TDDFT investigation of fluorescence properties of luciferin and oxyluciferin analogs bearing an amino group

Chemical origin of high chemiluminescence efficiency of firefly luciferin remains an intriguing subject. Developing more efficient fluorescent marked compounds based on the firefly luciferin is in full swing for application in gene expression and gene regulation, and imaging of live tissues fields. Thus, a series of luciferin analogs bearing an amino group of aminoluciferin (AL), quinolylaminoluciferin (QAL), naphthylaminoluciferin (NAL), coumarylaminoluciferin (CAL) and anthrylaminoluciferin (AAL), pyrenylaminoluciferin (PAL) and corresponding keto form oxides were studied in this work, and the ground and excited state properties of them have been calculated by using time dependent density functional (TDDFT) methods in the gas phase, dimethylsulfoxide (DMSO) solution and hydrophobic environment. The results suggested that replacing aminobenzothiazole moiety of firefly luciferin with the larger rigid moieties, the absorption spectra shift to the blue. Bio-heterojunction concept was employed to explain the higher degree of spatial overlap of corresponding orbitals for CAOL, AAOL and PAOL, an important factor to be of beneficial to enhance fluorescence efficiency. In order to further explain the high fluorescence efficiency of CAOL, the process of nonradiative decay is also investigated. Comparing the energy difference of the corresponding singlet and triplet excited states in intersystem crossing and internal conversion, the reasons is qualitatively explained for the high fluorescence efficiency of CAOL. The investigation result for their emission spectra suggested that the emission maximum of AAOL is similar to firefly in nature. The influence of substituent on fluorescent emission was discussed.