Role of spin-orbit and nuclear motion assisted spin-orbit coupling on the structural isomerism dependent intersystem crossing mechanism.

We investigate the intersystem crossing (ISC) mechanism of three isomeric molecules, namely, methyl 2-(9H-carbazol-9-yl)benzoate (o-MCBA), methyl 3-(9H-carbazol-9-yl)benzoate (m-MCBA), and methyl 4-(9H-carbazol-9-yl)benzoate (p-MCBA). Our analyses highlight the role of direct spin-orbit (DSO) coupling, Herzberg-Teller (HT) coupling, and spin-vibronic (SV) coupling in facilitating the ISC processes in these molecules. We have found that the dominant ISC pathway between the lowest excited singlet state (SCT1o/p) having charge-transfer (CT) character and the triplet state (TCT1+LE1o/p) with mixed CT and locally excited (LE) characters is primarily governed by both DSO and HT interactions in o-MCBA and p-MCBA, whereas for m-MCBA, the ISC pathway between the lowest excited singlet state (SLE1m) and the triplet state (TLE1m) is mainly assisted by HT and SV interactions. The time-dependent correlation function based method has been employed for the calculations of ISC rate constant (kISC). The computed DSO/HT assisted kISCs for the ISC pathway involving the lowest excited singlet and the triplet states of ortho, para, and meta isomers are found to be 0.87 × 106/4.16 × 106, 1.18 × 106/2.24 × 106, and 5.67 × 103/4.02 × 106 s-1, respectively, while the SV-aided kISC of m-MCBA is turned out to be 5.45 × 106 s-1. Our theoretical analysis reveals that while the SV interaction plays a crucial role on the ISC mechanism of meta isomer, the larger energy gaps between the lowest excited singlet state (SCT1o/p) and the higher lying triplet state (TCT2+LE2o/p) as well as that between (TCT2+LE2o/p) and the lowest excited triplet state (TCT1+LE1o/p) suppress the SV-driven ISC processes in ortho and para isomers. This study emphasizes that the origins of the same order of the experimental kISCs of the structural isomers are completely different.