Use of isomeric, aromatic dialdehydes in the synthesis of photoactive, positional isomers of higher analogs of o-bromo(hetero)acenaldehydes

Abstract

Synthesis of the pairs of positional isomers of the title compounds is based on utilization of two isomeric o,o-dibromo dialdehydes and a selective ortho-metallation/Friedel–Crafts cyclization sequence of the corresponding diacetals. Photophysical and electrochemical properties of the new group of fluorescent and photoresistant compounds have also been performed.

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Linearly fused polycyclic, aromatic and heteroaromatic hydrocarbons I, also called (hetero)acenes have been preferentially investigated in optoelectronics.¹

In the present research program, we envisaged the development of strategies for the synthesis of substituted (hetero)acenes II with donor–acceptor properties as optoelectronic materials which would be more resistant towards photooxidation due to the presence of the electron withdrawing aldehyde group. In two, very recent studies, Geerts and Stas have explicitly shown that aldehyde groups attached both to central^2a or peripheral^2b rings of the anthradithiophene core, make it more resistant by about a factor of two in comparison to the electron donating 1-[4-(diphenylamine)phenyl] group. According to our new concept, the aldehyde group was directly introduced onto the peripheral ring of the (hetero)acene during constructing of the polycyclic, aromatic core by a new modification of the Friedel–Crafts (F–C) cyclization,^3a–c in a one-pot procedure. Two isomeric, aromatic dibromodialdehydes IV and VII, both containing two ortho-bromo aldehyde moieties and aromatic monoaldehyde V were utilized for this purpose. Thus, both Br/CHO ortho-positional isomers VI and VIII of higher analogs of donor–acceptor o-bromo(hetero)acenaldehydes could be obtained for the first time (Scheme 1). o-Bromo aromatic aldehydes containing more than two aromatic rings remain so far an unknown group of acenes. Only, simply, o-bromo substituted aromatic aldehydes, like 2-bromobenzaldehyde or 2-bromo-1-naphthaldehyde⁴ as well as nonaromatic β-halovinyl aldehydes, all containing a Hal–C=C–C=O unit, have been explored and have previously found application as intermediates in the synthesis of more complex molecules for pharmaceutical industry and for organic synthesis in general and their chemistry has been recently reviewed.⁵ Thus, o-bromoanthraldehydes, such as 6a,a′,b/11a,b and their higher, four-membered ring heteroanalogs 6c,d/11c,d until now remain unknown. The second aim of the program was synthesis of new, bifunctional (Br, CHO) (hetero)acenes II which would enable synthesis of more complex molecules with spatially developed system of π-conjugated bonds.

Scheme 1 A new concept for synthesis of pairs of positional isomers of o-bromo(hetero)acenaldehydes VI/VIII from isomeric, aromatic dialdehydes IV/VII.

In our previous approach, we utilized two aromatic monoaldehydes of type V to obtain in a simple procedure electron rich (hetero)acenes III containing up to six alkoxy groups.^3a–c That approach and the present concept are based on intramolecular F–C type reaction which, together with the further modifications, such as the Bradsher aromatic cyclodehydration of diarylmethanes, represent synthetically outstanding C–C bond formation processes that lead to formation of new, five- or six-membered rings in (hetero)acenes. Synthesis of the new RO-Ar and bifunctional ortho-CHO/Br motifs in the latter is available only in this peculiar F–C modification.

Creation of one or more rings in one reaction step can be generally realized by acid-catalyzed generation of carbocations from C=O, C=C, C≡C, C–OH functional groups in aldehydes, acetals, epoxides, ketones, carboxylic acids, alcohols, alkene and alkynes.⁶ Herein, unique application of dialdehydes for formation of the new, six-membered aromatic ring, has been realized in a one-pot, selective cyclization process (vide infra).

The synthesis of the pairs of positional isomers of o-bromo(hetero)acenaldehydes VI/VIII, represented by examples 6a–d, 6a′/11a–d, was based on the use of isomeric dialdehydes 1 and 7 (Scheme 2). The aldehyde groups in each isomer were quantitatively protected as 1,3-dioxanes 2 and 8 by reaction with 1,3-propanediol in the presence of catalytic amount of Amberlite® IR 120 in refluxing toluene and used without further purification. Next, halogen/metal exchange then reaction with (hetero)aromatic monoaldehydes 3: 3,4,5-trimethoxybenz-aldehyde, 3,5-dimethoxybenzaldehyde, N-methylindole-2-carboxyaldehyde, benzo[b]thiophene-2-carbaldehyde, respectively, to afford isomeric diarylmethanols 4 and 9 were carried out. Protection of OH groups with benzyl bromide gave 1,3-dioxanes 5 and 10 and prevented formation of isobenzofurans under the acidic conditions used in the next and final step of the synthesis (Scheme 2). Thus, the use of aqueous 1 N HCl enabled transformation of protected 1,3-dioxanes 5 and 10, in one reaction step, into pairs of positional isomers 6 and 11 containing substituted anthracene, benzo[2,3-b]carbazole and benzo[b]naphtho[2,3-d]thiophene frameworks in up to quantitative yields (Scheme 2). Photostability of the obtained o-bromo acenaldehydes was relatively high showing 50% absorption decay after 40 min (6a, for 435 nm energy band; under exposure of broad band UV/vis light at maximum 370 nm in 10⁻⁵ mol L⁻¹ chlorobenzene solution at r.t.) in comparison to 55–58 s recorded for anthradithiophenes bearing either formyl or 5-formylthien-2-yl groups attached to peripheral or central ring of the acene core.² In the new approach, isomeric dibromodialdehydes 1 and 7 reacted selectively: (1) only one bromine atom underwent Br/Li exchange and a subsequent conversion to diarylmethanols 4 and 9 without formation of a competitive bis(diarylmethanol) and (2) only one aldehyde function cyclized to (hetero)acene 6/11 without formation of S_EAr byproducts derived from a possible presence of benzyl and/or dibenzyl carbocations which might be formed from two acetal/aldehyde and one diarylmethanol functions under acidic conditions.

Scheme 2 Realization of the new concept of synthesis of o-bromo(hetero)acenaldehydes 6 and 11 and examples of pairs of positional isomers of these compounds obtained under the concept.

Electrochemical cyclic voltammetry (CVs) was performed to determine the HOMO and the LUMO energy levels of 6. Cyclic voltammograms (CVs) are shown in Fig. 1. All the compounds 6 displayed an irreversible reduction peak as well as an irreversible oxidation peak. The molecular frontier orbital levels can be estimated from electrochemical analysis using the method described by Sun and Dalton.^7a

Fig. 1 Cyclic voltammograms of compounds 6a–c (at a concentration of 0.001 M) recorded in dichloromethane containing 0.1 M Bu₄N⁺PF₆⁻ as supporting electrolyte with a glassy carbon electrode. A platinum wire and an SCE were used as the counter and reference electrodes, respectively.

In order to calculate the absolute energies of LUMO (lowest unoccupied molecular orbital) and HOMO (highest occupied molecular orbital), the redox data was standardized to ferrocene/ferricenium couple which has a calculated absolute energy of −4.8 eV.^7b,c From the value of onset oxidation potential and onset reduction potential of the compounds, the HOMO and the LUMO as well as the energy band gaps (E_g) were calculated and are listed in Table 1. The optical absorption and fluorescence spectra of 6a–c in chloroform solution (Fig. 2 and 3), quantum fluorescence efficiency (0.13–0.51) and large Stokes shifts (4036–6943 cm⁻¹) were also measured and listed in Table 2.

Table 1 Measured HOMO–LUMO levels and energy band gaps of compounds 6a–c

Compd in CH2Cl2	Eox (V)	EHOMO (eV)	Ered (V)	ELUMO (eV)	Eg (eV)
6a	0.932	−5.514	−1.713	−2.869	2.645
6a′	0.910	−5.492	−1.718	−2.864	2.628
6b	0.764	−5.328	−1.791	−2.791	2.537
6c	1.004	−5.586	−1.883	−2.666	2.887

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Fig. 2 Normalized absorption spectra of 6a–c recorded in chloroform.

Fig. 3 Normalized emission spectra of 6a–c recorded in chloroform at λ_ex = 450 nm (6a), 450 nm (6a′), 413 nm (6b), 400 nm (6c).

Table 2 Absorption and fluorescence spectral parameters of 6a–c recorded in chloroform

No.	Absorption^a λmax [nm]	Fluorescence^a λmax [nm]	Stokes shift (ΔνST, cm^−1)	ΦF^b
a Spectra recorded in chloroform solution.b Quantum fluorescence efficiency (ΦF). Anthracene (ΦF = 0.11 in chloroform) and quinine sulfate dihydrate in 0.1 N HClO4 (ΦF = 0.59) were used as references.
6a	375, 390, 436	538	4348	0.13
6a′	376, 394, 440	535	4036	0.12
6b	373, 398, 420	550	6943	0.51
6c	397	503	5308	0.27

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The influence of the presence of different aromatic moieties derived from the corresponding aldehydes 3 on the position of the UV absorption and fluorescence bands of compounds 6a,a′–c is noticeable as in the moieties of 3a,b with two or three methoxy groups and in the N-methylindole moiety of the compound 3c. It should be also noted that the molecule 6c with the N-methylindole moiety shows the pronounced hypsochromic shift both in absorption and fluorescence spectra as compared to the molecules 6a,b. On the other hand, the observed influence of different R groups in 10-OR substituents at the newly formed, six-membered ring in 6a and 6a′ on those spectra is insignificant.

A comparison of physicochemical properties of methylene-1,3-dioxy substituted anthracenes^3c with 7-formyl substituted ones, presented here, possessing the same 10-benzyloxy-1,2,3-trimethoxyanthracene core, shows bathochromic shift in fluorescence spectra [from λ_max = 399, 424 nm (blue) to λ_max = 535 nm (yellow)], quantum fluorescence efficiency (Φ_F = 0.26 and 0.13, respectively) and large difference in Stokes effects (Δν_ST = 513 and 4348 cm⁻¹). Similar effects have been observed for methylene-1,3-dioxy substituted benzo[2,3-b]carbazoles^3b and formyl substituted ones, possessing the same 5-benzyloxy-6-methyl-benzo[2,3-b]carbazole core: bathochromic shift in fluorescence spectra [from λ_max = 410 nm (bluish) to λ_max = 503 nm (green)], quantum fluorescence efficiency (Φ_F = 0.21 and 0.27, respectively) and large difference in Stokes effects (Δν_ST = 860 and 5308 cm⁻¹).

In summary, we have synthesised pairs of positional isomers 6/11 of a previously unknown group of higher analogs of bifunctional o-bromo(hetero)acenaldehydes via a new modification of the acid-catalyzed cyclization of the Friedel–Crafts (F–C) type of isomeric O-protected o,o-diacetalo diarylmethanols 4/9. This selective modification utilized for the first time a combination of isomeric, aromatic o,o-dibromo dialdehydes 1/7 and aromatic monoaldehydes as substrates which led to the formation of chemically stable, photoactive products possessing photostability 40 times higher (6a) than that observed for anthradithiophenes.² The previous variant employed two aromatic monoaldehydes.^3a–c The RO-Ar and ortho-CHO/Br structural moieties in 6/11 are available only via this peculiar F–C modification. Further synthetic works, investigations of electrical properties of the new compounds and construction of OLEDs devices are under way.

Acknowledgements

The scientific work was financed from the Science Resources 2008–2013 as research grant N204 517139, 2012/05/N/ST5/00169 and from IEP grant (POIG) no. 10-047/10, Action 1.3.2: “Support for the protection of industrial property formed in scientific units as a result of R + D work”. DG acknowledges the support from the University of Łódź, through the grant no. 1155. Authors thank to Dr G. Wiosna-Sałyga, Technical University of Łódź, for measurement of influence of irradiation time on the absorption spectrum of 6a (Grant no. 2012/04/S/ST4/00128).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: General methods; synthetic protocols; ¹H-NMR, ¹³C-NMR, MS, HR-MS spectra. See DOI: 10.1039/c4ra14071k

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