Poly(acene)s, linear poly(benzenoid) hydrocarbons, consist of an aromatic linear array. The largest whose synthesis has been authenticated is hexacene, C26H16. However, all reported syntheses of hexacene are difficult to repeat. Synthesis of higher acenes (seven member and higher) have challenged chemists for a long time. Heptacene has been elusive by the attempted classical synthetic routes because such procedures instantly yield an array of dimers. Recently, pentacene and its derivatives have been shown to be excellent candidates with enhanced π-stacking ability for application in OFET and in other electronic devices. Hexacene and heptacene can be considered potential molecules for opto-electronic applications.
A classical synthetic procedure to produce heptacene was followed first, which involved Meerwein-Ponndorf-Verley (MPV) reduction of corresponding quinone. Product appeared to be a mixture of dimers of heptacene. In order to minimize dimerization processes, several reactions to substitute at the carbonyl group of quinone with a bulkier group were attempted. However, none of these reactions was successful. The insolubility of the precursor dione seems to be the primary reason for the failure of these reactions.
To enhance the solubility and stability of heptacene and precursors, substituted heptacenes retaining the polyacene backbone were designed. Symmetric quinones were considered as the key synthons. While many reduction methods failed to yield the final product from substituted quinones, the borane-THF complex reduced 6,8,15,17-tetraarylheptacene-7,16-quinones to the 7,16-dihydro derivatives. An alternative approach using coupling between in-situ generated dibenzyne and naphthofuran also failed to yield any heptacene core.
Dihydroheptacene derivatives emit in the region of 420 – 428 nm in several solvents (ΦF = 0.15 – 0.21 in CH2Cl2) and in the solid state (ΦF = 0.37 – 0.44). These compounds have good solubility in common organic solvents, are reasonably stable, and retain color purity even after annealing for 24 hours at 110 °C. Though their dilute solutions showed blue emission (λmax ~ 420 nm), they showed excimer emission (λmax ~ 480 and 510 nm) at higher concentration. The OLED devices containing 6,8,15,17-tetraphenyldihydroheptacene showed green emission (λmax ~ 515 and 550 nm) that is even further red shifted than the emission of excimer. This indicates that an inter-ion pair, electromer, is responsible for the electroluminescence. Pump-probe experiments of dihydroheptacenes revealed that the S1 state shows a broad absorption (~ 500-650 nm) in dichloromethane with a lifetime of ~ 0.23 – 0.33 ns.
Another synthetic strategy employed was photochemical expulsion of two molecules of carbon monoxide from α-diketones of ethano polyacenes. Photo-precursors of hexacene and heptacene were synthesized. The Strating-Zwanenburg photodecarbonylation of these photoprecursors in a poly(methyl methacrylate) matrix yielded the target hexacene and heptacene, respectively. The semi-rigid ploymer matrix enabled retention of highly reactive hexacene and heptacene through the prevention of thermal dimerization and oxidation. Heptacene was also generated in inert gas matrices at low temperature. Uv-vis-NIR absorption and IR spectra of heptacene were recorded in argon matrix at 10 K. When heptacene was generated in nitrogen matrix, it was stable up to 34 K. However, it was stable up to ~50 K, when generated in argon matrix.
Steady state photolysis, nanosecond laser flash photolysis, and femtosecond pump-probe experiments of α-diketone precursors of acenes were carried out to understand the mechanism of the Strating-Zwanenburg photodecarbonylation. It appears that both the singlet and triplet states of the diketones are involved in the decarbonylation process. These compounds have a small singlet-triplet energy gap (~ 4 kcal/mol). The lifetimes of the singlet excited states are in the range of 20-218 ps and decrease as the number of the benzenoid ring increases in the molecule. The triplet states are short lived (> 370 ps < 7 ns) and do not appear during the nanosecond experiments. It seems that the decarbonylation occurs within 7 ns. During the LFP experiment of heptacene precursor, the triplet state of the photoproducts, i.e., heptacene (λmax = 580 nm, τ ~ 11 µs), was also observed.
Rapid oxidation of heptacene occurs when a polymer film containing heptacene is exposed to air and this could be easily monitored by following gradual disappearance of its absorption in the visible region. The rates of disappearance of heptacene in different polymer films were observed to follow pseudo first order kinetics. Interestingly, those rates measured in the films of polystyrene (7.25 × 10-4 s-1), poly(ethyl methacrylate) (4.27 × 10-4 s-1), poly(methyl methacrylate) (1.60 × 10-4 s-1), and poly(vinyl chloride) (1.03 × 10-4 s-1) were found to correlate well with their oxygen permeability values. This indicates that the high reactivity of heptacene towards molecular oxygen can be used to determine the oxygen permeability of polymers.