[4]Cyclofluorene: Unexpected Influence of Alkyl Chain Length

Abstract

Presented here is the study of a new example of [4]cyclofluorene, with ethyl chains on the bridgeheads. Its molecular structure was established by solution NMR spectroscopy and single‐crystal X‐ray diffraction. Three successive oxidation processes and one reversible reduction were observed through cyclic voltammetry. The optical properties were characterized both in solution and thin film by UV/visible spectroscopy as well as stationary and time‐resolved fluorescence. It was found that this [4]cyclofluorene displays different characteristics compared with the other [4]cyclofluorenes substituted by methyl or propyl chains: a simple modification of the chain length induces a non‐negligible effect on the emission properties, which must be linked to the specific arrangement of the fluorene units. Furthermore, single‐crystal X‐ray diffraction reveals the formation of a pseudo‐tubular solid‐state arrangement of fully symmetrical ring structures, which was not observed for the other members of the [4]cyclofluorenes family. This finding could open the way to modulation of properties of cyclofluorenes through alkyl chain engineering.

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

“…The best conditions (Scheme 1) found are described below.T he intermediate 2,b ased on four square-shaped tetranuclearp latinum complexes,w as obtainedb ys tirring the bis-(pinacol)borane carbazole 1 with Pt(cod)Cl 2 and cesium fluoride in refluxing THFf or 24 h. Then, the carefully dried crude containing 2 was treated with triphenylphosphine in o-dichlorobenzene, 1hat room temperature and 48 ha t1 80 8Ct og ive [4]C-Et-Cbz with an improvedo verall yield of 27 %f rom 1. The calculated strain energy of [4]C-Et-Cbz (73 kcal mol À1 ,s ee Supporting Information for details on calculations, Figure S15) appears almost identicalt o that previously described for [4]C-diEt-F (72.4 kcal mol À1 ), [30] showingt he similari mpact of the Ca nd Nb ridges (substituted herein with ethyl chains) on this parameter.T hus, the overall yield of the formation of [4]C-Et-Cbz from its corresponding dibromocarbazole (22 %) has been increased threefold using the present optimized conditions compared to those described by Yamago and co-workers through ab is(trimethylstannyl)carbazole( yield of 7% for a N-methyl-substituted cyclocarbazole). The calculated strain energy of [4]C-Et-Cbz (73 kcal mol À1 ,s ee Supporting Information for details on calculations, Figure S15) appears almost identicalt o that previously described for [4]C-diEt-F (72.4 kcal mol À1 ), [30] showingt he similari mpact of the Ca nd Nb ridges (substituted herein with ethyl chains) on this parameter.T hus, the overall yield of the formation of [4]C-Et-Cbz from its corresponding dibromocarbazole (22 %) has been increased threefold using the present optimized conditions compared to those described by Yamago and co-workers through ab is(trimethylstannyl)carbazole( yield of 7% for a N-methyl-substituted cyclocarbazole).…”

“…The degree of tilt of the two other [4]-cyclocarbazoles [27] reported in the literature( substituted on the nitrogen atom by either methyl or phenylu nits) seems to be the same since they display similar H 1 chemical shifts (d = 6.45-6.51 ppm). [30] X-ray diffractiono fs inglec rystals of [4]C-Et-Cbz ( Figure 2), which were obtained by vapor diffusion of hexane in aconcentrated solution of chloroform, confirms the abab conformation assigned by NMR spectroscopy. [30] X-ray diffractiono fs inglec rystals of [4]C-Et-Cbz ( Figure 2), which were obtained by vapor diffusion of hexane in aconcentrated solution of chloroform, confirms the abab conformation assigned by NMR spectroscopy.…”

“…[32] [4]C-Et-Cbz formsadistorted ellipsoidal nanoring with axes of 11.1 and 10.6 Figure 2A, right), which is different than its fluorene analogue [4]C-diEt-F,w hich forms ap erfect circle (diameter = 11.0 ,C CDC reference:1 580867). [30] As shown in the NMR analysis, X-ray diffraction confirms that the upper part of each carbazole (including the nitrogen atom) is tilted towards the centreo ft he nanoring.T he shortestd istance between two cofacialh ydrogen atoms is hence measured between the two H 1 atoms (meand istance H 1 ÀH 1 : 8.05 ,F igure S16). Then, the corresponding distance between two cofacial hydrogen atoms gradually increases from H 3 ÀH 3 (mean distance:1 0.15 ,F igure S17) to H 4 ÀH 4 (meand istance: 11.87 ,F igure S18).…”

“…[33][34][35] Furthermore,t he bending angle for the carbazole units in [4]C-Et-Cbz is about1 298 (Table S8), whereas for the previously mentioned carbazoles it is about1 788 (Table S11), [33][34][35] close to planarity (1808). Nevertheless, it is importantt on ote that the mean displacement angle [3] measured for [4]C-Et-Cbz,7 .208 (Table S9), is smaller than that of non-bridge analogue [8]CPP, 9.588 (Table S10), [32] and almosti dentical to that of carbonbridged analogue [4]C-diEt-F,7 .108, [30] showingt hat the bridge (eitherc arbon atom in fluorene or nitrogen atom in carbazole) tends to flatten the biphenyl unit. Nevertheless, it is importantt on ote that the mean displacement angle [3] measured for [4]C-Et-Cbz,7 .208 (Table S9), is smaller than that of non-bridge analogue [8]CPP, 9.588 (Table S10), [32] and almosti dentical to that of carbonbridged analogue [4]C-diEt-F,7 .108, [30] showingt hat the bridge (eitherc arbon atom in fluorene or nitrogen atom in carbazole) tends to flatten the biphenyl unit.…”

“…PPh 3 in toluene 30 min at RT and then 24 ha t1 00 8C, Table S1) with ar elatively low overall yield (6 %). [30] As the synthesis of nanorings is ac entral point in the field, and particularly,i fa ne lectronic application is envisaged, synthetic investigations (screening of temperature, solventa nd reactiont ime, see Ta ble S1) for both the transmetallation and reductive elimination steps were performed. [30] As the synthesis of nanorings is ac entral point in the field, and particularly,i fa ne lectronic application is envisaged, synthetic investigations (screening of temperature, solventa nd reactiont ime, see Ta ble S1) for both the transmetallation and reductive elimination steps were performed.…”