Substituent Position Dependent Photophysical Properties of Indole – Barbituric and Thiobarbituric Acid based Molecular Rotors

A series of 2‐ and 3‐methylindole (MI)‐based barbituric acid (BA) and thiobarbituric acid (TBA) derivatives (2MIBA, 2MITBA, 3MIBA and 3MITBA) in Donor (D)‐π‐Acceptor (A) configuration having strong intramolecular charge transfer (ICT) interactions were synthesized by the simple and efficient route. Methyl substituent at the 2‐ and 3‐ position alters the coplanarity of the DA systems leaving 2MI derivatives as nonplanar whilst 3MI derivatives as planar one and the latter forms an intramolecular hydrogen bond between indole N−H and −C=O of BA or TBA, which facilitates the excited state intramolecular proton transfer (ESIPT) emission in 3MIBA and 3MITBA. Comparing the UV‐visible absorption spectra in the solution phase reveals that the ICT band of 2MITBA and 3MITBA are red‐shifted from the 2MIBA and 3MIBA due to the stronger electron‐accepting nature of C=S than the C=O. Since these molecules exert photo‐induced intramolecular charge transfer followed by a rotation around the bridging methylene group, only weak fluorescence is noted. Nevertheless, in an aggregated state, both 3MIBA and 3MITBA exhibit significant aggregation‐induced enhanced emission (AIEE) characteristics in their THF/water mixture, but the effect is not promising for 2MIBA and 2MITBA. Further, melamine‐3MITBA co‐assembly aided through an intermolecular hydrogen bonding network can also be used as a solid‐state emitter. A series of molecular rotors (2MIBA, 2MITBA, 3MIBA and 3MITBA) with D‐π‐A intramolecular charge transfer (ICT) characteristics were synthesized by simple and efficient route. The molecular geometry and photophysical properties of 3MIBA and 3MITBA are entirely different from the 2MIBA and 2MITBA due to the six membered intra molecular hydrogen bonding interactions between the indole −N−H and −C=O of BA or TBA groups.