N‐doped cycloparaphenylenes: Tuning electronic properties for applications in thermally activated delayed fluorescence

Abstract

We theoretically characterize a series of substituted cycloparaphenylene nanohoops to study the effect of incorporating an electron-withdrawing group into their cyclic structure. We systematically vary the nature, position, and number of nitrogen-containing acceptor groups in both neutral (pyridine) and charged forms (pyridinium and methylpyridinium) to provide insights into how this functionalization affects the structural, electronic, and optical properties of these systems. We focus also on the singlet-triplet energy difference, with low values found, which might pave the way to further applications in the field of devices for light-emitting applications providing a potential class of TADF-based emitters. K E Y W O R D S density functional theory, donor-acceptor cycloparaphenylenes, optoelectronic properties, TD-DFT, thermally activated delayed fluorescence | I N TR ODU C TI ONOrganic Electronics is a field of rapid growth attracting interest from interdisciplinary domains including electronics & engineering, chemistry, physics, and materials science, not to mention the great commercial interest toward its applications. [1,2] The facile tunability of organic molecules has paved the way for a wealth of organic-based materials to be synthesized with novel key properties. Already, small organic molecules find their place in photovoltaic cells, [3,4] light-emitting transistors, [5][6][7][8][9] and diodes, [10][11][12] to name just a few of these applications. The flexibility of organic materials over inorganic has enabled the birth of the "soft electronics" market, including devices that can twist, bend or mould to any surface or textile. With so many possibilities arising from unique properties of organic molecules, the future of organics appears bright: not only do organic materials promise more innovative technologies but also are far more sustainable (organic molecules are abundant, easily synthesized and potentially Int J Quantum Chem. 2018;118:e25562.

[4]Cyclo‐N‐ethyl‐2,7‐carbazole: Synthesis, Structural, Electronic and Charge Transport Properties

“…In addition and as for the two other nanorings, [8]CPP [28] and [4]C-diEt-F, [30] [4]C-Et-Cbz dis-play ab and tail at around 400 nm, which is due, according to TD-DFT (time-dependent density functional theory) calculations, to asymmetry forbidden( f = 0) HOMO!LUMO transition ( Figure 3B). The H-1, H-2, H-5 and H-6 orbitals are all delocalized all over the nanoring, including the nitrogen atoms, and are thus specific of chromophore [4]C-Et-Cbz.H owever,t he HOMO density resembles that of [8]CPP, [38,39] with no participation of the nitrogen atoms, whereas the LUMO density resembles the one of the molecule of carbazole Cbz itself, with no participation of the carbon atom in para positiono ft he nitrogen atom. LUMO and H-6!LUMO (l th = 340 nm).…”

Theoretical Insights for Materials Properties of Cyclic Organic Nanorings

“…The experimental synthesis of donor–acceptor (D–A) compounds [ 288–290 ] has propelled the study of the photophysical and semiconducting properties of this set of materials. [ 112,125,291 ] Figure 7 shows some functionalization strategies of CPPs leading to N‐doped systems. An inmediate consequence of the D–A fragments introduced is the localization of the HOMO and LUMO on different parts of the molecule, thus poorly overlapping and consequently reducing the HOMO‐LUMO energy gap with respect to the undoped CPP systems.…”

“…An inmediate consequence of the D–A fragments introduced is the localization of the HOMO and LUMO on different parts of the molecule, thus poorly overlapping and consequently reducing the HOMO‐LUMO energy gap with respect to the undoped CPP systems. This has important consequences on the lowest singlet‐ and triplet‐excited states of these compounds, [ 125,291 ] and also leads to a relatively low singlet‐triplet energy gap , allowing thus to theoretically explore the use of these molecules as thermally activated delayed fluorescence (TADF) emitters [ 292,293 ] : The full harvesting of both singlet and triplet excitons can facilitate light‐emission mechanisms beyond the spin statistical limit [ 294–297 ] for sufficiently small values (around 0.1–0.2 eV). Whereas the 8CPP system has a singlet‐triplet gap of 0.5 eV and a forbidden (for symmetry reasons) transition, thus precluding its use for any intended TADF application, the D–A strategy roughly halves the singlet‐triplet gap while keeping non‐vanishing oscillator strength values.…”

“…">2.The comprehension of the driving forces for the supramolecular packing and cohesive energies of these systems as a function of their size, [ 107 ] therefore opening a way to tune their supramolecular structure to access solid‐state CNT‐like structures via the adequate functionalization, [ 108–110 ] and the determination of the preferred charge‐transfer career paths [ 111,112 ] ; 3.The differences between pristine and substituted CPPs, [ 113–121 ] with emphasis on intra‐molecular donor‐acceptor compounds [ 122–124 ] displaying fluorescent and semiconducting properties upon specific functionalization [ 125 ] ; 4.The influence of structural units after changing phenylene by pyrenylene (i.e., CycloPYrelenes [CPYs]) or ever larger structural motifs [ 126–128 ] and its effect on the single‐molecule [ 129 ] and supramolecular properties [ 130 ] ; and finally …”

Charge transport parameters for carbon based nanohoops and donor–acceptor derivatives

“…[30] Several computational investigations have contributed to get insight on the unique properties of CPPs, covering structural, energetic, electronic, magnetic properties [9,10,17,31,32,33] along with their potential application in optoelectronics [34], for instance as thermally activated delayed fluorescence emitters. [35] In addition, charge transport properties of [n]CPPs were initially explored through the estimate of intramolecular reorganization energies and electronic couplings. [36] Prompted by the availability of the crystal structure of N,Ndimethylaza [8]CPP (hereafter labelled DMA [8]CPP, see also Fig.1 and Fig.…”