I am working on NLO studies of certain metallo porphyrins. I heard that metallation to the porphyrins induces π–π aggregation . So i want to know more of it. Is there any correlation of NLO property to π–π aggregation ?
porphyrins are discotic molecules and can self assemble to form long columns through pi-pi interactions (columnar self assembly). metallation makes the pi conjugation system of porphyrins electron deficient. this electron deficiency induce even stronger pi-pi interactions between porphyrin molecules which causes them to aggregate. now as the molecules aggregate in face to face manner to form long columns, this directional self assembly significantly affects different optical and other properties of these systems. for detailed insight into the matter you can search paper on porphyrin based liquid crystals.
NLO properties are related to the polarizability of the molecule (i.e., to the easiness of polarization of the molecule). Generally conjugated systems are easily polarizable and hence have good NLO properties.
Porphyrins form two types of aggregates. J-aggregates and H-aggregates
H-aggregates have molecules stacked on top of one another as in a column whereas J-aggregates have molecules arranged end-on.
So, what is the minimum value of dipole moment foe which a porphyrin can show NLO properties?
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