A comparative computational study of CN and CC bonding visible to NIR absorbing croconines

The lowest electronic excitations and charge transfer properties in two series of croconine dyes; 1) molecules with CN bonding, having an absorption in the visible region (400-600 nm) and 2) molecules with CC bonding, showing absorption in visible to near infrared (NIR) region (600-1100 nm) are analyzed by quantum-chemical calculations. The absorption maxima in CC bonding croconines (CCR) are always having 200-300 nm red shifted than its corresponding CN bonding croconines (NCR). The reason for this drastic red shift in CCR series than its corresponding NCR has been systematically studied by DFT, TDDFT and SAC-CI methods. It is found that, CCR series are with less charge transfer in nature and are having larger diradical character, whereas NCR series molecules showing larger charge transfer with lower diradical character. The change in bonding mode of central five membered croconate ring, from CN to CC, destabilization and/stabilization of HOMO LUMO levels were observed. This study may helpful in the design and synthesis of new visible to NIR absorbing croconine dyes which are useful in materials applications.