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fluorescence spectral experiment was tested to analyze the compounds. The superior catalytic performances could be
interactions between PDI and different additives. Figure S4 attributed to efficient irradiation of tDhOeI: 1r0e.a10c3ti9o/nD0CmCi0x1t1u2r7eD,
showed that the peak intensity and wavelength of PDI decreased mass transfer limitation as well as the strong
photocatalyt didn’t display the obvious changes after adding photoexcited reducing PDI•- anion. This novel method implies
DIPEA, HCOOH and the model molecule, confirming that the the potential of metal-free organophotocatalyst in the effective
absorption of visible light was mainly from PDI molecules. transformation of renewable resources since it is cheap,
Moreover, in the fluorescence spectra, neither HCO2H nor the commercial availability and low toxicity.
lignin model was shown to significantly reduce the
This work is supported by NSFC (21677098), Shanghai
photoluminescence intensity of PDI, even with a large amount
government (19SG42, 19520710700 and 18230742500) and the
of the reactant. However, DIPEA can gradually decrease the
Program for Professor of Special Appointment (Eastern Scholar)
photoluminescence intensity of PDI with the increasing addition
at Shanghai Institutions of Higher Learning (TP2016034).
amount, indicating that the excited state of PDI (PDI*) was
quenched by DIPEA. Since it was not quenched by the lignin
model, a reductive quenching mechanism was probably
occurred in which the excited state PDI* is generated via visible
Conflicts of interest
There are no conflicts to declare.
light absorption and then is quenched by DIPEA to produce the
catalytic active species PDI•− anion. Next, we used the isotope
experiment by using duterium formic acid and deuterium
acetonitrile instead of HCOOH additive and CH3CN solvent to
explore the proton source for the activation of the carbonyl
Notes and references
1
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reaction solvent CH3CN didn’t supply the proton in our system
(Figure S6-S7). Thus, the photocatalytic mechanism was
proposed, as shown in Scheme 2. In our PDI/DIPEA/HCOOH
catalytic system, PDI was a photocatalyt, DIPEA provided the
electron and HCOOH was a proton source. After visible light
illumination, the generated photoexcited *PDI oxidized DIPEA
to create the strongly reducing PDI•− anion (E1/2red = -2.14 V).
single-electron transfer (SET) from the PDI•− anion to the
lignin/formic acid complex produced the intermediate product
and meanwhile this process regenerated the ground state PDI.
The intermediate product underwent the selective C-O bond
cleavage to give the desired phenol fragmentation product
through protonation reaction by HCOOH and the other ketone
radical product. Meanwhile, the radical product went through a
hydrogen atom transfer process with the oxidized DIPEA to
afford the ketone product.[19]
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Scheme 2. Plausible mechanism of PDI photocatalytic
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In summary, we have demonstrated the first use of organic
molecule perylene diimide as a catalyst for visible-light-induced
selective depolymerization of oxidized lignin model by breaking
β-O-4 ketone bonds. Using a home-made continuous-flow
reactor can further shorten reaction time and decrease the
amount of catalyst and stoichiometric additives. Importantly, it
also showed the excellent capability for the depolymerization of
real lignin to the low mass weight monoaromatic or diaromatic
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4 | J. Name., 2012, 00, 1-3
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