Synthesis of π-conjugated polycyclic compounds by late-stage extrusion of chalcogen fragments

Abstract

The "precursor approach" has proved particularly valuable for the preparation of insoluble and unstable π-conjugated polycyclic compounds (π-CPCs), which cannot be synthesized via in-solution organic chemistry, for their improved processing, as well as for their electronic investigation both at the material and single-molecule scales. This method relies on the synthesis and processing of soluble and stable direct precursors of the target π-CPCs, followed by their final conversion in situ, triggered by thermal activation, photoirradiation or redox control. Beside well-established reactions involving the elimination of carbon-based small molecules, i.e., retro-Diels-Alder and decarbonylation processes, the late-stage extrusion of chalcogen fragments has emerged as a highly promising synthetic tool to access a wider variety of π-conjugated polycyclic structures and thus to expand the potentialities of the "precursor approach" for further improvements of molecular materials' performances. This review gives an overview of synthetic strategies towards π-CPCs involving the ultimate elimination of chalcogen fragments upon thermal activation, photoirradiation and electron exchange.

Keywords: arenes; chalcogens; extrusion; fused-ring systems; precursor approach.

Scheme 1

“Precursor approach” for the synthesis of π-conjugated polycyclic compounds, with the thermally- or photoinduced late-stage extrusion of carbon-based fragments resulting from retro-Diels–Alder and decarbonylation reactions. The seminal design of pentacene’s soluble precursor, as reported by Müllen and Herwig [14], is depicted in the top left.

Scheme 2

Valence isomerization of chalcogen heteropines and subsequent cheletropic extrusion in the case of sulfur, selenium and tellurium derivatives.

Scheme 3

Early example of phenanthrene synthesis via a chemically-induced S-extrusion (and concomitant decarboxylation) from a dibenzothiepine precursor [56].

Scheme 4

Top: Conversion of dinaphthothiepine bisimides 3a,b and their sulfoxide analogues 4a,b into PBIs 6a,b by S-extrusion triggered by electron injection, photo- and thermal activation. Bottom: Dinaphthothiepine S,S-dioxide 5 fails to deliver PBI 6a whatever the activation mode [57].

Figure 1

Top view (a) and side view (b) of the X-ray crystal structure of thiepine 3b showing its bent conformation. Thermal ellipsoids are shown at the 50% probability level, and all hydrogen atoms as well as propyl groups on imide substituents have been omitted for clarity. Reprinted with permission from [57]. Copyright 2020 American Chemical Society. This content is not subject to CC BY 4.0.

Scheme 5

Modular synthetic route towards dinaphthothiepines 3a–f and the corresponding S-oxides 4a–d, incorporating a variety of imide substituents [62].

Scheme 6

Top: Conversion of dithienobenzothiepine monomeric units into dithienonaphthalenes, upon S-extrusion triggered by electrochemical oxidation. Bottom: Exploitation of the S-extrusion process for peroxide sensing, taking advantage of the lability of oxidized dithienobenzothiepine to generate highly fluorescent dithienonaphthalene [63].

Scheme 7

Synthesis of S-doped extended triphenylene derivative 22 from 3-bromothiophene (17) with the thermally-induced ring contraction of thiepine 21 as key step [64].

Scheme 8

Top: Synthesis of thermally-stable O-doped HBC 26a. Bottom: Synthesis of S- and Se-based soluble precursors of seco-HBC 31 and their conversion by chalcogen extrusion upon thermal activation [65].

Scheme 9

Synthesis of dinaphthooxepine bisimide 33 and conversion into PBI 6f by O-extrusion triggered by electron injection [66].

Figure 2

Cyclic voltammogram of dinaphthooxepine 33, evidencing the irreversibility of the reduction process during the first cycle, leading to the formation of PBI 6f upon redox-triggered O-extrusion. Reprinted with permission from [66]. Copyright 2023 American Chemical Society. This content is not subject to CC BY 4.0.

Scheme 10

Top: Early example of 6-membered ring contraction with concomitant S-extrusion leading to dinaphthothiophene [69]. Bottom: Photoactivated S-extrusion occurring in natural product thiarubrine A [70].

Scheme 11

Examples of S-extrusion from annelated 1,2-dithiins under photoactivation (top) or thermal activation (bottom) [–72].

Scheme 12

Synthesis of dibenzo[1,4]dithiapentalene upon photoextrusion of SO₂ [78].

Scheme 13

Extrusion of SO in naphthotrithiin-2-oxides for the synthesis of 2,5-dihydrothiophene 1-oxides [79].

Scheme 14

SO-extrusion as a key step in the synthesis of fullerenes (C₆₀ and C₇₀) encapsulating H₂ molecules [80,82].

Scheme 15

Synthesis of diepoxytetracene precursor 56 and its on-surface conversion into tetracene upon O-extrusion. Inset in the top right shows a Laplace-filtered AFM image of a tetracene molecule produced on a Cu(111) surface. Inset adapted with permission from [83]. Copyright 2016 American Chemical Society. This content is not subject to CC BY 4.0.

Scheme 16

Soluble precursors of hexacene, decacene and dodecacene incorporating 1,4-epoxides in their hydrocarbon scaffold, and on-surface conversion into higher acenes by thermally-activated O-extrusion [–86].

Scheme 17

Synthesis of tetraepoxide 59 as soluble precursor of decacene [85].

Figure 3

Constant-height STM measurement of decacene on Au(111) using a CO-functionalized tip (sample voltage V = 20 mV), with the chemical structure of decacene superimposed in blue as a guide to the eye. Two different tip height domains were employed to resolve the Au(111) lattice as well as the adsorbed molecule with atomic resolution. Adapted with permission from [85], J. Krüger et al., “Decacene: On-Surface Generation”, Angew. Chem., Int. Ed., with permission from John Wiley and Sons. Copyright © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. This content is not subject to CC BY 4.0.