Y. Li et al.
Dyes and Pigments 175 (2020) 108177
Fig. 3. Absorption spectra (a), and fluorescence spectra (b) of V3 (10
μ
mol/L) with the addition of increasing concentrations of Cu2þ (0–12
μmol/L); Absorption
spectra (c) and fluorescence spectra (d) of V3 (10
μ
mol/L) with 400 nm light irradiation, on addition Cu2þ (10
μmol/L) in acetonitrile (Ex ¼ 400 nm).
XM500W power system by using suitable band-pass filter.
molecules photoisomerization lead to absorption spectra shift [21]. The
blue shift of ultraviolet absorption spectra is a typical feature of the
transition from trans-structure to cis-structure [22]. Therefore, pre-
liminary speculation is that only V3 can respond to 400 nm light and
isomerize. The retinal protonated Schiff bases with aromatic Schiff bases
showing no isomerization of the retinal skeleton has been reported [17].
Based on above conclusion, immine isomerization was taken as a
concern point. The NMR technique usually could provide required in-
formation, but the chemical shift is not significant enough in V3
(Fig. S1). This is attributed to the fact that there is only one proton at
both ends of imine C¼N. And it is difficult to characterize the isomeric
change with the change of the coupling constant [22].
3. Results and discussion
The synthetic route for retinal derivatives V1 (retinal-malononitrile),
V2 (retinal-diaminomaleonitrile), V3 (retinal-phenylenediamine) is
outlined in Scheme 1. The commercially available retinol acetate 1 was
hydrolyzed to produce retinol 2 according to the reported procedure
[18]. Next, retinal 3 was prepared through oxidation of retinol 2 using
MnO2 as oxidant. The preparation of V1 followed the Knoevenagel
condensation reaction of retinal and malononitrile [19]. V2, V3 were
prepared though the condensation reaction of retinal 3 and dia-
minomaleonitrile, o-phenylenediamine, forming Schiff base [20]. The
detail synthesis procedure andcharacterization information was pro-
vided in supporting information.
In order to solve above problem, the unique metal ion coordination
strategy was adopted to determine the photoisomerization reaction and
site. The diaminomaleonitrile and o-phenylenediamine in V2 and V3 are
classical coordination groups of copper ion, which were used as recog-
nition groups in copper ion chemical probe [20,23]. As shown in
Fig. 3a-3b, as the quantitative Cu2þ was added to V3 solution, the ab-
sorption peak regularly blue shift to short wavelength area; The fluo-
rescence emission peak gradually increased. The isosbestic point at 400
nm indicated the existence of an equilibrium between V3 and V3–Cu2þ
complex. To further explore the binding mechanism in detail, the
infrared spectrum of V3 and V3–Cu2þ were overlaid and compared
(Fig. S2). Firstly, two N–H stretching vibration peaks of the N–H2 group
at 3369 cmÀ 1 and 3474 cmÀ 1 appeared in the free V3 molecule. Sec-
The electrostatic interaction between retinal and opsin dominates
the charge distribution of retinal molecules in physiological environ-
ment. In absence of protein environment, different charge donor and
receptor groups would dominate charge distribution of retinal mole-
cules. As shown in Fig. 2a, the maximum absorption peaks of V1, V2, V3
in acetonitrile shift to short wavelength in order, corresponding to their
different electronic environment. From strong electron-withdrawing
group malononitrile, to medium electron-electron-donor group dia-
minomaleonitrile, and strong electron-donor group o-phenylenedi-
amine. The substituents vary led to retinal electron density shifting,
polyene chain molecular orbitals energy transition. Therefore, the ab-
sorption spectra of V1, V2, V3 showed regular changes. Among three
molecules, three charge distribution states, which one is more suitable
for photoisomerization?
ondly, after the N–H2 group on phenylene diamino binded with Cu2þ
,
the two stretching vibration peaks of N–H2 group disappeared. Instead,
the single stretching vibration of the N–H at 3428 cmÀ 1 appeared in the
coordination complex V3–Cu2þ
.
The preliminary photochemical reaction experiment of V1, V2 and
V3 was carried out in acetonitrile under �400 nm light irradiation. The
absorption spectra were recorded before and after irradiation as shown
in Fig. 2a, b, c. Upon irradiation with 400 nm light, the characteristic
absorption peaks, V1 and V2 decreased, V3 blue shifted. The chemical
shifts of V1 and V2 didn’t show regular changes, but new fragment
peaks appeared in NMR spectrum (Fig. S5). Generally, photo-bleaching
causes degradation of dye molecules leading to reduced absorption, dye
The same experiments were carried out after 400 nm light irradia-
tion. As shown in Fig. 3c, d, the absorption peak of V3 exhibited minimal
shift when Cu2þ was added. Similarly, the fluorescence emission peak of
V3 also didn’t enhance, indicating that the photoisomerization reaction
of V3 occurred in the coordination unit. The isomerized coordination
group can no longer coordinate with copper ions. Another convincing
proof is that the V2 still could respond to Cu2þ after 400 nm light
irradiation. As shown in Fig. S3, The absorption and fluorescence spectra
3
Li, Yang
