Angular ladder-type meta-phenylenes: synthesis and electronic structural analysis

Abstract

Herein, we report the synthesis of two new series of angular (all-syn) ladder-type meta-[n]phenylenes (LMP, n = 3–8). One series contains keto groups at the termini bridges, denoted angular keto (AKn), the second contains alkyl groups at all bridge sp³ carbons, denoted angular alkyl (AAn). Their electronic and structural properties were delineated by a combination of electrochemistry and spectroscopic (UV-Vis and emission) methods and further supported by DFT calculations. Interestingly, experimental and DFT data show that changing the bridging group at the termini from alkyl (AAn) to keto (AKn) gives an increase in the first reduction potentials and LUMO energies, as the π-system is extended. Also, the charge (de)localization behavior is different for these two species; while the AAn compounds stablize charge with Robin-Day class III, the AKn compounds show a clear switch from class III to class II. In comparison with the linear analogues (LKn and LAn), DFT results reveal a shape independency of the charge (de)localization mechanism in acceptor-π-acceptor series (AKn/LKn).

Graphical Abstract

First report of the synthesis and optoelectronic properties of angular ladder-type meta-phenylenes with two different bridging termini.

Introduction

Over the past few decades, significant strides have been taken toward the design of polycyclic aromatic hydrocarbons (PAHs) that offer a vast diversity of topologies along with unique and improved photoluminescence features, electrochemical/thermal stability, and effective charge transport.^1–8 Among PAH compounds, the synthesis of ladder-type poly-phenylenes (LPP)⁹ has been a topic of ever-growing interest mainly due to their conjugated, flat and conformationally rigid structures, which promise broad applications in electronic devices.^{4, 10–15}

The connectivity of phenylene moieties (meta or para) and position of the bridged sp³ carbons (syn or anti) are the source of structural diversity in LPPs (Chart 1). To date, most efforts have been devoted to the development of the linear para isomer and its derivatives (Chart 1). Other configurations like bent-shaped meta-phenylenes (LMPs) remain mostly unexplored,^{16, 17} as these cross-conjugated¹⁸ LPPs suffer from poor effective global conjugation^{19, 20} which has been attributed to the anti-resonance and destructive quantum interference (QI) phenomena.^21–24 It should be noted that the incorporation of meta-phenylene into the para-phenylenes in an alternating fashion follow the same trend and allow for a significant portion of cross-conjugation which has been observed and justified both experimentally and computationally.^25–27 Despite the acceptance of this behavior, Ratner revealed that cross-conjugated molecules, like polyenes, can show exceptional behavior in electron transmission.^{28, 29}

Chart 1.

The general structure of anti-ladder-type para-polyphenylene (LPPP), syn- and anti-ladder-type meta-phynylene (LMP) and all-syn LMPs in this work.

In addition to the overall architecture of LPPs, substituents have large effects on their electronic properties and applications. For instance, several applications of both electron-rich (e.g. polyfluorenes)^{4, 30} and electron-deficient (electron transporting) systems are well-established in different fields such as organic light-emitting diodes (OLEDs).³¹ One of the determining factors toward an efficient electron-transporting material^32–34 is its high electron affinity (low-lying LUMO) which can be achieved by incorporation of electron-withdrawing substituents.³⁵ Thus, the good performance of fluorenone as electron deficient unit has been acknowledged in the literature.^36–38 It should be noted that the incorporation of electron withdrawing groups are mainly done through the synthesis of electron donor and acceptor groups bridged by a π linkers (donor-π-acceptor, D-π-A),^{39, 40} rather than donor-π-donor (D-π-D) and acceptor-π-acceptor (A-π-A) systems.⁴¹

In this study, we report the synthesis of a series of angular (all-syn) ladder-type meta[n]phenylenes (LMPs). We aim to: (i) develop a new synthetic approach for diversifying LPPs, (ii) investigate structure-property relationships via comparison with their linear counterparts, and (iii) evaluate conjugation length effects on the A-π-A species. We prepared compounds with two different termini: angular keto-bridged (AKn) and angular alkyl-bridged (AAn, Chart 1). The structural and electrochemical properties of the synthesized molecules were determined experimentally and predicted using density functional theory (DFT) methods. Moreover, we compared their electronic and structural features to their linear isomers, linear ketos (LKn) and linear alkyls (LAn), using DFT methods. The results revealed an increasing evolution in the first reduction potentials and LUMO energies of AKn when the π-system is extended. This indicates the diminution of the charge delocalization in the A-π-A series. The details of these findings are discussed herein.

Synthesis

The 9,9-dihexylfluorene (1) was prepared on gram scale with the reaction of fluorene with 1-bromohexane in tetrahydrofuran (THF) using t-BuOk as the base (Scheme 1). The subsequent diacylated product (2) is obtained in excellent yield by treating 1 with two equivalents of 2-bromobenzoyl chloride and four equivalents of AlCl₃ in CS₂. For the cyclization reactions, we employed the palladium(II) acetate (Pd(OAc)₂) as the catalyst and tricyclohexylphosphinetetraflouroboric acid (PCy₃·HBF₄) as the ligand.⁴² The reaction mixture was heated in N,N-dimethylacetamide (DMA) to obtain the first even LMP (AK4). The synthesis of AA4 compound was accomplished by reducing the AK4 and alkylating following the same approach as for the synthesis of 1. The AK6 and AK8 molecules were obtained using similar sequence as used for AK4. For the synthesis of odd-membered compounds we reacted the 1 with one equivalent of 2-bromobenzoyl chloride in CH₂Cl₂ as the solvent to obtain the molecule 12 as an important precursor for the subsequent compounds. It should be noted that the AK3 molecule prepared following the procedure reported by Wei et al.⁴³

Scheme 1.

Synthesis of AKn and AAn; Reaction conditions: (i) t-BuOK/THF/1-bromohexane; (ii) AlCl₃/CS₂/2-bromobenzoyl chloride/0 °C to r.t.; (iii) Pd(OAc)₂/PCy₃·HBF₄/K₂CO₃/DMA; (iv) Pd/C (10%)/H₂/EtOAc/HClO₄; (v) AlCl₃/CH₂Cl₂/2-bromobenzoyl chloride/0 °C to rt.

The structures of all AKn and AAn compounds and the intermediate species were characterized by ¹H/¹³C NMR and mass spectrometry (see the electronic supplementary information for experimental details). Crystal structures were obtained for AA3, AA4, AA5 and AK5 (Fig. 1), which showed the expected angular and flat geometry. In addition, we were able to obtain the single crystal structures of the precursor of AK6 compound (Fig. S12, ESI).

Fig. 1.

Crystal structures (obtained at 100 K) of AA3, AA4, AK5 and AA5 molecules; hydrogen atoms and solvent molecules are deleted for clarity; thermal ellipsoids are set at 50% probability level; the C and O atoms are colored grey and red, respectively.

Continuation of adding more phenylenes leads to the synthesis of m-phenylene macrocycle (i.e. closed form with 10 phenyl units, AA10); however, we have not been able to accomplish that synthesis. The similar cyclic oligomers with 8 and 10 phenylene units were synthesized by Wu and coworkers which showed bowl-shaped and planar geometry, respectively.⁴⁴

They used hydrogen on the bridged sp³ carbons which allowed them to do the oxidative dehydrogenation reaction in the last step to obtain the interesting totally conjugated structures with unusual global aromaticity.^{44, 45}

Spectroscopy

The redox features of all AKn and AAn were collected by subjecting them to electrochemical reduction/oxidation at a platinum electrode as a 2 × 10⁻³ M solution containing 0.1 M tetra-butylammonium hexafluorophosphate (n-Bu₄NPF₆) as the supporting electrolyte. The cyclic voltammograms of all species showed reversible waves for the first reduction (AKn) and oxidation (AAn) processes (Fig. S3, ESI).

As depicted in the Fig. 2, The E_red1 is −0.98 V and −1.38 V for AK3 and AK8, respectively. These values indicate a significant increase (~400 mV) in the E_red1. Comparing the E_red1 for all AKn species (Table 1) shows that the E_red1 moves toward more negative values with increasing number of phenylene units. However, this change is more drastic when moving from AK3 to AK5 and converges for the rest (Table 1). These data show that for the studied A-π-A systems in this paper, extending the conjugation decreases the interaction between the ends and increases the LUMO level. It should be noted that the increasing trend of the LUMO level with extending the π system is observed before for different compounds. For example, the experimental data (electron affinities) for benzo-, naphtho-, and anthraquinones showed that the expansion of the π system reduces the electron affinity.⁴⁶ Moreover, this effect, increasing the LUMO energy by increasing the π system, has also been observed in the solution phase (CV studies) when electron withdrawing groups like nitro are attached to the larger aromatic system.^{47, 48} In addition, recently, our group investigated the end cap substitution effects on polypara-phenylenes wherein experimental and DFT results confirmed that the electron rich groups (alkoxy and alkylamine) reverse the evolution of HOMO energies with respect to the number of phenylene units and in turn reverse the expected oxidation potentials.^{41, 49}

Fig. 2.

Cyclic (—) and square-wave (…) voltammograms of AK3 and AK8 (left, in 1:1 MeCN:C₆H₆) and AA3 and AA7 (right, in CH₂Cl₂) compounds at a scan rate of 100 mV s⁻¹ and 22 °C; The CV and SW of other species are provided in the ESI.

Table 1.

Experimental and DFT calculated values of the first redox potential (E_red1/E_ox1, V vs. SCE), maximum absorption and emission (λ_max, nm), energy of LUMO and HOMO (eV) and optical band gap (E_g, eV).

	Ered1a	absorptionb	emission	ELUMOc	Egd		Eox1a	absorptionb	Emission	EHOMOc	Egd
AK3	−0.98(−0.97)	329(311)	530	−3.42(−2.38)	3.63(4.28)	AA3	1.43(1.44)	339 (298)	342	−5.83(−6.01)	3.58(5.50)
AK4	−1.18(−1.21)	434(430)	487	−3.22(−2.21)	2.57(4.13)	AA4	1.30(1.28)	352 (311)	357	−5.70(−5.86)	3.43(5.28)
AK5	−1.35(−1.35)	419(419)	506	−3.05(−1.94)	2.58(4.19)	AA5	1.23(1.22)	360 (318)	365	−5.63(−5.77)	3.37(5.16)
AK6	−1.35(−1.36)	422(421)	513	−3.05(−1.92)	2.56(4.06)	AA6	1.18(1.20)	364 (322)	369	−5.58(−5.72)	3.33(5.09)
AK7	−1.36(−1.37)	430(421)	513	−3.04(−1.90)	2.56(3.96)	AA7	1.17(1.17)	367 (325)	371	−5.57(−5.69)	3.32(5.05)
AK8	−1.38(−1.37)	430(421)	516	−3.02(−1.90)	2.54(3.90)	AA8	---(1.17)	--- (327)	---	---(−5.67)	---(5.02)

In parentheses are:

^aDFT values of first redox potential (E_red1/E_ox1, scaled to experimental data)

^bDFT maximum absorption (λ_max nm)

^cDFT energy of LUMO/HOMO (eV) and

^dDFT E_g (E_LUMO–EHOMO, eV)

all DFT values are reported at BLYP35/6-31+G(d,p)+SMD(CH2Cl₂) level of theory.

On the other hand, the first oxidation potential (E_ox1) of AAn compounds decreases with an increase in length, as expected. The E_ox1 is ~1.43 V and ~1.17 V for AA3 and AA7, respectively (Table 1 and Fig. 2), in accordance with an increase in HOMO energy with respect to length. Having these data, we calculated the LUMO and HOMO energies for AKn and AAn molecules respectively (Table 1) by employing E_1/2 of the first redox event according to E_HOMO/LUMO = −[E_1/2 + 4.4] eV.^{50, 51}

To explore the frontier molecular orbitals (FMOs) of our molecules, we collected the absorption and emission spectra of both AAn and AKn compounds (Table 1 and Fig. 3 and 4). As listed in Table 1, the absorption maxima for AA3-AA7 clearly show a bathochromic shift from AA3 (339 nm) to AA7 (367 nm). The same trend is observed for the emission bands, which is a common optical behavior of conjugated systems.⁵² However, cross conjugation causes very little shift in optical spectra.^53–55

Fig. 3.

Compilation of the absorption (—) and emission (…) spectra of the AA3-AA7 (in CH₂Cl₂) at 22 °C.

Fig. 4.

Compilation of the absorption (—) and emission (…) spectra of the AK3-AK8 (in 1:1 MeCN:C₆H₆) at 22 °C.

Comparing the AAn with AKn reveals that the AKn series does not show any correlation with increasing number of phenylenes; however, show us the red shift of both absorption (except the AK3) and emission spectra by changing the termini bridges from alkyl to keto. Interestingly, this changing leads to huge difference between the Stokes shift values. While the AAn compounds indicated very small value (~341 cm⁻¹), the AKn species showed much higher average value (~6016 cm⁻¹, Table 1 and Fig. 3 and 4). These larger values of AKn series can be useful in the optical materials as they benefit from lower self-quenching.^56–58

The optoelectronic properties of fluorenone was the subject of both experimental and computational studies where the results showed that the observed bands in the UV-vis spectrum correspond to the π to π* transition and n to π* transition is symmetrically forbidden.^{59, 60} Therefore, we believe that our AKn systems are showing the same behaviour.

Computational

We performed comparative DFT calculations on our molecules, using the non-standard BLYP35⁶¹ functional which gives reliable results for radical species⁶² with double-zeta quality basis set (6–31+G(d,p)) for all optimizations and subsequent studies. It should be noted that some of the calculations are repeated with four other functionals and the results are reported and compared in the ESI; the computational details are provided in the ESI. The calculated HOMO/LUMO energies (Table 1 and Fig. 5) show a decreasing energy gap (E_g = E_LUMO–E_HOMO) with increasing number of phenylene units for both the AKn and AAn systems. Comparing these values with experimental optical E_g (Table 1)⁶³ shows acceptable correlation. However, it seems that the DFT overestimates (~2 eV) the E_g values for both AKn and AAn species. The obtained values from the other functionals showed same trend and all of them overestimate the E_g values (Tables S6 and S7, ESI).

Fig. 5.

HOMO (filled) and LUMO (hollow) energies at BLYP35/6–31+G(d,p)+SMD(CH₂Cl₂) level in eV for A) angular keto (AKn), B) angular alkyl (AAn), C) linear keto (LKn) and D) linear alkyl (LAn) species.

We employed the time-dependent density functional theory (TD-DFT) methods to compare the calculated and experimental UV-Vis data. As listed in Table 1, the TD-DFT calculations underestimate the observed λ_max by around 40 nm for the AAn species. On the contrary, the TD-DFT data for the AKn compounds show good agreement with the experimental. Analyzing the contributions of the frontier orbitals to the observed λ_max revealed that the HOMO-to-LUMO transition percentage in the highest observed wavelength (λ_max) decreases by increasing the conjugation for both AKn and AAn.⁶⁴ This HOMO-to-LUMO transition decrease is more dramatic for AKn species, from 87% to 22%, and much smaller for AAn, from 90% and 71% (Fig. S17–S28, ESI). Therefore, the absorptions of AKn compounds are less sensitive to the E_g in comparison to the AAn compounds and do not show a clear red-shift trend as observed for the AAn series.

In order to understand the charge delocalization of our compounds and comparing them with the Robin-Day classification⁶⁵ of charged species, we calculated spin density distributions.⁶⁶ Robin and Day classified the polaron (i.e. hole or electron) into three different classes based on the magnitude of the electronic coupling (V₁₂) and structural reorganization energy (λ_reorg). In class I, the two redox centers of repeating units are completely localized and act as isolated units with no electronic coupling between them (V₁₂ = 0). In class II, the electronic coupling is smaller than the reorganization energy (λ_reorg > 2V₁₂) and the system can show the dynamic hoping of polaron. While the compounds following the class III indicate strong electronic coupling and evenly and static charge delocalization (2V₁₂ > λ_reorg).

As illustrated in Fig. 6, the spin density of ≥AK5 anion radical gravitates to one terminal despite the totally symmetrical LUMOs of neutral species. This means that the injected electron into the system prefers to localize at one terminus. Indeed, comparing the reduction potential of ≥AK5 with fluorenone supports the DFT results; the E_red1 of ≥AK5 and fluorenone is around −1.36 V and −1.37 V, respectively (Table 1 and Fig. S3). In contrast, the AK3 and AK4 compounds show charge delocalization. This charge (de)localization change (not following the LUMO pattern) can be attributed to the interplay between the electronic coupling (V₁₂) and structural reorganization energy (λ_reorg) from Marcus theory.⁶⁷ According to these data, electronic coupling is larger than the reorganization energy (2V₁₂ > λ_reorg) for AK3 and AK4; they are therefore Robin-Day class III compounds.⁶⁵ While, the AK5-AK8 compounds belong to Robin-Day class II (2V₁₂ < λ_reorg)⁶⁷ with a dynamic charge hopping mechanism in these acceptor-bridge-acceptor (A-π-A) systems (Fig. S29, ESI).^68–70

Fig. 6.

Isosurface representations (0.02 au) of HOMO and LUMO (for neutral, S₀) and spin density (0.001 au, for anion/cation radical, D₀) for AA3-AA8 (left) and AK3-AK8 (right) molecules at BLYP35/6–31+G(d,p)+SMD(CH₂Cl₂) level.

On the other hand, the spin density distribution of the AAn compounds does not show any charge (de)localization switch. The hole is symmetrically distributed (limited to ~4 phenylene units) at the center for all of cation radials, as is typical of conjugated oligomers.^{66, 71, 72} The lack of close correspondence between the HOMO and spin density distributions can be attributed to the energetic penalty of λ_reorg.^{73, 74}

Finally, in order to investigate the effects of the shape of AKn/AAn compounds, we carried out a series of comparative DFT calculations for the linear para isomers with both keto (LK3-LK8) and alkyl (LA3-LA8) bridged termini (Fig. 7A). The results show that the increasing LUMO energy trend is not due to the shape of AKn compounds and can be observed also in the linear keto-bridged species LKn (Fig. 5C). Therefore, we concluded that the presence of electron withdrawing groups (keto) on the termini of the rigid phenylene backbone is responsible for the observed trends in FMO energies and the first reduction potentials.

Fig. 7.

A) structures of linear LKn and LAn compounds; B) calculated bond lengths [BLYP35/6–31+G(d,p)+ SMD(CH₂Cl₂)] of phenylene linking single bonds for AA5 and LA5 in angstrom.

Moreover, our calculations for oxidation potential of linear compounds (LAn) show that the oxidation energies are systematically lower (5.28 → 4.84 eV, from LA3 to LA8) compared to bent-shaped structures (5.41 → 5.20 eV, from AA3 to AA8). Fig. 7B illustrates the optimized geometries of AA5 and LA5. The C-C bond lengths between phenylene units remain relatively constant for both angular and linear series and are independent of the conjugated length (bond_C-C=1.47±0.01 Å). This implies that the pi electrons are localized within Clar sextets and there is no bond contraction/elongation.

Conclusion

In summary, we successfully developed the synthesis of a series of π-conjugated angular ladder-type meta-phenylenes with two different substituents on their termini, alkyl (AAn) and keto (AKn). Electrochemistry, spectroscopy and DFT methods were used to analyze the electronic properties of all compounds. The results indicated an increase in the first reduction potential (~400 mV) moving from AK3 to AK8. DFT calculations on both synthesized compounds and their linear analogues (LK3-LK8 and LA3-LA8) reveal the progressive increase in LUMO energies and charge localization with the increase of phenylene units in the keto-containing compounds regardless of their shape (linear or angular). This shows us that replacing the alkyl with keto substituents can switch the charge transfer mechanism from the static delocalization (class III) in AAn/LAn to dynamic hopping (class II) in AKn/LKn species. Therefore, we believe that this work shed a new light on the importance of substituent effects on the π-conjugated systems and structure-dependent charge delocalization in oligomers.

Abbreviations

AK

angular keto

AA

angular alkyl

LK

linear keto

LA

linear alkyl