Increased Antiaromaticity through Pentalene Connection in [n]Cyclo-1,5-dibenzopentalenes

Abstract

Conjugated nanohoops incorporating nonalternant hydrocarbons have altered optoelectronic properties compared to [n]cycloparaphenylenes or other purely aromatic hoops. We synthesized [n]cyclo-1,5-dibenzopentalenes (n = 4, 5), in which nonalternant dibenzo[a,e]pentalenes are connected through their pentalene units. This leads to an increase in antiaromatic character and low-lying LUMO energies. Calculations show puckered or entangled conformations of the precursor macrocyclic Pt-complexes. Our study proves dibenzopentalene as a versatile nonalternant building block for conjugated nanohoops with modifiable antiaromaticity and optoelectronic properties.

Conjugated Nanohoops with Dibenzo[a,e]pentalenes as Nonalternant and Antiaromatic π-Systems

“…In the structurally related indeno[1,2-b]fluorenes, transient absorption spectroscopy measurements showed that the excited-state lifetimes were shorter than the timescale at which fluorescence is usually observed. 84 We also did not observe fluorescence for those hoops, in which the -system is dominated by DBPs, namely [n]CDBPs 2 and 3, 16 [n]C-5,10-DBPs 4 and 5, 18 51 On the other hand, in [2]DBP [12]CPP 6 13 and DBP[n]CPPs 8 and 9, 17 which consist of DBPs alternating with [6]-or [7]paraphenylene linker(s), we were able to detect fluorescence, albeit of low intensity. This fluorescence stemmed from the paraphenylene units, which explains the similar wavelengths and shapes of the emission bands, with maxima at 471 nm (6), 487 nm (8), and 479 nm (9), corresponding to a bathochromic shift with increasing strain (the spectra of 8 and 9 are shown in Figure 8B).…”

“…We also accessed [ n ]C-1,5-DBPs 4 and 5 connected through the 5- and 10-positions of the five-membered DBP rings to compare the two conjugation pathways (Scheme 1 B). 18 As described above, the use of boronic ester 14 in the Pt-mediated cyclization led to success and allowed us to isolate tetrameric 4 and pentameric 5 . At 0.8 and 1.9%, respectively, the yields were lower than those of the [ n ]CDBPs 2 and 3 , probably due to the lower solubility of the intermediate Pt complexes.…”

“…12 Despite the tremendous diversity of nanohoops that have been reported in the past ten years, carbon nanohoops containing antiaromatic units had remained unexplored until recent reports from our group. [13][14][15][16][17][18] The electronic properties of antiaromatic molecules are markedly different from those of aromatic compounds, manifested in higher HOMO and lower LUMO energies and higher reactivities, among others. When we entered the field, little was known regarding how bending and cyclic conjugation would alter the properties of antiaromatic systems.…”

“…In the first, we employed difunctionalized prebuilt DBP units, and we relied on established synthetic methods for nanohoop formation, namely Approaches 1 and 2 in Figure 2 . These provide access to all-antiaromatic hoops consisting exclusively of DBP units, namely [ n ]cyclodibenzopentalenes 2 and 3 and [ n ]cyclo-1,5-dibenzopentalenes 4 and 5 , 18 as well as the larger DBP–CPP conjugates 6 and 7 . 13 In a second and new synthetic strategy, we used the folded diketone building block 1 as a DBP precursor in our nanohoop synthesis, and we made use of its naturally folded structure (Approach 3; Figure 2 ), leading to hoops 8 – 12 .…”

“…Calculations showed that the structures of the tetra- and pentameric Pt complexes as precursors to 4 and 5 assume a puckered shape or an entangled conformation, respectively, rather than a planar conformation, as might be expected (Figure 3 ). 18…”