The spectra of absorption and fluorescence of 3 and 4 are
shown and outlined in Fig. 2 and Table 1, and the excitation
wavelength (ex) is 700 nm. Due to the heavy atom effect, both
compounds 3Br (Φf < 0.01) and 4Br (Φf = 0) are very weakly
fluorescent, in comparison with 3H (Φf = 0.34) and 4H (Φf =
0.29). Dye 3H absorbs and emits at 704 and 733 nm, respectively.
Changing the phenyl to naphthyl group at 3,5-positions in 3H, aza-
BODIPY 4H shows long-wavelength absorption and emission
waves (abs/em = 718/754 nm). The absorption maximum of 4Br
is blue-shifted to 698 nm comparing to that of the corresponding
parent aza-BODIPY 4H (abs = 718 nm). NIR-absorbing Dyes 3
and 4 have high extinction coefficients (ε = 88000-95000 L mol1
cm1). Attaching of the naphthyl group at 3,5-positions provides a
broader full width at half maximum for 4Br (FWHM = 76 nm)
compared with 3Br (FWHM = 64 nm).
Fig. 3. Frontier molecular orbitals of aza-BODIPYs 1, 2 and 4 at the
B3LYP/6-31G(d) level with Gaussian 09. For 1, and 4,
HOMO/LUMO (eV) = 5.33/3.27, 4.99/3.02, and 4.99/3.04,
respectively.
2
In comparison with the maximum absorption of aza-BODIPY
2H bearing the phenyl groups at 1,3-positions (abs = 706 nm) [31],
a substituent change from phenyl to naphthyl group in dye 4H
(abs718 nm) leads to be 12 nm of bathochromic shift. However,
only attaching naphthyl groups at 1,7-positions or 3,5-positions in
1H, their photophysical properties 2H and 3H are nearly same,
except the FWHM. Additionally, the molecular geometries of aza-
BODIPYs 1H, 2H and 4H were optimized by using density
functional theory (DFT) at the B3LYP/6-31G(d) level of theory
[38]. The calculated HOMO and LUMO orbital energy levels are
summarized in Fig. 3. The HOMO and LUMO were mainly
localized in the aza-BODIPY core. Changing the substituent from
phenyl to naphthyl group resulted in a remarkable bathochromic
shift (abs = 688 nm for 1H, abs =706 nm for 2H, abs =718 nm for
4H), owing to a narrowing of the HOMO–LUMO band gap for the
lowest-energy absorption bands in dye 4H (1.95 eV) relative to
those of dye 1H (2.06 eV) or dye 2H (1.97 eV), according to MO
calculations (Fig. 3).
The investigation of the singlet oxygen generation was
performed to assess the ability of naphthyl-containing aza-
BODIPYs 3Br and 4Br as a PS in toluene. To simulate a deep
tissue penetration and to reduce the normal cell-damage, the
monochromatic light [39-41] at 690 nm by using a 150 W xenon
lamp at 0.5 mW/cm2 was selected to irradiate the toluene solution.
Singlet oxygen generation was estimated by 1,3-
diphenylisobenzofuran (DPBF), a singlet oxygen indicator [42].
The experiments were performed at initial concentrations of 6.5 ×
10-6 mol/L of 3 and 4, and 6 × 10-5 mol/L of DPBF. When the
oxidation of DPBF with the reactive singlet oxygen happened, the
absorption maximum of DPBF at 416 nm was found to be
decreased. The results demonstrate that the singlet oxygen
generation of 4Br with the dibromo groups at 2,6-positions (Figs.
4 and 5) was more effective than that of 4H. A 14.5-fold rate
enhancement is observed for 4Br compared to 4H (Fig. 5 and Fig.
S1 in Supporting inforamtion). And, the singlet oxygen generation
of 4Br (4.2-fold rate) is more effective than that of 3Br (Fig. 5 and
Fig. S1). Moreover, one notices that aza-BODIPY 4Br with the
naphthyl groups at 1,7-positions is a stronger PS than 2Br with the
phenyl groups at 1,7-positions, having a 2.2-fold rate enhancement
for generating the singlet oxygen (Fig. S2 in Supporting
information) [31]. Additionally, no photobleaching of 4Br was
found during this experiment, based on the absorption intensity
(abs = 705 nm in toluene) (Fig. 4). These results indicated that the
NIR PS 4Br was able to be used for the generation of the singlet
oxygen.
Fig. 2. Absorption and fluorescence spectra of 3 and 4 in CH2Cl2 at
298 K. ex = 700 nm for 3 and 4.
Table 1
Photophysical properties of aza-BODIPYs 3 and 4 in CH2Cl2 at 298
K.
(L mol-1
cm-1)
FWHM
(nm)
max for absorption
and emission (nm)
Dye
Φf
1H[30]
1Br[30]
2H[31]
2Br[31]
3H
3Br
4H
4Br
688/715
679/714
706/733
695/728
704/733
692/734
718/754
698/NA
55
57
67
69
58
64
79
76
85000
75000
89000
82000
90000
88000
95000
92000
0.36
0.1
0.33
0.12
0.34
<0.01
0.29
0
Fig. 4. DPBF (initial concentration at 6 × 10-5 mol/L) degradation
profile in toluene by aza-BODIPY 4Br (6.5 × 10-6 mol/L) under the
light. Monochromatic light (690 nm at 0.5 mW/cm2) was used. The
figure displays time-dependent decrease (0, 1, 3, 5, 10, 15, 20, 25, 30,
40, 50 and 60 min) of the absorbance at 416 nm by oxidation of DPBF
with 4Br.