Linear and quadratic response TDDFT methods for the excited-state absorption in oligofluorenes

The excited-state absorption in a set of fluorene oligomers are simulated based on the LR-TDDFT and QR-TDDFT calculations. It is shown that the results obtained from LR-TDDFT are in good agreement with the data of QR-TDDFT and the experimental results. The advantage of the LR-TDDFT over QR-TDDFT method is that the computational cost is scaling as only N4. In addition, it is noticed that the influences of the used exchange correlation functional in LR-TDDFT and QR-TDDFT for the simulations of ESA also show similar effects. Accordingly, LR-TDDFT can be used as an alternative to QR-TDDFT for studying the ESA of conjugated polymers.