The cyanide anion sensing mechanism of 1,3,5,7-tetratolyl aza-BODIPY (1) has been rigorously investigated using density functional theory and time dependent-density functional theory methods. Mulliken charge distribution and Natural Bond Orbital analysis reveals that cyanide addition may occur at both electrophilic centers with equal probability. The molecular orbital analysis reveals that first excited state (S-1) of 1 is a local excited state with pi-pi transition, whereas for 2 (the cyano form of 1), S-1, a charge-separation state, is found to be responsible for the intramolecular charge transfer (ICT) process which in conjunction with partial configuration change induces fluorescence stimulation in 2. (C) 2016 Elsevier B.V. All rights reserved.