Benzo-TCNQ

The molecular structures of compounds 45, 46 and 47 were determined by X-ray analysis [115] and showed a butterfly shape (Figure 1.26) although much less distorted than the benzene-fused TCAQ molecule and close to those of the benzo-TCNQ [110]. 

Figure 1.24. Molecular orbital diagram showing the energies, symmetries, and atomic orbital composition of the HOMO and LUMO of TCNQ, benzo-TCNQ and 1,4-TCAQ. The atomic orbital (AO) coefficients are given in units of 10. ...

The correlation diagram sketched in Figure 1.51 summarizes the effects of joining the donor benzodithiin moiety to the acceptor 9,9,10,10-tetracyano-1,4-naphthoquinodimethane (benzo-TCNQ) unit to obtain 72a. The LUMO of benzo-TCNQ shows an atomic orbital composition similar to that of the LUMO of 72a and is destabilized from —5.66 eV to —5.44 eV in passing from benzo-TCNQ to 72a. 

The HOMO of benzo-TCNQ is also spread over the TCNQ moiety and should be correlated with the HOMO-1 of 72a. Figure 1.51 also shows that the HOMO of 72a has no, corresponding orbital in benzo-TCNQ. This molecular orbital is furnished by the donor unit and gives rise to the new charge-transfer band. 

Figure 1.51. Calculated VEH one-electron energies diagram showing the HOMO and LUMO of benzo-TCNQ and 72a.

We tried to improve the donor-acceptor properties obtained for previously reported compounds [26,162] by combining a strong electron-acceptor moiety such as benzo-TCNQ or benzo-DCNQI with the 1,4-benzoxazine system as the donor fragment. The synthesis of compounds 92 and 94 was carried out according to Scheme 1.17 by reaction of 2,3-dichloro-1,4-naphthoquinone with 2-methylaminophenol in pyridine yr) at 100°C to yield the novel A(-methylben-zo[6]phenoxazin-6,11-dione (91) in good yield. 

A similar analysis has been done91 on CT-complexation of the series benzene, benzothiophene, benzo[l,2-b, 4,3-b ]dithiophene ([3]SBS), and the homologs [5]SBSBS and [7]SBSBSBS with TCNQ as the acceptor. Beside association constants, also AH- and AS-values were determined by measurements at different temperatures (Table 21). It is striking that the gradual increase of the association constant in the series is interrupted at [5]SBSBS the next member has even a lower value. 

Interestingly, previous attempts to prepare TCNQ derivatives (76) from benzo[h]naphto[2,3-e][l,4]diox-in-6,11-quinones (74) were unsuccessful, leading to a tricyano derivative (75) instead of the expected TCNQ derivative [160]. 

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