donor units, respectively, even though the exact mechanism of chargeseparation is debated [3][4][5][6][7][8][9][10][11][12].…”

Mapping charge-transfer excitations in Bacteriochlorophyll dimers from first principles

“…Photoinduced charge-transfer excitations are of central importance to the primary processes of natural photosynthesis and for photovoltaic and photocatalytic applications [1,2]. In organic semiconductors, charge-transfer excitations are believed to be important intermediates between excited states localized on donor molecules and charge-separated electron-hole states on acceptor and donor units, respectively, even though the exact mechanism of charge-separation is debated [3][4][5][6][7][8][9][10][11][12].…”

PAH/PAH(CF3)n Donor/Acceptor Charge‐Transfer Complexes in Solution and in Solid‐State Co‐Crystals

“…[38,39] However,C F 3 is am uch stronger electron-withdrawing group (EWG) than F. [22,40,41] For example, the DFT-predicted EAso fp erfluoroanthracene (ANTH(F) 10 )a nd ANTH(CF 3 ) 10 are 1.84 [29,30] and 4.01 eV, [24] respectively.W ith only six CF 3 groups,t he experimental EA of 2,3,6,7,9,10-ANTH(CF 3 ) 6 (ANTH-6-1), at 2.81(2) eV, [26] is more than 1eVh ighert han ANTH(F) 10 and is the same as the 2.78 (6) eV EA of the common charge-transfer electron acceptor chloranil (2,3,4,5-tetrachlorobenzoquinone; Cl 4 BQ). [42] Another differenceb etween CF 3 and Fs ubstituents on PAHs is exemplified by the order of photostability of 9,10-ANTH(X) 2 derivatives in aerated cyclohexane:C F 3 > H > F. [43] Al-thoughP AH/PAH(F) n co-crystals have been studied for many years (e.g.,c o-crystals of perfluoronaphthalene with pyrene [44] and fluorene [45] ), to our knowledge there are no reports of PAH/PAH(CF 3 ) n co-crystals other than our briefc ommunication of the structures of co-crystals containing pyrene and either 1,2,3,5,7-azulene(CF 3 ) 5 (AZUL-5-1) [46] or ANTH-6-1 [47] (both of which will be discussed in greater detail in this paper) and the co-crystal structure of two complementary p-bowls, corannulene/1,3,5,7,9-corannulene(CF 3 ) 5 . [48] We have studied the synthesis and physicochemical properties of 53 PAH(CF 3 ) n derivatives prepared by substituting H atoms in twelve unsubstituted PAHs using CF 3 radicals generated from CF 3 Ia th igh temperature (n = 1-8).…”

“…Organic co‐crystalline materials containing electronically‐ and structurally‐tuneable aromatic donors and acceptors exhibit a wide range of physicochemical properties that have found, or are expected to find, use in molecular electronic applications . Of particular interest are the ways that strong electron‐withdrawing groups can affect not only the electronic coupling of donors and acceptors but the degree of π–π overlap and one‐dimensional stacking .…”

“…Organicc o-crystalline materials containing electronically-and structurally-tuneable aromatic donors and acceptors exhibit a wide range of physicochemical properties that have found, or are expectedt of ind, use in molecular electronic applications. [1][2][3][4][5][6][7][8][9][10] Of particular interesta re the ways that strong electron-withdrawing groups can affect not only the electronic coupling of donors and acceptors but the degree of p-p overlap and one-dimensional stacking. [11][12][13] In particular,F atoms [14][15][16][17][18][19] and perfluoroalkyl (R F )g roups [20][21][22][23] can have desirable electronic and structural effects.…”