Unveiling Charge Dynamics in Acetylene-Bridged Donor-π-Acceptor Covalent Triazine Framework for Enhanced Photoredox Catalysis

Xingwang Lan, Xiaopeng Liu, Yize Zhang, Qing Li, Juan Wang, Qianfan Zhang,* and Guoyi Bai*

Research Article

Unveiling Charge Dynamics in Acetylene-Bridged Donor−π−

Acceptor Covalent Triazine Framework for Enhanced Photoredox

Catalysis

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ABSTRACT: Covalent triazine frameworks (CTFs) with donor−

acceptor motifs have been identiﬁed as prospective semiconducting

materials for photocatalysis. Though donor−acceptor motifs can

favor forward intramolecular charge separation, some cases still suﬀer

from backward charge recombination, resulting in the decrease of the

photocatalytic activity. Herein, acetylene-bridged CTFs bearing an

extended donor−π−acceptor motif was fabricated to prompt exciton

dissociation. Experimental investigations and density functional

theory calculations prove that the acetylene moiety can suppress

backward charge recombination, minimize exciton binding energy,

and enhance charge carrier lifetime, thereby prompting forward

charge transfer/separation in comparison to the analogous one

without acetylene. Thus, the acetylene-bridged CTFs showcased a

higher photocatalytic activity for metal-free photocatalytic oxidative

amines coupling with oxygen under visible-light irradiation, and apparent quantum eﬃciency at 420 nm was achieved up to 32.3%,

that is, twofold higher than the one without acetylene. Furthermore, the acetylene moieties can adsorb oxygen molecules and provide

active sites to lower the energy barrier and thus signiﬁcantly enable the photoredox catalysis. This work provides alternative insights

into the design and construction of high-performance CTFs, with prospective applications in solar-to-chemical energy conversion.

KEYWORDS: acetylene, covalent triazine frameworks, donor−π−acceptor, charge transfer/separation, photocatalysis

INTRODUCTION

interests in various ﬁelds because of their ﬂexible structural

■

designability, high chemical stability, and tunable function-

Photoredox catalysis has become deemed as a promising

alternative strategy in facilitating organic reactions to value-

14,15

ality.

Their inherent semiconducting properties make them

particularly attractive candidates for applications in photo-

added products, as it oﬀers the possibility of using solar

catalysis. CTFs, analogous to the chemical structure of g-

C N , can be precisely fabricated using diﬀerent nitrile

energy to enable numerous challenging chemical trans-

−5

formations,

even under mild conditions. Besides being

17,18

thermodynamically allowed for photoredox catalysis, in

precursors even at a low temperature.

By rationally

general, an eﬃcient photocatalyst also needs to possess enough

advantages in kinetic aspects, such as adsorption and

choosing electron-donor building blocks connected to

electron-acceptor triazine units, an intriguing donor−acceptor

motif can be directionally constructed with modiﬁed band

activation for reactants, charge-separation/transfer eﬃciency,

and subsequent charge utilization. Notwithstanding such

guidance, attempting to precisely control these complicated

kinetic behaviors is quite challenging. Tremendous eﬀorts have

been devoted on the pursuit of exploring some regulating

alignment and photoelectric property. Along this line, recent

years have also witnessed their promising potentials in

0−27

photocatalysis, especially for photocatalytic H evolution,

28−33

34−38

CO reduction,

and organic transformations

under

−10

strategies

and developing eﬃcient photocatalysts in recent

visible-light irradiation. Though donor−acceptor dyads enable

decades, especially for heterogeneous photocatalysis. However,

to a certain degree, more or less restrictions, for example,

tedious synthetic procedures, hampered redox ability, as well as

limited charge utilization, are still involved in some cases.

Consequently, constructing eﬃcient heterogeneous photo-

catalysts with additional developments is urgently required.

Received: April 20, 2021

Revised: May 24, 2021

Published: June 8, 2021

11−13

Covalent triazine frameworks (CTFs),

emerging as a

class of porous organic materials, have aroused extensive

2021 American Chemical Society

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ACS Catal. 2021, 11, 7429−7441

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Figure 1. (a) Schematic diagram of the photoexcited electron-transfer process in the D−A system and D−π−A system. (b) Scheme of synthesis of

CTF-1 and A-CTF-2 following acid-catalyzed trimerization.

to favor forward intramolecular charge separation, most

motif can eﬃciently optimize the kinetics in electronic and

adsorptive behaviors for enhanced photoredox catalysis.

examples still suﬀer from backward charge recombination

simultaneously due to their charge attraction as well as weak

optical absorption in the visible-light region. Thus, it is

essential to minimize the exciton binding energy and prompt

charge separation in CTFs to attain a high quantum

EXPERIMENTAL SECTION

■

Synthesis of CTF-1. A 50 mL Schlenk ﬂask was charged

with triﬂuoromethanesulfonic acid (TfOH) (400 μL) in 2 mL

eﬃciency. The unique π-conjugated structure with the

donor−acceptor dyad endows them with a rather enhanced

light-harvesting ability and mitigated charge recombination

because of the larger delocalization and the extending

of CHCl , which was cooled to 0 °C. Compound 1 (109 mg)

in 8 mL of chloroform (CHCl ) was added dropwise into the

acid solution with stirring for 30 min at the temperature of 0

C under an Ar atmosphere. The solution was stirred for

9,41,42

migrating distance of charge carriers (Figure 1a).

another 1 h at 0 °C and then heated to 35 °C for 24 h to

complete the trimerization of the terminal cyano groups. Once

completed, a certain amount of NH ·H O solution was added

Hence, we envisioned whether extending the conjugated

structures or incorporating chromophores into the framework

might modulate the electronic band structure and further

promote charge transfer/separation in the skeleton by

modulating the charge-transfer pathway.

In this work, two CTFs bearing donor−π−acceptor (D−π−

A) motifs without or with acetylene as the bridge were

fabricated by acid-catalyzed trimerization at a low temperature

in the resulting solution until the reaction mixture became

neutral or alkaline, and the mixture was subsequently stirred

for another 2 h. The resultant solid precipitate was ﬁltered;

sequentially washed with excess water, acetone, tetrahydrofur-

an (THF), and dichloromethane (CH Cl ); and dried at 70 °C

under vacuum overnight, ﬁnally aﬀording CTF-1 as a yellow

powder (100 mg, 92%).

(Figure 1b). Their structures consisted of a central electron-

rich tertiary amine as the donor and electron-deﬁcient triazine

as the acceptor, which were linked by an extended π-

conjugation unit regardless of the acetylene (−CC−)

moiety. Outcomes from comprehensive investigations and

density functional theory (DFT) calculations reveal that the

acetylene moiety in CTFs is crucial to modulate the electronic

structure and photoelectric property of CTFs. In essence, the

extended π-conjugation by the acetylene unit can not only

inhibit backward charge recombination and thus energetically

favor the separation and transfer of charge carriers but also

boost the charge carrier migration lifetime from the donor to

acceptor, giving rise to an enhanced photocatalytic activity. As

suggested in these results, acetylene-bridged CTFs exhibited

much enhanced photocatalytic performances for the metal-free

photocatalytic oxidative homocoupling of primary amines

using oxygen under visible-light irradiation, compared to that

of CTFs without the acetylene linker. Last but not least, DFT

calculations suggest that the acetylene moiety can also act as an

adsorption site to facilitate oxygen adsorption and provide

active sites to lower its energy barrier in the visible-light-driven

oxidative coupling process. Consequently, it is believed that

the strategy by integrating an alkyne moiety into the D−π−A

Synthesis of A-CTF-2. A 50 mL Schlenk ﬂask was charged

with TfOH (100 μL) in 10 mL of CHCl , which was cooled to

°C. Compound 2 (125 mg) in 8 mL of CHCl was added

dropwise into the acid solution with stirring for 40 min at the

temperature of 0 °C under an Ar atmosphere. The solution

was stirred for another 1 h at 0 °C and then heated to room

temperature for 24 h to complete the trimerization of the

terminal cyano groups. Once completed, a certain amount of

NH ·H O solution was added in the resulting solution until

the reaction mixture became neutral or alkaline, and the

mixture was subsequently stirred for another 2 h. The resultant

solid precipitate was ﬁltered; sequentially washed with excess

water, acetone, THF, and CH Cl ; and dried at 70 °C under

vacuum overnight, ﬁnally aﬀording A-CTF-2 as a dark yellow

powder (104 mg, 83%).

General Procedure for the Photocatalytic Oxidative

Coupling of Amines. The photocatalytic oxidative coupling

of amines was conducted in a self-designed stainless steel

reactor with a water bath for controlling the temperature, and a

300 W xenon lamp (15 A, PLS-SXE 300) with a 400 nm cut

ﬁlter was used as a light source that illuminated the reaction

mixture from the top of the reactor (the distance was

approximate 8 cm). In a typical procedure, a photocatalyst

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ACS Catal. 2021, 11, 7429−7441

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Figure 2. (a) ¹³C CP/MAS NMR spectra of CTF-1 and A-CTF-2. (b) N adsorption−desorption isotherms of CTF-1 and A-CTF-2. SEM images

of (c) CTF-1 and (d) A-CTF-2. TEM images of (e) CTF-1 and (f) A-CTF-2.

(

8 mg) and 0.2 mmol amines were ﬁrst added into 2 mL of

nitrogen adsorption−desorption at 77 K on an Autosorb-iQ-

MP analyzer . Surface area was obtained by the Brunauer−

Emmett−Teller (BET) method, and pore size distribution was

acetonitrile in a quartz glass vial, which was ultrasonically

dispersed for 5 min. Three parallel vials were sealed in the

reactor placed on a magnetic stirring apparatus, and ﬁve

successive vacuum/oxygen cycles were conducted to remove

derived from the N desorption branch using nonlocal DFT

(NLDFT). Scanning electron microscopy (SEM) was recorded

using a Hitachi SU8010 instrument. Transmission electron

microscopy (TEM) was conducted using a JEOL JEM-2100F

microscope. Photoluminescence (PL) spectra of the samples in

solvents and solid powders were recorded at room temperature

on a steady-state spectroﬂuorometer Edinburgh FS5. The

ultraviolet−visible diﬀuse reﬂectance spectroscopy (UV/vis

DRS) was performed on a Shimadzu UV-3600 spectropho-

air and charged with O (99.99%) to 0.1 MPa. Prior to light

illumination, the mixture was magnetically stirred for 30 min in

darkness to reach an adsorption−desorption equilibrium and

then irradiated by the 300 W Xe lamp for 4 h. For product

analysis, the conversion and selectivity were quantiﬁed by an

Agilent 7820A gas chromatography (GC) system with ﬂame

ionization detection using the area normalization method. All

the products were identiﬁed by an Agilent 5975C gas

chromatography−mass spectrometry (GC−MS) system.

Characterization. Fourier transform infrared spectra (FT-

IR) in the 4000−400 cm regions were measured on a Nicolet

iS10 spectrometer in transmission mode at room temperature.

The solid-state ¹³C cross-polarization magic-angle spinning

nuclear magnetic resonance (CP/MAS NMR) spectroscopy

was carried out on a Bruker AVANCE III model 600 MHz

NMR spectrometer. X-ray photoelectron spectroscopy (XPS)

spectra were recorded on a Thermo ESCALAB 250xi

spectrometer. Powder X-ray diﬀraction (PXRD) patterns

were obtained using a Bruker D8 AVANCE, with the Cu Kα

radiation source from 2 to 40° with a scanning rate of 1°

tometer tested from 200 to 800 nm using BaSO as a reference.

The electron spin resonance (ESR) measurements were

analyzed by a Bruker A300 spectrometer at room temperature,

−

with light irradiation using a 300 W Xe lamp. O temperature-

programmed desorption (TPD) with the TCD tests was

conducted on an AutoChem II 2920 instrument from the

temperature range of 50 to 500 °C.

Electrochemical and Photoelectrochemical Measure-

ments. All the electrochemical and photoelectrochemical

measurements were carried out on an electrochemical

workstation (CHI 760E, CH Instruments Inc., Shanghai) via

a standard three-electrode system. The working electrode was

prepared as follows: 2 mg sample was dispersed in 500 μL of

dimethylformamide and then the mixture was ultrasonically

dispersed for 30 min. Following that, 100 μL of the slurry was

−

min . Thermogravimetric analysis (TGA) was carried out on

STA449C/QMS403C/ TENSOR27 from 30 to 800 °C under

a nitrogen atmosphere with a heating rate of 10 °C min .

−

taken out and dropped on 1 × 1 cm indium tin oxide and

Surface area and pore size distributions were measured by

dried at room temperature. Furthermore, Ag/AgCl electrode

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ACS Catal. 2021, 11, 7429−7441

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−

Figure 3. Fluorescence spectra of (a) CTF-1 and (b) A-CTF-2 dispersed in various solvents (1.0 mg mL ) under excitation at 365 nm.

(

saturated KCl), platinum wire, and 0.2 M Na SO solution

nitrogen triple (CN) stretching vibration at 2221 or 2226

−

were used as the reference electrode, auxiliary electrode, and

electrolyte solution, respectively. Mott−Schottky (M−S) plots

were recorded at a scan rate of 5 mV/s in the dark at diﬀerent

frequencies of 800, 1000, and 1500 Hz. The on−oﬀ transient

photocurrent responses were recorded with a sampling interval

of 20 s using a 500 W Xe lamp and a certain applied bias of 0.5

V. Electrochemical impedance measurements were recorded

cm in comparison with their nitrile monomers also indicates

the transformation of −CN into the triazine-conjugated

structure. Solid-state C CP/MAS NMR spectroscopy exhibits

the characteristic signals of sp carbon atoms in triazine rings at

∼169 ppm (Figure 2a). Meanwhile, the sp C−N at ∼144 ppm

for CTF-1 or A-CTF-2 and the sp carbon atom in acetylene for

A-CTF-2 also apparently validate the integrity of their

48,49

over a 0.01−10 Hz frequency range with a 5 mV amplitude.

backbones, respectively.

These results were further

DFT Calculation Methods. DFT calculations and the

nonadiabatic molecular dynamics (NAMD) simulation were

performed using a GPU-based code with a plane-wave basis,

conﬁrmed by XPS analyses (Figure S2 ). High-resolution XPS

spectra of C 1s and N 1s of CTF-1 and A-CTF-2 showcase the

carbon (285.48 or 285.42 eV) and nitrogen (398.80 or 398.76

PWMAT. Full structure relaxation was performed until the

forces became lower than 0.01 eV/A. As a large vacuum

spacing (20 Å) was used in our simulations, the planar-

averaged electrostatic potential converged to a constant value

far from the surface. The norm-conserving pseudopotentials

and the generalized gradient approximation of the Perdew−

eV) of sp -hybridized CN bond, indicating the formation of

0−52

the triazine ring.

Moreover, sp (CC)-hybridized

carbons at 285.07 eV can be deconvoluted from the C 1s

peak of A-CTF-2, further reﬂecting the presence of acetylene

moiety in A-CTF-2. The PXRD patterns of CTF-1 and A-

CTF-2 clearly suggest their amorphous nature with partial π−π

Burke−Ernzerhof (PBE) functional were adopted. Mean-

while, we had employed the Heyd−Scuseria−Ernzerhof

stacked structures (Figure S3 ). TGA under N reveals that

both CTF-1 and A-CTF-2 are chemically stable and possess

good thermal stability up to 350 °C (Figure S4 ).

The porosities and speciﬁc surface areas of CTF-1 and A-

(

HSE06) hybrid functional to obtain the accurate band

structure. The plane-wave cutoﬀ energy was 50 Ry, and

the k-point mesh is 1 × 1 × 1. Test calculations showed that

this cutoﬀ and k-mesh were suﬃcient to obtain converged

energies, nonadiabatic coupling coeﬃcients, and force.

Molecular dynamics (MD) simulations were performed at

CTF-2 were assessed by N adsorption−desorption isotherms

measured at 77 K, as shown in Figure 2b. CTF-1 displays a

steep uptake in the low-pressure region (P/P < 0.01) and a

subsequent more gradual increase at high-pressure values,

which is a typical type I isotherm with hysteresis. The

hysteresis in the isotherm can be ascribed to the pores swelling

with the increasing gas pressure. The curve of CTF-1

demonstrates the presence of micropores, and the pore size

mainly distributed in 0.52 and 0.88 nm was calculated by

NLDFT (Figure S5 ). The BET surface area was calculated to

∼

300 K under the NVE ensemble with the Verlet algorithm.

The time step for MD simulations was 2.0 fs. About 50 states

were used in the valence band to expand the hot-carrier wave

function.

RESULTS AND DISCUSSION

■

−1

be 924 m g . However, A-CTF-2 presents a type IV

isotherm, suggesting the existence of mesopores and macro-

Synthesis and Characterization. The two CTFs without

acetylene (CTF-1) and with the acetylene unit (A-CTF-2)

were synthesized by TfOH-catalyzed trimerization of tris-

−1

pores, with the BET surface area being only 24 m g . It is still

unclear why the surface area of A-CTF-2 is so low, but similar

observations are found for the CTFs prepared by acid-

,4′,4″-(4-cyanophenyl)triphenylamine (1) and tris-4,4′,4″-(4-

cyanophenylethynyl) triphenylamine (2), respectively (Figure

b). Various measurements were performed for their structural

characterizations. The FT-IR spectra in Figure S1 show the

25,48

catalyzed strategy in previous works.

We suppose that the

low surface area is probably caused by its rapid reaction and

relatively low reaction temperature to avoid undesired side

reactions; also, compound 2 is rather sensitive in the extremely

acidic TfOH, leading to its carbonization, which can likely

−

characteristic signals of triazine ring at 1510 and 1358 cm for

CTF-1 or 1502 and 1322 cm⁻for A-CTF-2.

18,47

The

concomitant attenuation in the intensity of the carbon−

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ACS Catal. 2021, 11, 7429−7441

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Figure 4. Optical and electronic properties of CTF-1 and A-CTF-2. (a) UV/vis DRS spectra (the inset digital pictures show the color of CTF-1

up) and A-CTF-2 (down)), (b) band gap determined from the Kubelka−Munk-transformed reﬂectance, (c,d) M−S plot measured in 0.2 M

Na SO , with Ag/AgCl as the reference electrode in dark, (e) experimental estimated band structure diagrams, (f) steady-state PL emission spectra,

(

(g) I−t curves, EIS Nyquist plots (h) in darkness and (i) under light illumination of CTF-1 and A-CTF-2.

cause the low surface area. The morphologies of CTF-1 and A-

CTF-2 were then observed by SEM and TEM (Figures 2c−f

and S6). CTF-1 presents the randomly stacked chunks with an

angular shape in a wide range of sizes, whereas A-CTF-2

exhibits regular cluster-like nanospheres, with the dimensions

in the ∼460 nm range. By carefully observing the high-

resolution TEM images in Figure S7 , microporous channels

can be clearly discerned in CTF-1; however, such channels are

maxima was found with an increase in the polarity of the

solvent (Figure 3a). In contrast, the suspensions of A-CTF-2 in

the same solvents exhibited weak ﬂuorescent colors, and

varying the polarity of the solvents only resulted in a slight

bathochromic shift (Figure 3b). Such a very weak solvato-

chromic behavior of A-CTF-2 probably originates from the

extended conjugated nature and periodical arrangement of the

framework, which can stabilize well the excited-state intra-

54,55

hardly seen in A-CTF-2, consistent with the N sorption

molecular charge transfer.

Additionally, the ﬂuorescence

results.

intensity of A-CTF-2 is much weaker than that of CTF-1, likely

arising from the greater π-stacking in the latter structure.

Photoelectric Properties. The electronic properties of

CTF-1 and A-CTF-2 were systematically investigated by

photophysical and electrochemical analyses. Both CTFs

contain a central tertiary amine donor with a π-cross-linker

unit connected to the triazine acceptor. The chemical structure

is favorable for electronic absorption and emission; thus, their

optical properties were ﬁrst investigated by ﬂuorescence

spectroscopy in solvents with various polarities. Compounds

Solid-state UV/vis DRS spectra exhibit an intrinsic

absorption extending to the visible-light region, originating

from their D−A interactions and π−π* electron transition of

the conjugated ring. Apparently, the absorption edge of A-

CTF-2 is remarkably red-shifted in comparison with that of

CTF-1 (Figure 4a), indicating that the extended π-conjugated

structure along with the incorporation of the acetenyl group

into the skeleton can enhance the light-harvesting ability in the

visible region. The optical band gaps of CTF-1 and A-CTF-2

and 2 were highly soluble in moderate polarity solvents and

exhibited blue-green emissions under excitation at 365 nm

Figures S8 and S9 ). Upon dispersing the powder of CTF-1 in

7,58

were calculated by the Kubelka−Munk function.

The band

diﬀerent solvents, orange or light green ﬂuorescence was

observed, and a distinct bathochromic shift of the emission

gaps were measured to be 2.46 eV for CTF-1 and 2.35 eV for

A-CTF-2 (Figure 4b). To further determine the electronic

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ACS Catal. 2021, 11, 7429−7441

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Figure 5. Energy band gap structures of (a) CTF-1 and (b) A-CTF-2 using DFT calculations. (c,d) Spatial distributions of VBM and CBM along

with the projections of 3D band structures of (c) CTF-1 and (d) A-CTF-2 for the VBM and CBM in the K −K plane, respectively. All energies

were calculated with reference to the vacuum level. The brown, gray, and white balls represent C, N, and H atoms.

structures of CTFs, M−S measurements were conducted at

three diﬀerent frequencies to estimate their conduction band

valence band potentials (E ) of CTF-1 and A-CTF-2 can be

calculated to be 1.58 and 1.64 V versus Ag/AgCl, respectively,

potentials (E ). The positive slopes regardless of the varied

according to the formula E_C_B= E_V_B− E . The corresponding

frequencies suggest the typical n-type semiconductor charac-

band structure alignments of CTF-1 and A-CTF-2 are

schematically illustrated in Figure 4e.

teristic for CTFs. The derived ﬂat band potentials (E ) of

CTF-1 and A-CTF-2 were extrapolated to be about −0.88 and

To reveal the kinetics of the photogenerated charge carriers,

steady-state PL emission spectroscopy, transient photocurrent

response (I−t) analysis, and electrochemical impedance

spectroscopy (EIS) were comparatively conducted for CTF-1

and A-CTF-2. PL intensity in the solid state for A-CTF-2 is

remarkably lower than that of CTF-1 (Figure 4f), which in

−

0.71 V versus Ag/AgCl, respectively (Figure 4c,d). In general,

it can be accepted that the E potential is approximately equal

to the E potential in n-type semiconductors. Thus, E_C_Bof

CTF-1 and A-CTF-2 are −0.88 and −0.71 V, respectively.

Combined with the aforementioned optical band gap, the

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principle indicates the higher eﬃciency on the separation and

zone, while the lowest point in the conduction bands of A-

CTF-2 is close to the γ point, as shown in Figure 5. The

indirect band gap structures of both CTFs in Figure 5a,b

suggest that the electronic transition process requires phonons

to participate in changing the electron momentum and thus

inhibit the recombination of electrons and holes. Such energy

band structure signiﬁcantly aﬀects the migration of carriers.

The eﬀective carrier masses for the edge states of CTF-1

samples are m* = 1.71m and m* = 0.844m , while those for A-

transfer of photogenerated charge carriers for A-CTF-2. The

transient photocurrent responses in Figure 4g also exhibit a

higher photocurrent density on A-CTF-2, which further

illustrates the more eﬃcient separation of photogenerated

electron−hole pairs, indicating the promoted transfer kinetics

of charge carriers. The enhanced charge separation and

transfer eﬃciency were also conﬁrmed by EIS in the dark and

under visible light. Obviously, A-CTF-2 presents a smaller arc

radius and a lower resistance in charge transportation than that

of CTF-1 (Figure 4h), and the arc radius is further decreased

under visible light (Figure 4i), strongly suggesting that A-CTF-

CTF-2 samples are m* = 1.45m and m* = 0.352m (m

denotes the electron rest mass). These results suggest that the

incorporation of alkynyl groups causes a faster migration of the

A-CTF-2 band-edge carriers, resulting in their easier transfer to

the molecules from its framework. Furthermore, the larger

relative eﬀective mass (m*/m*) of A-CTF-2 also reﬂects its

possesses superior charge-transfer eﬃciency, in line with its

eﬃcient charge separation eﬃciency. These photoelectrochem-

ical analyses provide solid proof that A-CTF-2 shows enhanced

separation and migration of charge carriers, which can be

attributed to the extended π-conjugated structure and

migration distance of photogenerated charge carriers because

of the incorporation of the acetenyl group into the skeleton.

DFT Calculations. For an in-depth understanding of the

electronic structural features at the atomic level, the

monolayers of CTF-1 and A-CTF-2 were investigated by

DFT calculations. The light absorption and exciton formation

processes from the electronic structure of the two CTFs were

ﬁrst simulated. The calculated band structures with an ideal

inﬁnite model shown in Figure 5a,b (upper panel) suggest the

indirect band gaps of 2.55 and 2.36 eV for CTF-1 and A-CTF-

lower carrier recombination eﬃciency, which strongly

indicates a superior photocatalytic activity for A-CTF-2.

Additionally, in order to evaluate the lifetimes of the

photoexcited carriers, we thus calculated the transition dipole

moments (P_a_→_b= φ |−r|

φ_j) between the ground state and

ﬁrst excited state of the two COFs. As shown in Figure 5a,b

lower panel), it is obvious that the electrons on the VBM of

(

A-CTF-2 have greater transition probability and oscillator

strength. The ﬂuorescence lifetimes of COFs are simulated via

τ =

w h e r e t h e o s c i l l a t o r s t r e n g t h

2 * f * ΔE

f = ΔE|φ| − r|

φ| and ΔE is the energy diﬀerence between

, respectively, which are in good agreement with the

state i (φ ) and state j (φ ), only considering the ﬁrst excited

experimental values. In order to better understand the

diﬀerence between the electronic structures of the two

CTFs, we plot the total density of states (Figure S10).

Compared with CTF-1, as the alkynyl group in A-CTF-2

reduces the energy of the conduction band minimum (CBM)

state, electrons in the valence band maximum (VBM) state are

more likely to be excited to form electron−hole pairs under

light illumination. The reduction of the band gap can facilitate

the utilization eﬃciency of the visible light. The simulated

CBM and VBM (vs vacuum) of CTF-1 locate at −2.626 and

state, suggesting the ﬂuorescence lifetimes of 11.2 and 13.5

ns for CTF-1 and A-CTF-2. The prolonged lifetime for A-

CTF-2 further demonstrates the higher stabilization of carriers

because of the existence of alkynyls. Furthermore, the charge

separation and mobility were simulated by NAMD, with a hot

hole distribution in VBM and a hot electron distribution in

CBM (Figures S12 and S13). The results reveal that charge

carriers directly migrate from the donor to acceptor along with

the skeleton (Figure 6a,b), resulting in the construction of an

internal electric ﬁeld; also, the time of charge carrier mobility

in A-CTF-2 is distinctly prolonged compared with that of

CTF-1 due to the extended π-conjugation. For both the COFs,

the hot hole relaxation time is much longer than the hot

electron relaxation time, which proves that the relaxation

process of the holes is the rate-determining step of electron−

−

5.176 eV, and of A-CTF-2 at −2.828 and −5.233 eV,

respectively (Figure S11).

Moreover, the VBM electron-state density distribution of

CTF-1 spatially overlaps and is mainly distributed in the

triphenylamine donor along with the benzene π-conjugation

units. The CBM is only located over triazine acceptors (Figure

hole pair recombination. Correspondingly, the lifetime of the

generated hot electrons and holes in A-CTF-2 is also longer

than that of CTF-1. From the perspective of the hot carrier

dynamic process, the hot carriers of A-CTF-2 have a thermal

equilibrium time. The hot carriers would not immediately cool

from the excited state to the ground state, as shown in Figure

6c,d. The alkynyl group of A-CTF-2 not only acts as a bridge

for carrier transfer but also becomes a stable group for hot

carriers. The incorporation of the alkynyl group in A-CTF-2

leads to a longer carrier migration distance, implying a better

stabilization of electrons and holes in the excited state. The

above discussion on the nonequilibrium kinetics of excitons

provides us with a clear theoretical picture, which proves that

the alkynyl group of A-CTF-2 is an important reason for its

excellent photocatalytic performance.

c). This means that the charges can migrate from the tertiary

amine and π-conjugation units to the triazine moiety under the

light irradiation. A similar phenomenon can also be observed

from A-CTF-2 (Figure 5d). Nevertheless, alkynyls also

contribute some charges to the VBM; also, partial electrons

can stay on the alkynyls in the CBM, which means that the

alkynyl can disperse the charge concentration of the triazine

unit. The donor part and acceptor part of the two CTFs are

separated in space, so photogenerated electrons need to

undergo a long-range charge transfer to recombine with holes.

From the projections of the three-dimensional (3D) energy

band structure of CTF-1 and A-CTF-2, the electronic

structures of the VBM state of both are almost the same, but

the compositions of the CBM state have distinct diﬀerences.

Although the VBM and CBM of both COFs are composed of

the p orbitals of the C and N atoms, the energy band

Photocatalytic Activities. With the experimental and

theoretical investigations on the photoelectric properties of

CTF-1 and A-CTF-2, a visible-light-driven oxidative coupling

reaction of primary amines was also conducted to evaluate

dispersion relationship of A-CTF-2 containing alkynyl groups

is very diﬀerent from that of CTF-1. The lowest point in the

conduction bands of CTF-1 is at the edge of the ﬁrst Brillouin

435

ACS Catal. 2021, 11, 7429−7441

alternative for oxidative coupling because of its environ-

Research Article

7−69

mentally benign process and mild conditions.

We ﬁrst

selected the conversion of benzylamine (BAN) into N-

benzylidenebenzylamine as a model reaction (Figure 7a).

Either CTF-1 or A-CTF-2 could proceed smoothly and aﬀord

good conversions of BAN. The apparent quantum eﬃciency at

420 nm was estimated to be about 16.9% for CTF-1 and 32.3%

for A-CTF-2. Apparently, A-CTF-2 showed a higher catalytic

eﬃciency than the former. Control experiments revealed that

the photocatalyst, visible light, and oxygen were all

indispensable for this oxidation; otherwise, only little

conversions were observed. The initial O pressure was further

investigated for the oxidation of benzylamine, and a slackened

change was found with a gradually increasing O pressure,

suggesting that the O pressure is not the key factor for this

oxidation (Figure S14 ). In sharp contrast, g-C N only gave a

28.7% conversion under identical conditions. In addition, the

photocatalyst concentration and diﬀerent solvents were also

investigated in detail. These results suggested the optimal

concentration as 4.0 g/L (Table S1) and CH CN as the

optimal solvent (Table S2 ).

Apart from the excellent activities, catalyst recycling is also

signiﬁcant for heterogeneous catalysis; thus, the recyclability of

our superior photocatalyst A-CTF-2 was examined for the

model reaction after it was separated by simple ﬁltration from

the reaction mixture. A-CTF-2 remained highly active for at

least ﬁve cycles (Figure 7b), and the FT-IR spectrum of the

used A-CTF-2 was almost consistent with that of the fresh one

(Figure S15), suggesting its excellent photochemical stability.

Next, various substituted amines were tested for photocatalytic

oxidative coupling with CTF-1 and A-CTF-2 under the

Figure 6. (a,b) Isosurface of the electron orbitals of magenta VBM

and blue CBM (upper panel) and the charge transport processes

(

lower panel) for (a) CTF-1 and (b) A-CTF-2. (c) Hot electron and

d) hole carrier cooling time of CTF-1 and A-CTF-2 estimated by

NAMD calculations.

their photocatalytic activities. Photocatalytic selective aerobic

oxidation of primary amines to imines is a more fascinating

Figure 7. (a) Visible-light-driven oxidative coupling of benzylamine. (b) Stability of the A-CTF-2 photocatalyst. (c) Visible-light-driven oxidative

coupling of various primary amines over CTF-1 and A-CTF-2. Reaction conditions: 0.2 mmol amines, 8 mg catalyst, 2 mL of CH CN, 0.1 MPa O ,

4.0 h, room temperature, visible light (λ > 400 nm); conversion and selectivity are determined by GC and GC−MS.

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ACS Catal. 2021, 11, 7429−7441

Research Article

Figure 8. (a) Proposed mechanism for the photocatalytic oxidative coupling of benzylamine. (b) Adsorption energy barriers of O in diﬀerent sites

for CTF-1 and A-CTF-2. (c) Adsorption model of O in the acetylene unit of A-CTF-2. The brown, gray, pink, and red balls represent C, N, H, and

O atoms, respectively.

that both O ^•and O as reactive oxygen species (ROS)

−

optimal conditions, respectively (Figure 7c). Interestingly, all

the amines bearing electron-donating or electron-withdrawing

groups as substrates could furnish the corresponding imines in

high conversions over A-CTF-2, whereas CTF-1 exhibited an

obviously lower photocatalytic activity for these substrates.

Furthermore, time-dependent experiments on the oxidative

coupling of para-substituted BAN, including H, OCH , CH , F,

2 2

participated in this photocatalysis. The addition of t-BuOH

and catalase exhibited a negligible decrease in the conversion,

which might rule out the existence of the hydroxyl free radical

( OH) and hydrogen peroxide (H O ) during the photo-

catalytic process. To further verify the ROS O₂and O , the

•

−

ESR measurement was exploited with 5,5-dimethyl-pyridine-N-

Cl, and CF₃groups, were conducted using A-CTF-2 to

investigate the reaction kinetics (Figure S16a ). Kinetic

experiments showed a zero-order rate equation for the

oxidative coupling of amines. Their Hammett plots have no

reasonable linear relationship between log(k /k ) and the

oxide (DMPO) and 2,2,6,6-tetramethyl-4-piperidone (TEMP)

as trapping agents (Figure S17). ESR signals for O₂

captured by DMPO obviously increased under light illumina-

tion for 3 min, and the signals were decreased after adding

BAN into the mixtures; also, the characteristic ESR signals of

•−

Brown−Okamoto constant σ (Figure S16b), which can be

attributed to the negligible inﬂuence of the substituent at the

oxidation potential of amines, that is, all the benzylic amines

the TEMPO radical, a stable nitroxide that was produced by

the reaction between TEMP and O , also showed similar

•

−

results. These observations collectively corroborate that O

have relatively low oxidation potentials. These results also

and O₂synergistically serve as the main ROS for the

photocatalytic oxidation reaction.

indicate that the steric eﬀect is the main inﬂuencing factor for

the reaction rate rather than the electronic eﬀect.

Based on the experimental and computational results, a

tentative mechanism for the photocatalytic oxidative coupling

of benzylamine with diﬀerent CTFs is illustrated in Figure 8a.

Both CTFs are ﬁrst photoexcited to produce excitons, bound

electron−hole pairs, that further dissociated into free charge

carriers under visible-light irradiation. In CH CN, O is

To gain insights into the key active species involved in the

photocatalytic oxidation, a series of trapping experiments using

diﬀerent scavengers were conducted, as shown in Table S3.

After adding KI as a hole scavenger, the BAN conversion was

drastically decreased, suggesting that the photogenerated holes

reduced to O₂^•with photogenerated electrons, which is

−

were vital for the coupling reaction. With AgNO as the

electron scavenger, the reaction was also inhibited. Further-

more, evident decreases in BAN conversions were observed in

the presence of p-benzoquinone as a superoxide radical anion

followed by the oxidation of the latter with the remaining hole

to O on the surface of CTFs, whereas the photogenerated

holes mainly participate in the oxidization of BAN into its

cationic radical form. Both CTF-1 and A-CTF-2 possess

suﬃciently reductive and oxidative potentials to support the

•

−

(

O₂) scavenger and 2,2,6,6-tetramethylpiperidine-1-oxyl

(

TEMPO) as a singlet oxygen ( O ) scavenger, indicating

437

ACS Catal. 2021, 11, 7429−7441

ACS Catalysis

https://pubs.acs.org/doi/10.1021/acscatal.1c01794.

Research Article

redox process. In this photoredox process, the adsorption of

high-performance CTFs, with prospective applications in solar-

to-chemical energy conversion.

O and benzylamine on CTFs is a crucial step to reduce or

oxidize them into active species; thus, DFT calculation was

performed to examine their adsorption energies (E ). The

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

abs

■

E_a_b_svalues of A-CTF-2 for O molecules in the tertiary N and

triazine N active sites are slightly lower than that of CTF-1

(Figure 8b). Notably, the calculation results reveal that the

acetylene moieties in A-CTF-2 can adsorb O molecules and

provide active sites to lower the energy barrier of O (−0.958

Materials and experiment methods, structural analyses of

compounds 1 and 2, and additional results and

discussion (PDF )

eV), which is much lower than that of the N active sites

(Figure 8b), thus indicating that acetylene moieties are more

active for adsorbing O molecules and promoting the ROS

formation by the reduction of electrons (Figure 8c). The

AUTHOR INFORMATION

adsorption of O on these CTFs can also be conﬁrmed by O -

■

TPD (Figure S18 ). This peak at about 400 °C corresponding

Corresponding Authors

to the chemisorption of O species in A-CTF-2 is much

Qianfan Zhang − School of Materials Science and Engineering,

Beihang University, Beijing 100191, P. R. China;

orcid.org/0000-0001-8121-7727; Email: qianfan@

buaa.edu.cn

Guoyi Bai − Key Laboratory of Chemical Biology of Hebei

Province, College of Chemistry and Environmental Science,

Hebei University, Baoding, Hebei 071002, P. R. China;

orcid.org/0000-0002-0986-6455; Email: baiguoyi@

hotmail.com

stronger than that of CTF-1, further indicating that the alkynyl

group can provide additional adsorbtion sites for oxygen

species. Furthermore, the E_a_b_svalue of A-CTF-2 for benzyl-

amine (−0.765 eV) is also lower than that of CTF-1 (−0.368

eV). As a consequence, incorporating alkynyl groups can

simultaneously promote the adsorption of O and benzylamine

on A-CTF-2, which is favorable for enhancing the photo-

•

−

catalytic eﬃciency. The resulting O₂and O reacted with

the formed cationic BAN with the generation of hydroperoxy

(

phenyl)methanamine intermediate with H O . The trapping

Authors

experiment using N,N-diethyl-1,4-phenylenediamine without

or with A-CTF-2, and for the model reaction, was then

conducted to verify this process, which was monitored by an

Xingwang Lan − Key Laboratory of Chemical Biology of Hebei

Province, College of Chemistry and Environmental Science,

Hebei University, Baoding, Hebei 071002, P. R. China; Key

Laboratory of Photochemical Conversion and Optoelectronic

Materials, Technical Institute of Physics and Chemistry,

University of Chinese Academy of Sciences, CAS, Beijing

100190, P. R. China

Xiaopeng Liu − School of Materials Science and Engineering,

Beihang University, Beijing 100191, P. R. China

Yize Zhang − Key Laboratory of Chemical Biology of Hebei

Province, College of Chemistry and Environmental Science,

Hebei University, Baoding, Hebei 071002, P. R. China

Qing Li − Key Laboratory of Chemical Biology of Hebei

Province, College of Chemistry and Environmental Science,

Hebei University, Baoding, Hebei 071002, P. R. China

Juan Wang − Key Laboratory of Chemical Biology of Hebei

Province, College of Chemistry and Environmental Science,

Hebei University, Baoding, Hebei 071002, P. R. China

UV/vis spectrometer (Figure S19 ). The gradually increasing

intensity of the absorption peak at 500 nm was observed; also,

the signal intensity with A-CTF-2 was obviously higher than

that without A-CTF-2, suggesting that A-CTF-2 can assist the

formation of H O . The resulting benzylimine then reacts with

another BAN to give N-benzyl-1-phenylmethanediamine as the

intermediate. After this, the ﬁnal coupled product is generated

through the elimination of ammonia.

CONCLUSIONS

■

In summary, two new CTFs containing D−π−A motifs

without or with acetylene cross-linkers were successfully

synthesized via the trimerization of nitrile monomers. Their

chemical structures consisted of a central electron-rich tertiary

amine as the donor and electron-deﬁcient triazine as the

acceptor, which were connected by an extended π-conjugation.

The comprehensive investigations based on photoelectric

properties and DFT calculations suggest that the acetylene

moiety as a π-cross-linker unit incorporated into the CTF

skeleton is crucial to modulate the electronic band structures

and photoelectric properties of CTFs and prompt exciton

dissociation, which in essence inhibits backward charge

recombination and minimizes exciton binding energy, thereby

improving the separation and transfer of charge carriers, thus

giving rise to the improvement of the photocatalytic activity.

Photocatalysis experiments for the metal-free photocatalytic

oxidative coupling of primary amines using oxygen show

signiﬁcantly enhanced photocatalytic performances for acety-

lene-bridged CTFs in comparison to that of CTFs without the

acetylene linker. DFT calculations reveal that the acetylene

moiety can not only improve the photoelectric property but

also facilitate oxygen adsorption in the visible-light-driven

oxidative coupling reaction. These present results further

provide important insights into the design and construction of

Complete contact information is available at:

https://pubs.acs.org/10.1021/acscatal.1c01794

Author Contributions

Xingwang Lan and Xiaopeng Liu contributed equally to this

work. The manuscript was written through contributions of all

authors. All authors have given approval to the ﬁnal version of

the manuscript.

⊥

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

This work was supported by the National Natural Science

Foundation of China (21908038 and 22078080), Natural

Science Foundation of Hebei Province (B2019201341), and

Open Topic Program of Key Laboratory of Photochemical

Conversion and Optoelectronic Materials of Technical

Institute of Physics and Chemistry (PCOM202105).

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ACS Catal. 2021, 11, 7429−7441

ACS Catalysis

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