Organic Donor-Acceptor Complexes As Potential Semiconducting Materials.

In this review article, the synthesis, characterization and physico-chemical properties of the organic donor-acceptor complexes are highlighted and a special emphasis has been placed on developing them as semiconducting materials. The electron-rich molecules, i. e., donors have been broadly grouped in three categories, namely polycyclic aromatic hydrocarbons, nitrogen heterocycles and sulphur containing aromatic donors. The reactions of these classes of the donors with the acceptors, namely tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), tetracyanobenzene (TCNB), benzoquinone, pyromellitic dianhydride and pyromellitic diimides, fullerenes, phenazine, benzothiadiazole, naphthalimide, DMAD, maleic anhydride, viologens and naphthalene diimide are described. The potential applications of the resulting DA complexes for physico-electronic purposes are also included. The theoretical investigation of many of these products with a view to rationalise their observed physico-chemical properties is also discussed.

A new cross-conjugated mesomeric betaine.

Cross-conjugated mesomeric betaine (CCMB) has been defined as the dipolar species in which positive and negative charges are exclusively restricted to different parts of the molecule. In contrast to indolizine which undergoes [8+2] cycloaddition with dimethyl acetylenedicarboxylate (DMAD), its 1-aza analogue, namely imidazo[1,2-a]pyridine reacts with the same reagent to afford the first representative of the CCMB isoconjugate with the odd non-alternant hydrocarbon anion. The structure of the product could be assigned on the basis of the NMR and HRMS results. Furthermore, the spectral studies indicated the presence of additional DMAD molecules in CCMB, possibly in the form of a charge-transfer (CT) complex. The whole sequence of reactions initiated by the attack of imidazo[1,2-a]pyridine on DMAD could be rationalized on the basis of the computational study of a model reaction sequence at the DFT (B3LYP/6-31+G(d)) level indicating the formation of a new CCMB derivative. The electronic excited states of the product were investigated by time-dependent density functional theory (TDDFT) calculations at the wB97XD/6-311++G(d,p) level, which indicate low-lying charge transfer that is characteristic of the CCMBs.

Synergy between Experimental and Theoretical Results of Some Reactions of Annelated 1,3-Azaphospholes.

Computational calculations have been used successfully to explain the reactivity of the >C=P- functionality in pyrido-annelated 1,3-azaphospholes. Theoretical investigation at the Density Functional Theory (DFT) level shows that the lone pair of the bridgehead nitrogen atoms is involved in extended conjugation, due to which electron density increases considerably in the five-membered azaphosphole ring. The electron density in the azaphosphole is further enhanced by the presence of an ester group at the 3-position making the >C=P- functionality electron-rich. Thus, 1,3-azaphospholo[5,1-a]pyridine, i.e., 2-phosphaindolizine having ester group at the 3-position only does not undergo Diels-Alder (DA) reaction with an electron rich diene, such as 2,3-dimethyl-1,3-butadiene (DMB). However, an ester group at 1-position acts as electron-sink, due to which transfer of the electron density to the >C=P- moiety is checked and DA reaction occurs across the >C=P- functionality. The coordination of the Lewis acid to the carbonyl group at the 3-position raises the activation barrier, while it is lowered remarkably when it is coordinated to the P atom. Furthermore, the attack of 1,3-butadiene on the Si face of the P-coordinated (o-menthoxy)aluminum dichloride complex is a lower activation energy path. Fukui functions could be used to explain relative reactivities of indolizine and 2-phosphaindolizine towards electrophilic substitution reactions.

Diels-Alder Reaction of phosphaethene with 1,3-dienes: an ab initio study.

Computations on Diels-Alder (DA) reactions of phosphaethene with 1,3-butadiene and with isoprene reveal asynchronous transition structures. The DFT (B3LYP/6-311+G) activation energies of these reactions, 12-14 kcal/mol, are much lower than that of the parent ethene-butadiene reaction, 28 kcal/mol, even though the exothermicities of all lie in the same range, from -29 to -33 kcal/mol. The transition states (TSs) for the phosphethene-butadiene or isoprene DA reactions are earlier than the TSs of the butadiene-ethene cycloaddition. Due to the weakness of the C=P pi bond compared to the C=C pi bonds, the energies required to reach the phosphaethene TSs are much less than the carbocyclic cases. The computed (1)H NMR chemical shifts and nucleus independent chemical shifts (NICS) quantify the aromatic character of the transition states. Regioselectivities of the neutral phosphaethene-isoprene DA reactions are modest, at best. However, computations on radical cation DA reactions of phosphaethene with isoprene, which proceed stepwise with open chain intermediates, can account for the high regioselectivities that have been observed in some cases.