Investigation of photophysical properties of mer-tris(8-hydroxyquinolinato) aluminum (III) and its derivatives: DFT and TD-DFTcalculations

Optimized structures and photophysical properties of mer- and fac-Alq3 have been generated by using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). Investigating the substitution effect in the Alq3 derivatives, the role of the electron-donating (CH3- and NH2-) and electron-withdrawing (F-, CN-, NO2- and phenyl-) groups with 2- to 7-substitution have been analyzed. According to the calculation results, the 4- and 5- substituted Alq3 exhibit an apparent spectral shift relative to the non-substituted Alq3. The HOMO, LUMO, Eg (the energy gap between LUMO and HOMO), λabsmax (maximum absorption wavelength) and f (the relative oscillator strength) of mer-Alq3 with the 4- or 5-phenyl substitution on the quinoline ligand in the ground electronic state were calculated by using the DFT/B3LYP/6-31G(d) and TD-DFT methods. 5-phenyl substituted mer-Alq3 with an electron-donating substituent showed an increase in the π-delocalization as compared to the 4-phenyl substituted mer-Alq3 derivatives. Similarly, 4-phenyl substituted mer-Alq3 with electron-withdrawing substituents also exhibits increased π-delocalization in the pyridine ring as compared to the non-substituted Alq3. Replacing the CH group at the 4, 5 and 4,5 positions of the quinoline ligand of mer-Alq3 with the aza group (nitrogen atom) gives three Alq3 analogous: AlX3, Al(NQ)3 and Al(NX)3; the calculated energy gap Eg of these derivatives decreases in the order Al(NQ)3>Al(NX)3>AX3. Four quinoline with group III metals Mq3 complexes were investigated for the photophysical properties; the calculated energy gap Eg decreases in the order Tlq3>Inq3>Gaq3>Alq3. The photophysical properties of 4-hydroxy-8-methyl-1,5-naphthyridine (mND) chelated with group III metals (MmND3 complexes) were investigated also; their calculated Eg have the opposite order as those of Mq3 complexes.