Synthesis and Properties of Achiral and Chiral Dipyrenoheteroles and Related Compounds

I n the design of organic electronics, there is particular interest in polycyclic aromatic hydrocarbons (PAHs) based on fluorenone and dibenzoheterole precursors. Derivatives of these compounds have found their way into the design of organic field effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic photovoltaics (OPVs) through expansion of their conjugated π-electron systems. [1][2][3] The resulting PAHs can act as charge transport materials and allow devices that are thinner, more flexible, and more versatile than their inorganic counterparts.

Also of interest are PAHs, in which the conjugated system has been constructed in a helical fashion, leading to chirality and interesting optical properties such as circularly polarized luminescence (CPL). [4][5][6][7] Recently, CPL-active materials have attracted more interest in the design of high-efficiency OLEDs, chemical and biological sensors, and optical information processing technologies. 8,9 In recent years, our groups have reported the synthesis of chiral perylene bisimides and structurally related chiral peropyrenes. [10][11][12][13][14] The latter were synthesized via 4-fold alkyne benzannulation (Figure 1a) wherein the bay regions of the peropyrene are functionalized by bulky aryl groups, creating a barrier to racemization that is sufficient for the separation of enantiomers by chiral HPLC. More recently, the Nevada team reported the use of alkyne benzannulation to laterally expand the π-electron systems of [4]helicenes while functionalizing the resulting cove regions (Figure 1b,c). 15,16 This functionalization increases the barrier of enantiomerization well beyond that of normal [4]helicene, allowing for the separation and analysis of enantiomers.

Herein, we expand upon this work, incorporating both fluorenone and dibenzoheterole subunits to access novel PAHs with bent and chiral, [5]helicene-like geometries around a central, conjugated bridging group. We have synthesized two regioisomers to examine the effects of various bridging groups and substitution patterns on the reactivity of the intermediates and the interesting changes in the optical and electronic properties of the final products.

Tetrayne precursors 3a-c were obtained in good yields (72-76%) via 2-fold Suzuki cross-coupling of 2 with Letter pubs.acs.org/OrgLett commercially available 1a-c (Scheme 1). 17 Fourfold alkyne benzannulation was achieved by treating tetrayne precursors with InCl 3 and AgNTf 2 , and the resulting dipyrenoheterole nanographenes 4a and 4b were obtained in good yields of 76% and 81%, respectively. 18,19 The calculated structures of these compounds exhibit a twist in the backbone. However, the barriers of enantiomerization are believed to be so low as to render the compounds achiral. Nanographene 4c was not obtained due to the electronics of the core. The presence of a carbonyl ortho and para to the benzannulation reaction sites withdraws electron density from these positions, making them too unreactive for complete benzannulation to occur and resulting in a complex mixture of products.

Tetrayne precursors 6a-c were obtained in good yields (80-85%) via 2-fold Suzuki cross-coupling of 2 with commercially available 5a-c (Scheme 2). Fourfold alkyne benzannulation was achieved for nanographenes 7a-c in good yields (73-83%). Compared to those in 3c, the reaction sites in 6c experience a smaller electron-withdrawing effect from the carbonyl, allowing for facile benzannulation to compound 7c, albeit with a reaction time that is longer than those of electronrich analogues 7a and 7b.

In all cases of successful benzannulation, we observed a bathochromic shift in the absorbance and fluorescence spectra of 4a, 4b, and 7a-c, relative to their tetrayne precursors 3a, 3b, and 6a-c, respectively (Figure 2). This bathochromic shift is expected, due to the expansion of the π-electron systems. Notable, however, is the degree to which the spectra were shifted for the different regioisomers (Figure 2b,c). In 2016, the Chalifoux group showed that there is an absorbance bathochromic shift of approximately 110 nm when we expand the π-electron system of pyrene to peropyrene and again when we expand it to teropyrene. 17 In each case, we are adding three fused rings to the system. However, in the case of this work, the π-electron systems of our isomers are identical in size, yet we observed drastic changes in the absorbance and fluorescence.

When comparing 4a and 7a, we observed bathochromic shifts of 95 and 83 nm in the absorbance and fluorescence, respectively. Similarly, between 4b and 7b, we observed bathochromic shifts of 111 and 64 nm in the absorbance and fluorescence, respectively. This phenomenon is known for isomers of relatively small PAHs wherein one isomer has a greater degree of symmetry. The less symmetrical isomer tends to be bathochromically shifted, corresponding to a smaller optical band gap. 20 The structures of compounds 4a, 4b, and 7a-c were calculated at the B3LYP/6-31G(d) level of theory (Figure SI-6). The twisting of the backbone in the fjord region leads to decreased symmetry in 7 relative to the achiral isomer, 4, so our findings are in agreement with the aforementioned trend. These results demonstrate that we can exercise a great deal of control over the optical properties of PAHs through simple changes in the geometry of the backbone, rather than changes in the size of the backbone, to give us an additional means of tuning the properties of a PAH for a given application.

The emission lifetimes and fluorescence quantum yields were determined for each benzannulated compound, and the results are listed in Table 1. In comparing 4a and 4b to 7a and 7b, we see that the chiral isomers exhibit fluorescence with shorter lifetimes, but with quantum yields much larger than those of their respective achiral isomers. In the case of 7c, we observed a very short lifetime with a very low quantum yield.

The electrochemical properties of 4 and 7 were determined by cyclic voltammetry (CV) (Figure 3) and differential pulse Scheme 2. Synthesis of Helicene Compounds 3, and the results are summarized in Table 2. In all cases, these compounds exhibited two reversible oxidations. However, conclusive reduction peaks were observed for only 7. Compound 7c exhibited two reduction peaks, the first of which was reversible, whereas 7a and 7b showed only one irreversible reduction peak. In changing the geometry of the isomers, we observed a slight decrease in the HOMO values of 7a-c relative to those of 4a and 4b and a decrease in the LUMO values of 7a-c relative to those of 4a and 4b because no reduction waves were observed in the probed electrochemical window. From these results, we have further confirmation that changing the shape of the conjugated backbone gives us a meaningful way to tune the optical and electronic properties of these molecules.

The enantiomers of 7a-c were separated by HPLC on a semipreparative chiral column. The circular dichroism (CD) and CPL properties of the enantiomers were then studied (Figure 4). Compound 7a showed a weak Cotton effect at the lowest-energy absorbance (g abs = 1.1 × 10 -3 ; 503 nm), a stronger Cotton effect at the maximum absorbance (g abs = 5.1 × 10 -3 ; 351 nm), and an average g lum of 1.1 × 10 -3 from 480 to 620 nm. Compound 7b showed a weak Cotton effect at the lowest-energy absorbance (g abs = 8 × 10 -4 ; 487 nm), a stronger Cotton effect at the maximum absorbance (g abs = 3.6 × 10 -3 ; 344 nm), and an average g lum of 8 × 10 -4 from 480 to 620 nm. Compound 7c showed a weak Cotton effect at the lowestenergy absorbance (g abs = 8 × 10 -4 ; 479 nm) and a stronger Cotton effect at the maximum absorbance (g abs = 3.3 × 10 -3 ; 322 nm). Unfortunately, the fluorescence quantum yield of 7c was too low to obtain CPL data. These values compare favorably to those reported for other highly contorted, chiral organic compounds. 11,13,15,16,[22][23][24][25][26] In summary, we have synthesized novel PAHs through 4fold alkyne benzannulation onto dibenzoheteroles and fluorenone to explore the effects of incorporating heteroatoms and carbonyls into the backbones of our PAHs as well as the effects of changing the geometries of PAH isomers. We found that there is little difference between the sulfur and oxygen derivatives, though substitution of the heteroatom with a carbonyl results in a large bathochromic shift in the fluorescence of the molecule, albeit with a very short lifetime Half-wave potentials were determined by CV and referenced against the Fc/Fc + redox couple. b Reduction of 4a and 4b was not observed in the electrochemical window. c For irreversible processes, the peak value was determined by DPV. d Calculated using the equations E LUMO = -[E(M/ M -) + 5.15 eV] and E HOMO = -[E(M/M + ) + 5.15 eV], assuming that the energy level of Fc/Fc + with respect to the vacuum level is -5.15 eV. 21 e Calculated from redox potentials. f Absorption wavelengths of the first absorption onset. g Estimated from the UV-vis spectra.