Influence of microcrystal formation on the aggregated state emission behaviour of pyrene substituted phthalonitrile positional isomers

Pyrene, due to its peculiar electronic properties is ideal for D-A type charge transfer fluorophores. Herein we synthesized pyrene and phthalonitrile incorporated D-A architectures 3-PyPN and 4-PyPN with large stoke's shift of 184 nm and 168 nm respectively (methanolic solution). 3-PyPN exhibited weak emission compared to 4-PyPN in solution and PMMA film with quantum yields of Φ_F = 0.15 (solution), 0.22 (film) for 3-PyPN and Φ_F = 0.59 (solution) and 0.54 (film) for 4-PyPN. 4-PyPN suffered solid state emission decrement compared to 3-PyPN with the corresponding quantum yields being 0.12 and 0.24 respectively. Further, 3-PyPN showed AIE behaviour in contrary to the ACQ observed with 4-PyPN. Morphological and size analysis of aggregates displayed formation of ordered microcrystals of 510 nm average particle size with 3-PyPN and irregular fibrous particles of 805 nm with 4-PyPN. Plausible explanation towards this behaviour may be that 3-PyPN experiences incremental restriction of intra-molecular rotation (RIR) owing to steric hindrance and attains semi-propeller geometry. This allows staggered J- type aggregation of pyrene leading to microcrystalline morphology. Quantum chemical calculations demonstrates changes in aggregated state emission as the outcome of dihedral angle changes between 3-PyPN (θ1 = 67.44°, θ2 = 110.76°) and 4-PyPN (θ1 = 56.92°, θ2 = 125.80°) due to steric hindrance exerted by 3-PyPN due to pyrene and nitrile group proximity.