PPP calculations by Klessinger showed that the first excited state of indigo is more polar than the ground state, so that positive solvatochromism might be expected. This was in fact observed by Reichardt (see Table 2.1) [8].
Table 2.1 |
Absorption |
maximum |
of indigo |
in various media |
||
Medium |
Steam |
CCl4 |
Xylene |
Ethanol |
DMSO |
Solid (KBr) |
^max nm |
540 |
588 |
591 |
610 |
620 |
660 |