Theoretical Study of Cyclic Pyrene Oligomers and Their Resemblance with Cyclic Paraphenylenes: Disclosing Structure–Property Relationships for Cyclic Nanorings

Abstract

We theoretically discuss here the relationships between the structure of recently synthesized nanorings, dubbed as cyclo-2,7-pyrenylene (CPY) and formed upon bending and bonding a finite number of pyrene units until self-cyclation, and a set of chemically relevant properties such as the induced structural and energetical strain, the electronic and optical properties, or the response to charge injection, as well as their transport mechanism through a concerted migration of charge-carriers. We also compare these properties, and their evolution with the number of pyrene-linked units, with those obtained for the closely related cycloparaphenylene (CPP) compounds, trying to disclose the underlying structure-property guidelines. To do it, we always employ dispersion-corrected DFT methods to systematically include the key effects affecting all the properties tackled. A correct match with some available experimental results, for the [4]CPY compound (the only one synthesized so far), anticipates the accuracy of the calculations done for the rest of compounds. Finally, since this kind of systems are envisioned as possible precursors for the fine-tuned and controlled synthesis of carbon nanotubes, we also address the stability of the dimers found in their crystalline structure, and the associated cohesive energy, which may drive the synthesis of the corresponding nanotubes after an adequate dehydrogenation reaction.

Theoretical studies on the structure and thermochemistry of cyclicparaphenylenediazenes

“…We have chosen the B3LYP model, since it produces reasonably good p-conjugated structures, and appropriate for the prediction of electronic structure of polycyclic hydrocarbons and their energies. 21,27,34,35,40,[45][46][47][48] The combination of B3LYP 42 with 6-31G(d) and 6-31+G(d,p) basis sets has been tested by other groups and results were found to be in good agreement with experimental values. 21,40,46,48 Therefore, we believe the same combination will provide reasonably accurate geometrical parameters and vibrational frequencies for [n]CPPDs.…”

Theoretical Insights for Materials Properties of Cyclic Organic Nanorings

“…The substitution of phenylene by larger pyrene units [ 127,128 ] does not change the preferred herringbone packing of the formed CPY nanorings, observing again a limited number of configurations: parallel, herringbone, and tubular‐like dimers. [ 130 ] Similarly to the CPP case, the slipped tubular‐like dimer is favored with respect to the other two main dimers found, but the higher number of the latter in the unit cell dominates the final crystalline packing, and leads now to a cohesive energy of 69 kcal mol −1 and thus slightly larger than those calculated for CPPs of approximately the samediameter.…”

“…Briefly, for these nanorings, the inter‐layer charge transport mechanism (due to the favorable slipped‐lateral dimer arrangements found) is more efficient than the intra‐layer counterpart. The introduction of pyrene units (CPYs) has also been studied, [ 130 ] for both 3‐cyclo‐2,7‐pyrenylene (3CPY) and 4‐cyclo‐2,7‐pyrenylene (4CPY) compounds, leading to smaller reorganization energy values with respect to CPPs of the same diameter but also to smaller electronic coupling values. The pyrene units, on the other hand, offer a more versatile template for substitutions in ipso and ortho positions, with the effect of electroactive atoms (i.e., F, Cl, Br) and groups (CN) recently studied allowing thus to anticipate the use of these systems as organic (ambipolar) molecular semiconductors.…”

“…">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 5.The process of physisorption and molecular friction of these systems on, for example, graphite surfaces, [ 131 ] which could modulate a highly directional growth of solid‐state samples, as well as the emerging semiconducting properties of the deposited monolayer upon diffusion of the charge carriers injected from the surface.…”

Nature (Hole or Electron) of Charge-Transfer Ability of Substituted Cyclopyrenylene Hoop-Shaped Compounds

“…In this regard, we have focused in last years on the differences and similarities between the cycloparaphenylene (CPP) and cyclopyrenylene (CPY) compounds of increasing size, or [ n ]­CPY with n representing the number of monomers, first validating and next applying a set of adequate theoretical methods for that purpose. , Actually, the standarization of the rich chemistry afforded by these cyclic compounds would systematically need: (i) to explore the differences between the affordable constituent units, e.g., phenylene, naphthylene, phenacene, or pyrenylene rings, , to name just a few examples; (ii) to examine the effect of functionalizing the structure, with the incorporation of electroactive atoms within the rings or substituents at their periphery; , and (iii) to evaluate consequently the impact of these functionalization on selected target properties with respect to the pristine compounds. Note also that the solid-state semiconducting properties (including estimates of hole mobilities of the set of [ n ]­CPPs compounds (with n = 5–12) have been very recently studied with values reaching 2 cm 2 V –1 cm –1 .…”