5
04
H.L. Ma et al. / Spectrochimica Acta Part A 74 (2009) 502–508
Table 2
The maximum absorption positions of TPPS in ILs and aqueous solution.
Medium
Species of TPPS
Soret bands (nm)
Q bands (nm)
Ref.
−
H2O
H2O
H2O
H2TPPS4
413
434
434, 490
420
448
515, 550, 589, 645
645
702
515, 550, 589, 645
661
702
[37,38,39]
[37,38,39]
[37,38,39]
Here
Here
Here
H4TPPS2 monomer
−
H4TPPS2 J-aggregate
−
−
[MBIM]BF4
[HBIM]TS
[HBIM]BF4
H2TPPS4
H4TPPS2 monomer
−
H4TPPS2 J-aggregate
−
437, 483
[
37–39]. While in aprotonic IL [MBIM]BF4 medium, the spectral
Q bands into one new band at 665 nm. No TPPS aggregation band
appeared in the presence of [HBIM]MS, even the concentration of
[HBIM]MS was increased to higher level. Moreover, an ill-isobestic
point at ca. 430 nm revealed that the equilibrium between TPPS
property of TPPS was closely similar to that in neutral aqueous
solution except that Soret band changes from 423 nm to 420 nm.
However, in the other two protonic ILs medium, i.e., [HBIM]BF and
4
4
− 2−
free base (H TPPS ) and the protonated TPPS (H TPPS ) existed
2 4
[
HBIM]TS, absorption of TPPS show obvious change. TPPS has D4h
symmetry in [HBIM]TS and the Soret band red-shifted from 420 nm
to 448 nm, while Q bands was intense at 661 nm, very weak at
under the experimental environment. In other words, the proton
2−
transfer from [HBIM]MS to TPPS (H TPPS ) occurred efficiently in
4
5
55 nm and 605 nm. It was suggested that TPPS existed in the proto-
AILs.
2−
nated species (H TPPS ) in [HBIM]TS. Whereas in the [HBIM]BF4,
Just as expected, the absorption features of TPPS changed in
4
besides red-shifted band at 437 nm, a new band at 483 nm occurred
in parallel with a red-shifted Q band at 702 nm, indicating that TPPS
J-aggregates were formed in [HBIM]BF4 medium. Although both
very similar trend upon the titration of [HBIM]BF , [HBIM]TS
4
and [HBIM]DS into [MBIM]BF4 (the absorption spectra were not
showed here). A well-isobestic point at ca. 430 nm was observed
upon the addition of three PILs aforementioned, respectively, also
[
HBIM]BF and [HBIM]TS are usually strong acid, TPPS J-aggregates
4
4
−
showing that the equilibrium between the free H TPPS and the
2
can be induced in [HBIM]BF4 medium, rather than in [HBIM]TS
medium.
2−
protonated H TPPS species was established. The absorbance of
4
In addition, it was noteworthy that the splitting extent of char-
acteristic absorption spectra of TPPS J-aggregates relative to their
Soret band were smaller in single [HBIM]BF4 compared to aqueous
solution, that is, ꢀꢁ = 46 nm (from 437 nm to 483 nm) in [HBIM]BF4
and ꢀꢁ = 56 nm (from 434 nm to 490 nm) in aqueous solution
TPPS at 446 nm experienced a substantial increase to a platform
with increase in PILs concentration to a certain degree, namely,
0.30 mol/L for [HBIM]BF , 0.60 mol/L for [HBIM]TS and [HBIM]DS,
4
0.80 mol/L for [HBIM]MS. More importantly, the change degree of
the absorbance of protonated TPPS was slightly dependent on the
properties of PILs under same concentration. Based on these obser-
vations, the ability of PILs transfer proton to TPPS seemly obey the
following order: [HBIM]BF4 > [HBIM]DS > [HBIM]TS > [HBIM]MS.
[
37–39].
3.2. The effects of PILs on the absorption spectra of TPPS in AILs
PILs can act as proton-donors besides as solvents. This is
3.3. The effects of PILs on the fluorescence spectra of TPPS in AILs
important in acid–base catalytic reaction. Therefore, next atten-
tion was focused on the effect of PILs on the protonation of TPPS in
In general, the fluorescence spectra of porphyrin free base and
its derivatives exhibited a dual emission band in aqueous solution
which appeared at ca. 645 nm and ca. 702 nm, respectively. The two
emission bands merged to one centered at near 665 nm upon the
[
MBIM]BF medium. Fig. 2 displayed typically the absorption spec-
4
tral changes of TPPS upon the increase of [HBIM]MS in [MBIM] BF4
medium.
Upon the addition of [HBIM]MS, the Soret band of TPPS produced
the expected red-shift to 446 nm, going with the collapse of the four
2−
formation of the protonated H4TPPS species.
Fig. 3 illustrated that the fluorescent properties of H2TPPS4
−
were sensitive to the presence of the four PILs in [HBIM]BF4
medium. It can be seen that TPPS free base also exhibited the
distinct dual emission patterns located at 649 nm and 710 nm in
[
MBIM]BF . When PILs was progressively added into the TPPS-
4
[
MBIM]BF4 system, initially, the fluorescence intensity of TPPS
decreased at both 649 nm and 710 nm. Whereas further addition
of PILs, apparently, the broad band at 710 blue-shifted and grad-
ually increased accompanying continuous decrease blue-shift of
5
nm of the band at 648 nm until ultimately disappearance. The
enhancement degree of TPPS emission at longer wavelength was
greatest in the titration of [HBIM]TS and smallest in the titration
of [HBIM]BF , as shown in Fig. 3. Besides, an isoemission point at
4
ca. 670 nm was observed for [HBIM]TS titration. These suggest that
TPPS free base in neutral [MBIM]BF4 has been converted to its pro-
tonated form upon the addition of the four PILs herein, respectively.
Another property was that the fluorescence intensities of TPPS were
much lower than that observed in conventional solvents either in
the absence or presence of PILs in AILs medium.
3
.4. Resonance light scattering spectra
Fig. 2. Absorption spectra of TPPS in [MBIM]BF4 upon the titration of [HBIM]MS. The
concentration of TPPS in [MBIM]BF4 was kept constantly at 4 M. The inset was the
amplified absorption spectrum of TPPS in [MBIM]BF4 upon the titration of [HBIM]MS
in the Q band region of 500–750 nm. The concentration of [HBIM]MS used here were
Fig. 4 displayed typically the RLS spectra of TPPS in the pres-
ence of various concentrations of [HBIM]TS in [MBIM]BF . When
4
0
.0 mol/L, 0.30 mol/L, 0.60 mol/L, 0.90 mol/L, 1.2 mol/L and 1.8 mol/L, respectively.
[HBIM]TS was added into the TPPS/AILs system, initially, the RLS