Intramolecular Electronic Interactions between Nonconjugated Arene and Quinone Chromophores

The novel surprisingly colorful dark blue and orange-red molecular clips 1 and 2 containing a central p-benzoquinone spacer-unit and anthracene or napththalene sidewalls were synthesized by DDQ oxidation of the corresponding colorless hydroquinone clips 7 and 8. The colors of the quinone clips result from broad absorption bands in the visible range (1, λmax = 537 nm and 2, λmax = 423 and λshoulder =515 nm) showing bathochromic shifts of 112 and 90 nm, respectively, compared to the similarly tetraalkyl-substituted duroquinone 31, even though the clips 1 and 2 only contain insulated π systems as chromophores, a central tetraalkyl-substituted p-benzoquinone spacer-unit and two anthracene or two naphthalene sidewalls. To elucidate the electronic properties of these clips, we prepared the compound 3, the anti-configured isomer of clip 2, and the benzene-, naphthalene-, and anthracene-substituted quinones 4, 5, and 6, the so-called “half-clips”. The “half-clips” 6 and 5 show a similar color change and the same trend in the UV/vis absorption spectra as the anthracene and naphthalene clip 1 and 2. This finding already rules out that the color of these systems is a result of “through-space” π−π interactions between the aromatic sidewalls in the molecular clips 1 and 2. Quantum chemical ab initio calculations provide good evidence that the bathochromic shift of the absorption band at the longest wavelength observed in the UV/vis spectra of the clip quinones 2, 3, and 1 and the “half-clip” quinones 4, 5, and 6 with an increasing number of rings in the anellated aromatic unit (from benzene to anthracene) is the result of an increasing configuration interaction between a n → π* excitation of the quinoid component and a π → π* excitation with intramolecular charge transfer (CT) character. The initial π orbitals involved here and in higher lying transitions mainly stem from through-space interactions between π orbitals of the aromatic sidewalls and π orbitals of the quinone moiety with varying degree of mixing. The configuration interaction in the excited states can be considered to be a homoconjugation, that is, the relevant charge transfer states are formed across an allegedly insulating aliphatic bridge. The UV/vis spectra of the molecular clips 1−3, the “half-clips” 4−6, and the quinones 32 and 33 simulated by means of quantum chemical ab initio calculations agree well with the experimental spectra.