Sensitized Emission of Luminescent Lanthanide Complexes Based on 4-Naphthalen-1-yl-Benzoic Acid Derivatives by a Charge-Transfer Process

• Published in: Chemphyschem 2006-01, Vol.7 (1), p.213-221
• Main Authors: Kim, Yong Hee, Baek, Nam Seob, Kim, Hwan Kyu
• Format: Article
• Language: English
• Subjects: Atomic and molecular physics; benzoic acid derivatives; charge transfer; energy transfer; Exact sciences and technology; Fluorescence and phosphorescence spectra; Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion); lanthanides; luminescence; Molecular properties and interactions with photons; Physics
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200500291

The photophysical properties of 4‐naphthalen‐1‐yl‐benzoic acid ligands and their EuIII‐cored complexes were systematically investigated to elucidate the effective energy‐transfer pathway in luminescent lanthanide complexes. A series of 4‐naphthalen‐1‐yl‐benzoic acid ligands, such as 4‐naphthalen‐1‐yl‐benzoic acid (NA‐1), 4‐[4‐(4‐methoxyphenyl)‐naphthalen‐1‐yl]‐benzoic acid (NA‐2), and 4‐{4‐[4‐(4‐methoxyphenyl)‐naphthalen‐1‐yl]‐benzyloxy}‐benzoic acid (NA‐3), were synthesized and utilized for the synthesis of their EuIII‐cored complexes, corresponding to NAC‐1, NAC‐2, and NAC‐3. The fluorescence spectra of NA‐1 and NA‐2 show large Stokes shifts with increasing solvent polarity. These large Stokes shifts might be dominantly due to the formation of an intramolecular charge transfer (ICT) complex in the excited state. Also, the intensive luminescence of the EuIII ions by the photoexcitation of the ligand in NAC‐1 and NAC‐2 in polar solvents supports that the energy transfer from the ligand to the EuIII ion takes place efficiently. In the case of NA‐3, which has a CH2OPh group that acts as a blocking group, there is no dependence of the fluorescence spectrum on the solvent nature and no luminescence of the EuIII ions by the photoexcitation of the ligand, indicating no formation of the ICT state. This can be due to the fact that the formation of the ICT state in NA‐3 was prevented because the OCH2 group acts as a blocking group by interrupting the π‐conjugation between the benzoic acid and the naphthalene unit. From these photophysical studies, we suggest that the ICT state plays a very important role in the energy‐transfer pathway from the ligand to the EuIII ion. To our best knowledge, this is the first demonstration of sensitized emission of luminescent lanthanide complexes based on 4‐naphthalen‐1‐yl‐benzoic acid derivatives by the charge‐transfer process. Charge transfer: An intensive luminescence of EuIII ions by photoexcitation of the luminescent ligand in EuIII‐chelated complexes is observed only in polar solvents whereas no emission is observed in nonpolar solvents. It has been demonstrated for the first time that the energy transfer from the ligand to the lanthanide ion takes place through the charge‐transfer state of the ligands in the EuIII‐chelated complexes (see scheme).