When a single molecule absorbs or emits light, it does so perpendicular to the direction of the respective transition dipole. In principle, the directions of the dipoles for absorption and emission can be different, due to different electronic configurations of the ground and excited states of the molecule, but in practice the difference is very small. Photons are thus preferentially reemitted into the same plane in which absorption took place.
A large angle between these two dipoles would probably require a significant change in the molecular structure when the molecule is excited. I have heard of some fluorescent proteins which exhibit large angles between absorption and emission directions, but I do not know any details. (Edit: I found this interesting paper, which reports negative fluorescence anisotropy in YFP and a large angle between the absorption and emission dipoles.)
I would be interested in specific compounds which have a large angle between the two transition dipoles. Particularly, are there any simple compounds (less than ~100 atoms) that show such a behavior?
I have read about twisted intra-molecular charge transfer (TICT), in which a segment of the molecule rotates by ~90 degrees upon excitation. I could imagine that in such a case the emission might take place in a plane perpendicular to the original plane of absorption, but I haven't found any confirmation of this either.
Update: to illustrate, I have created a small sketch that outlines what I have in mind. The molecule I drew is just for illustration and does not necessarily show that behavior; I also don't know how exactly such a restructuring process actually takes place (the one I drew would correspond to a rotation of the rings on the right around the single bond). I further assumed that the dipole directions coincide roughly with the positions of the two rings, which might not actually be the case.
That said, the process I have in mind looks as follows:
- A photon with a polarization vector parallel to the dipole direction arrives (its direction of propagation is necessarily perpendicular to the dipole direction)
- The photon is absorbed and the molecule is left in an excited state
- In the excited state some internal rearrangement of the molecular structure takes place, such that the dipole direction changes
- A fluorescence photon is reemitted perpendicular to the new dipole axis. The photon has been effectively "redirected" (and shifted to higher wavelengths)