J.-F. Zheng et al.
Dyes and Pigments 185 (2021) 108946
heptaazaphenalene (Heptazine 4): Cyameluric chloride (1.10 g, 3.98
mmol) was added slowly to a suspension of 25 mL of dichloromethane,
4-Methylbiphenyl (2.66 g, 15.8 mmol) and anhydrous AlCl3 (2.1 g, 15.8
mmol) with stirring at 40 ◦C in a flame-dried 50 mL round bottom flask.
After 12 h, the flask was removed from heat, and the solvent was
removed with a rotary evaporator. Then, 30 mL H2O was added, the
yellow solid was filtered and washed with water. The product was pu-
rified by column chromatography (eluent: dichloromethane/hexane =
2:1) and removal of the solvent with a rotary evaporator, and yellow
product was obtained (0.78 g, yield: 29%). 1H NMR (400 MHz, CDCl3):
δ = 8.65 (d, J = 8.5, 6H), 7.75 (d, J = 7.8, 6H), 7.60 (d, J = 8.3, 6H), 7.29
1–4 have been obtained by the incorporation of four different electron-
rich aryl substituents at three peripheral positions of the heptazine cores
through Friedel-Crafts alkylation reactions. The chemical structures of
heptazines 1–4 have been confirmed by solution 1H, 13C NMR, high-
resolution mass (HRMS), fourier transform infrared (FTIR) spectros-
copy and single-crystal X-ray diffraction techniques. Unfortunately, due
to the low solubility, it is rather hard to obtain the satisfied 13C NMR
spectra for heptazines 1, 3 and 4. For the same reason, only the single
crystals of 2 and 4 suitable for X-ray crystallographic analyses were
obtained by the slow diffusion from their chloroform solutions. The X-
ray single-crystal diffraction analyses (Fig. 1) revealed that both single
crystals belong to the triclinic space group P1. In the both structures,
both heptazine cores are planar with the peripheral C–N bond lengths
varied between ~1.32 Å and ~1.34 Å, and the C–N distances involving
the central nitrogen atoms are ~1.40 Å. The dihedral angles between
heptazine cores and adjacent benzene rings are observed as ~5.4◦,
~18.7◦, ~3.0◦ for heptazine 2 and ~8.9◦, ~29.3◦, ~3.7◦ for heptazine
4, respectively, and the dihedral angles between the benzene ring and
next benzene ring for heptazine 4 were ~14.1◦, ~31.3◦, ~31.6◦. Owing
to one more benzene ring in the substituents, heptazine 4 possesses a
more twisted “propeller” structure, which is in contrast to that of hep-
tazine 2. Notably, there is a remarkable hydrogen-bonding interaction
between N atoms of heptazine cores and H atom of chloroform solvents
in both structures of the heptazine 2 and 4 (~2.52 Å and 2.43 Å,
respectively), which may play an essential role in their subsequent
photocatalytic hydrogen evolutions [20,21]. In addition, both crystals
(d,
J = 7.4, 6H), 2.42 (s, 9H). ESI-MS m/z calculated for
[C45H33N7+H+]+: 672.2870, [M]+ found: 672.2729.
2.3. X-ray crystallography studies
Suitable single crystals of heptazine 2 and 4 were mounted on glass
fiber for the X-ray measurement. The single crystals of heptazine 2 and 4
were obtained by dissolving each of them in a chloroform solvent, fol-
lowed by slow evaporation of the solvents within several days. Unfor-
tunately, we did not get the single crystals of heptazine 1 and 3, since
they were much easier to precipitate out from the solvents. Diffraction
data were collected on a Rigaku-AFC7 equipped with a Rigaku Saturn
CCD area-detector system. The measurement was made by using graphic
monochromatic Mo Kα radiation (λ = 0.71073 Å) under a cold nitrogen
stream. The frame data were integrated and absorption correction using
a Rigaku Crystal Clear program package. All calculations were per-
formed with the SHELXTL-97 program package [29], and structures
were solved by direct methods and refined by full-matrix least-squares
against F2. All non-hydrogen atoms were refined anisotropically, and
hydrogen atoms of the organic ligands were generated theoretically onto
the specific atoms. Detailed single crystal data for heptazines 2 and 4
could be found in the Supporting Materials.
adopt a two-dimensional (2D) “brickwork” arrangement with strong π-π
stacking interactions to next layer (~3.34 Å and ~3.36 Å for heptazine
2, ~3.37 Å and ~3.41 Å for heptazine 4, respectively), which not only
makes them low solubility in water environments and thus they acts as
the heterogenous catalysts in water, but also have significant influences
on their following photophysical and photochemical properties.
3.2. Photophysical properties in the solutions
2.4. Density functional theory (DFT) calculations and time-dependent
density functional theory (TD-DFT) calculations
The structural properties revealed for the substituted heptazines 1–4
are also reflected in their UV/Vis absorption and fluorescence properties
in dichloromethane (DCM). As illustrated in Fig. 2a, all four heptazines
All DFT calculations were performed using the hybrid B3LYP func-
tional, as implemented in Gaussian 09 software package [30]. A
6-311G+ (d, P) basis set was used. For geometry optimizations, all
minima were verified via a calculation of vibrational frequencies,
ensuring that no imaginary frequencies were present. TD-DFT calcula-
display high molar extinction coefficients (
ε
~ 8.4–11.5 × 104 Mꢀ 1
cmꢀ 1, the molar extinction coefficients and band maxima are given in
Table S1). And the main absorption peaks for heptazines 1–4 (at 320,
340, 370, 380 nm) could be assigned to the π-π* transition of their
tions were carried out by using M062×/6-31
+
G(d)/PCM
π
-conjugated system, and the slightly differences is attributed to the
(Dichloromethane).
electronic effects of the side groups, which is indeed supported by the
TD-DFT calculations (see Tables S5–S10, Figs. S17–20). Compared with
the absorption spectrum of phenyl substituted heptazine 1, those of
heptazines 2–4 with larger conjugated substituents are bathochromi-
cally shifted from 326 nm to 384 nm. In comparison, the low-intensity
2.5. Photocatalytic hydrogen evolution measurements
Photocatalytic hydrogen production experiments and the determi-
nation of apparent quantum yields (AQYs) were carried out as the pre-
viously reported method [31]. Generally, first, 10 mg heptazine-based
small-molecule photocatalyst was fully dispersed in a mixed solvent of
water and TEOA (110 mL, 10:1 in vol%). Afterwards, an aqueous solu-
tion of chloroplatinic acid (H2PtCl6) was added as the precursor for the
in-situ photodeposition of Pt cocatalyst (3 wt%). The reaction was per-
formed in a Pyrex top-irradiation reaction vessel maintained at 12 ◦C by
a flow of cooling water, and was irradiated with a 300 W Xe lamp
equipped with a 420-nm cut-off filter after being completely degassed.
An online gas chromatography system (GC-8A, Shimadzu) was applied
to monitor the amount of hydrogen produced was for each sample (10
mg) within 4 h, and argon was used as the carrier gas.
absorption band with a maximum at around 450 nm (
ε
~500 Mꢀ 1
cmꢀ 1) is attributed to the n-
π* transition involving the lone pairs of N
3. Results and discussions
Fig. 1. The single-crystal structure of heptazine 2 (a) and heptazine 4 (b),
including the CHCl3 solvent molecule, respectively (C atoms, grey; N atoms,
blue; Cl atoms, green; H atoms, white). (For interpretation of the references to
colour in this figure legend, the reader is referred to the Web version of
this article.)
3.1. Syntheses and characterizations
As shown in Scheme 1 and the experimental section, the heptazines
3
Zheng, Jun-Feng
