organic compounds
Ê
double-bond character in NBD-chloride [1.349 (9) A; Suzuki
et al., 1988], whereas in the present systems, this bond has
considerable single-bond character, with a mean bond length
benzoxadiazole does not contribute to electron ¯ow, which is
evident from the fact that its geometry does not change much
on substitution with an amino group. Hence, the dipole
moment is directed from the amino to the nitro group,
removing the complexity and controversy about the involve-
ment of the benzoxadiazole unit.
Ê
of 1.406 (3) A. Similarly, the C5ÐC6 bond in NBD-chloride is
Ê
1
1
.435 (9) A, whereas in (I)±(III) it has an average length of
Ê
.383 (1) A, indicating that the reversal of bond order of these
compounds is due to substitution with a stronger donor.
The C4ÐN4 bond, which is very important for photo-
physical behaviour, is found to be considerably shortened in
each of (I), (II) and (III). In similar donor±acceptor molecules
with ICT states, for example, 3,5-dimethyl-4-dimethylamino-
benzonitrile, the corresponding C4ÐN4 bond length is
Comparing the derivatives studied, it can be seen that (I)
has the shortest C4ÐN4 bond. This is due to the maximum
overlap of the amine lone pair with the benzoxadiazole
moiety, resulting from the favourable nitrogen conformation
retarding rotation around the bond. This observation supports
the dynamic NMR data that indicate (I) has a higher barrier to
rotation compared with other compounds (Saha & Samanta,
1998).
Ê
ꢀ
1
.414 (3) A, and at 173 K, it is also twisted by 59.3 (2) (Heine
et al., 1994). However, this is shorter than the corresponding
bond in the salt 4-aminobenzonitrile hydrochloride
Ê
The cyclic amino moiety in (II) is found to be in the stable
chair conformation. The three angles around the amine N
atom indicate the pyramidality at the N atom; in these deri-
vatives, the amino N atom is almost planar. Furthermore, it
can be seen from the data that (III) is more planar than the
others. This is due to the lower barrier to inversion for larger
[
1.467 (2) A], which is considered to be a pure single bond
(Colapietro et al., 1981). In the case of 4-dimethylamino-
benzonitrile (DMABN), the corresponding bond length is
Ê
ꢀ
.367 (3) A, with a twist angle of 10.8 (2) (Heine et al., 1994),
where considerable double-bond character has been predicted
1
Ä
rings (Oki, 1985), and also supports the photophysical obser-
vations that (III) has the maximum non-radiative rate-
constant value (Saha & Samanta, 1998).
on the basis of photophysical properties. For the present NBD
Ê
derivatives, the mean C4ÐN4 length is 1.336 (5) A.
Comparing this with all the data mentioned above, we can
conclude that there is a considerable charge ¯ow from donor
to acceptor, making the bond appreciably shorter. The double-
bond character of C4ÐN4 can also be explained by a formal
valence-bond approach.
There is a small increase in the twist angle between the
amine and NBD moieties on going from (I) to (III). The
ꢀ
maximum twist angle detected for (III) is 10.9 (5) [ꢁ(C8Ð
N4ÐC4ÐC5)], which indicates that the excited state is unli-
kely to be twisted, contrary to the speculation that a twisted
intramolecular charge-transfer (TICT) state is involved
(Forgues et al., 1993). This supports our earlier ®ndings (Saha
& Samanta, 1998).
In summary, from the structure determinations on these
three NBD derivatives, we have been able to extract valuable
data about the effect of amino substitutions, and some of these
data supplement photophysical observations.
Another noteworthy point is that the N5ÐC7 bond is
considerably shorter in the present NBD derivatives [mean
Ê Ê
.422 (4) A] compared with NBD-chloride [1.470 (8) A;
1
Suzuki et al., 1988]. This indicates that the nitro group acts as
the acceptor, a conclusion which is supported by the obser-
vation that the nitro group is almost coplanar with the
benzoxadiazole unit. The adjacent ®ve-membered ring of the
Figure 2
A view of the molecular structure of (II) showing the atom-labelling
scheme. Displacement ellipsoids are drawn at the 50% probability level
and H atoms are shown as small spheres of arbitrary radii. One H atom on
each of C9 and C11 is wholly obscured by the parent atom.
Figure 3
A view of the molecular structure of (III) showing the atom-labelling
scheme. Displacement ellipsoids are drawn at the 50% probability level
and H atoms are shown as small spheres of arbitrary radii.
ꢁ
Acta Cryst. (2002). C58, o174±o177
Satyen Saha
10 10 4
C H N O
, C11H N
3 12 4
O
3
and C12H
14
N O
4 3
o175