S. Biju et al. / Inorganic Chemistry Communications 10 (2007) 393–396
395
1
The excitation spectra of the complexes 1 and 2 moni-
5
7
5
tored around the peak of the intense D4 ! F5 transition
of the Tb3+ ion, exhibits a broad band between 250 and
450 nm (kmax = 370 nm) which can be assigned to p–p*
electron transition of the ligands [8]. A small peak at
490 nm observed as a result of f–f absorption transition
D
4
5
(7F6 ! D4) of Tb3+ ion. This transition is much weaker
than the absorption of organic ligands, which proves that
luminescence sensitization via excitation of the ligand, is
more effective than direct excitation of the Tb3+ ion. The
room-temperature normalized emission spectra of Tb3+
complexes 1 and 2 (Fig. 2) shows characteristic emission
bands of Tb3+ (kex = 370 nm) centered at 490, 545, 585
and 620 nm, resulting from the deactivation of the 5D4
excited state to the corresponding ground state 7FJ
(J = 6, 5, 4, 3) of the Tb3+ ion. The strongest emission is
centered on 545 nm, which corresponds to the hypersensi-
tive transition of D4 ! F5 [13,14]. The broad emission
peaks obtained may be due to the greater non-homogeneity
for Tb3+ local coordination site due to the presence of
water molecules.
The overall quantum yields (Uoverall) of terbium com-
plexes (1 and 2) were measured at room-temperature using
the technique for powdered samples described by Bril and
De Jager-Veenis [15]. The overall quantum yields of com-
plexes 1 and 2 were calculated as 11% and 22%, respec-
tively, and were found to be promising as compared to
the recently reported Tb(PMIP)3(H2O)2 (where PMIP
2
1
0.5
1.0
1.5
Time (ms)
Fig. 3. Decay profile of the complexes 1 and 2 monitored at 545 nm and
excited at 370 nm.
5
7
decays of complexes could be described by mono-exponen-
tial kinetics, which suggests that only one species exists in
the excited state of these complexes. In combination with
the data for the overall quantum yields, the data for the
luminescence lifetimes show that longer the luminescent
lifetimes, higher the quantum yields of the complexes.
The singlet and triplet energy levels of HPBI and HTPI
were estimated by referring to the wavelengths of UV–Vis
absorbance edges (HPBI and HTPI are 365 and 360 nm)
and the lower wavelength emission edges (HPBI and HTPI
are 450 and 442 nm) of the corresponding phosphorescence
spectra of the complexes Gd(PBI)3(H2O)2 and Gd(TPI)3-
(H2O)2 (Supplementary data). The triplet energy level of
HTPI (22620 cmꢀ1) was found to be moderately higher
than HPBI (22220 cmꢀ1), may be due to the presence of
electron-donating group (–CH3) on the benzoyl moiety of
the HPBI system. Generally, the sensitization pathway in
luminescent terbium complexes consists of the excitation
of the ligands from the ground state to their excited singlet
states, and subsequently through the intersystem crossing
of the ligands to their triplet states, following the energy
transfer from the triplet state of the ligand to the central
ion. In this process, the 4f electrons of the Tb3+ ion are
= 1-phenyl-3-methyl-4-isobutaryl-5-pyrazolone; Uoverall
=
29.7 · 10ꢀ3%) [16] and terbium-1-phenyl-3-[G-3]-4-pheny-
lacetyl-5-pyrazolonates (G stands for poly aryl ether; Uoverall
= 2.26%) [17]. Among complexes 1 and 2, the later shows
better quantum yields due to the presence of electron-
releasing group (–CH3) on the benzoyl moiety.
The luminescence lifetimes (s) were also investigated for
terbium complexes and found to be 400 and 472 ls, respec-
tively for 1 and 2 (Fig. 3). The measured luminescent
2.0x107
7F5
5D4
2
5
excited to the DJ manifold from the ground state, finally
1.5x107
the Tb3+ion emits when the 4f electrons undergo a transi-
7F6
5
tion from the excited state of D4 to the ground state. It
has been noticed that the energy gaps DE(1pp*–3pp*)
1.0x107
between the pp* and pp* levels are 5180 and 5160 cmꢀ1
for HPBI and HTPI, respectively. According to Rein-
houdt’s empirical rule [18] that the intersystem crossing
process will be effective when DE(1pp*–3pp*) is at least
5000 cmꢀ1, thus the intersystem crossing processes are
effective for all the ligands. According to Latva’s empirical
rule [19], an optimal ligand-to-metal energy transfer pro-
cess for Tb3+ is when DE(3pp*–5D4) > 2000 cmꢀ1. It can
be concluded that the transfer process is effective from
HTPI to Tb3+ and that HTPI is a suitable as a sensitizer
for Tb3+ (DE(3pp*–5D4) > 2220 cmꢀ1). On the other hand,
1
3
1
5.0x106
7F4
7F3
0.0
450
500
550
600
650
Wavelength (nm)
Fig. 2. Room-temperature emission spectra of the complexes 1 and 2
excited at 370 nm.
Biju
