3.4. UV–Vis absorption spectra
UV–Vis absorption spectra of HL1 and the Cu(II) complex were
determined in 50 lM CH2Cl2 solution. Absorptions of the Cu(II)complex were significantly different from those of HL1 as shown in
Fig. 4. Compared with the Cu(II) complex, an important feature of
the absorption spectrum of HL1 was that three absorptions were
observed at 240, 305 and 363 nm (e = 20,020, 8212, 3920 mol1 -
L cm1) respectively. The latter absorption peaks were all shifted
in the spectrum of the Cu(II) complex. The absorption was at
240 nm of HL1 shifts to 267 nm (e = 19,960 mol1 L cm1) in the
Cu(II) complex, which can be assigned to p–p⁄ transition of phenyl
rings [38]. A new conspicuous absorption peak at 425 nm
(e = 5418 mol1 L cm1) and weak broad absorption at 475 nm
(e = 1460 mol1 L cm1) were observed in the Cu(II) complex,
which could be assigned to 2B1g?2A1g and 2B1g?2B2g transitions
of distorted square pyramidal geometry of the Cu(II) complex [39].
These observations were in line with common spectral features of
d-block metal complexes. The absorption edge (kedge) of HL1 was at
412 nm, corresponding to an optical band gap (Eg
opt) of 3.010 eV.
The absorption edge (kedge) of the Cu(II) complex was determined
to be 526 nm yielding an optical band gap (Eg
opt) of 2.357 eV. With
the increase of conjugated system, the band gap became narrower
from the ligand HL1 to the Cu(II) complex.