Biradicals Stabilized by Intramolecular Charge Transfer: Properties of Heterosubstituted Pentalene and Cyclooctatetraene Biradicals † Shmuel Zilberg* and Yehuda Haas* Department of Physical Chemistry and the Farkas Center for Light Induced Processes, The Hebrew UniVersity of Jerusalem, Jerusalem, Israel ReceiVed: December 2, 2005; In Final Form: March 27, 2006 Intramolecular charge transfer can lead to substantial stabilization of singlet ground state and a corresponding increase of the singlet-triplet gap for molecules isoelectronic with the dianions of antiaromatic hydrocarbons. The formal biradicals 2,5-di-heterosubstituted-pentalenes and 1,5-di-heterosubstituted-cyclooctatetraenes are theoretically predicted to have the potential to be stable, persistent non-Kekule ´ molecules, as supported by high-level quantum chemical calculations. The singlet-triplet energy gaps and the S 0 -S 1 excitation energies of these molecules are similar to those of aromatic molecules rather than standard biradicals. These formal biradicals have a pronounced zwitterionic character, having a singlet ground state. The marked stabilization of the ground-state singlet for these non-Kekule ´ molecules is accompanied by a significant destabilization of the highest occupied molecular orbital (HOMO), leading to a low ionization potential (IP). This apparent inconsistency is explained by analyzing the electronic structure of the molecules. In the case of di-aza- pentalene, the energy of the first electronic excited state is only slightly lower than the ionization potential, making it a candidate for molecular autoionization. I. Introduction Substantial efforts have been made to design and prepare persistent nonclassical molecules such as diradicals 1 and triplet 2 or singlet carbenes, 3 yet “bottleable” non-Kekule ´ molecules remain a challenge. A design principle to achieve this goal via a unique substitution of two carbon atoms in formal antiaromatic molecules by heteroatoms is proposed. A highly stable formal aromatic [4n + 2] π-system is formed, even though, as shown by calculations, a possible Kekule ´ structure is sacrificed. Non-Kekule ´ molecules are molecules with a completely conjugated π-system, for which no classical Kekule ´ structures can be written. 4 A pair of nonbonding molecular orbitals (NBMOs) that are occupied by two electrons is the MO portrayal of the diradical’s electronic origin in non-Kekule ´ hydrocarbons. 5 Tetramethyleneethane (TME) is one of the prototype non-Kekule ´ molecules. Other types of fully π-conjugated diradicals are antiaromatic species such as cyclobutadiene (CBD), cyclooc- tatetraene (COT), or pentalene (PNT). Borden and Davidson predicted the violation of Hund’s rule for diradicals that have disjoint NBMOs. 5,6 Thus, the ground state of TME is a singlet with a modest singlet-triplet gap ΔE ST ) E S - E T )-2 kcal/ mol, as verified experimentally by negative ion photoelectron spectroscopy. 7 Incorporation of the TME fragment into a five- membered ring, as in the case of 3,4-dimethylenecyclopentane- 1,3-diyl, Ia, changes the ground state to a triplet. 8 The singlet- triplet gap ΔE ST for hydrocarbon disjoint diradicals is usually small, consistent with the high reactivity of these molecules. Qualitative MO arguments suggest that the lowest singlet state can be stabilized by replacing a carbon atom by a heteroatom. 9 Thus, the heteroderivatives of Ia (Figure 1) show a substantial stabilization of singlet vs triplet, as has been shown computa- tionally 10,11 and experimentally. 12 Lahti et al. 13 used semiempirical quantum chemical calcula- tions to assess the properties of diradicals (type I) and the related bicyclic derivatives of pentalene (type II). Figure 1 shows their structure and the numbering convention used in this paper. The large ΔE ST computed for type II derivatives led them to discuss criteria for determining whether these systems should be described as biradicals or zwitterions. Through the use of configuration interaction, it was found that the ratio (C 1 /C 2 ) 2 of the squares of the coefficients of the two leading configura- tions in the electronic wave function of the ground-state singlet is a useful criterion for this purpose. If a single configuration is dominant (i.e., C 1 . C 2 ), then the system has a pronounced zwitterionic character. Thus, the ratios were found to be 8.05, 3.28, and 2.46 for IIb, IIc, and IId, respectively, showing that IIb is best described as a zwitterion. Cava and co-workers isolated the tetraphenyl-substituted derivative of IId, tetraphenylthieno(3,4-c)thiophene (hereafter designated as IIe), leading to the elucidation of its electronic spectrum, 14 X-ray structure, 15 and photoelectron spectrum. 16 Derivatives of IIb and IIc, as well as of III, have not yet been † Part of the “Chava Lifshitz Memorial Issue”. * To whom correspondence should be addressed. E-mail: yehuda@ chem.ch.huji.ac.il. Figure 1. Structures of molecules discussed in the text (X: a ) CH2, b ) NH, c ) O, and d ) S). 8397 J. Phys. Chem. A 2006, 110, 8397-8400 10.1021/jp057027v CCC: $33.50 © 2006 American Chemical Society Published on Web 05/02/2006