Effects of the acceptor pattern and substitution position on the properties of N-phenyl-carbazolyl based donor-acceptor-donor molecules

Article abstract of DOI:10.1039/c7ra01449j

Four optoelectronic compounds based on the same donor (N-phenylcarbazole) and different acceptors of benzothiadiazole or benzoselenadiazole were designed and synthesized to investigate the effects of the substitution position and effects of the acceptor heteroatom on the properties of N-phenyl-carbazolyl based organic optoelectronic compounds. The results demonstrated that these four compounds have band gaps (2.25 to 2.59 eV) that are lower than that of carbazole (3.20 eV). For the acceptor moiety, the replacement of benzothiadiazole with benzoselenadiazole was beneficial for the polarization of the D-A compound. Different acceptors substituted at the 3-position of N-phenylcarbazole led to molecules with a higher degree of planar conjugation than molecules derived from acceptors substituted at the 2-position. The increase in polarity and conjugation degree of the D-A compounds gave rise to lower band gaps and redshifts in the UV-vis absorption spectra and fluorescence emission spectra compared to the corresponding compounds. Solvatochromism effects were observed for these four compounds in different polar solvents, accompanied by redshifts in the fluorescence emission spectra, which ranged from 509 nm to 596 nm. Using thermogravimetric analysis, the decomposition temperatures of all four materials were found to be above 400 °C.

Full text of DOI:10.1039/c7ra01449j

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Effects of the acceptor pattern and substitution
position on the properties of N-phenyl-carbazolyl
based donor–acceptor–donor molecules†
Cite this: RSC Adv., 2017, 7, 18189
Xuejing Song, Min Wang, Lingqian Kong and Jinsheng Zhao *d
a
b
c
Four optoelectronic compounds based on the same donor (N-phenylcarbazole) and different acceptors of
benzothiadiazole or benzoselenadiazole were designed and synthesized to investigate the effects of the
substitution position and effects of the acceptor heteroatom on the properties of N-phenyl-carbazolyl
based organic optoelectronic compounds. The results demonstrated that these four compounds have
band gaps (2.25 to 2.59 eV) that are lower than that of carbazole (3.20 eV). For the acceptor moiety, the
replacement of benzothiadiazole with benzoselenadiazole was beneficial for the polarization of the D–A
compound. Different acceptors substituted at the 3-position of N-phenylcarbazole led to molecules
with a higher degree of planar conjugation than molecules derived from acceptors substituted at the 2-
position. The increase in polarity and conjugation degree of the D–A compounds gave rise to lower
band gaps and redshifts in the UV-vis absorption spectra and fluorescence emission spectra compared
to the corresponding compounds. Solvatochromism effects were observed for these four compounds in
different polar solvents, accompanied by redshifts in the fluorescence emission spectra, which ranged
from 509 nm to 596 nm. Using thermogravimetric analysis, the decomposition temperatures of all four
Received 4th February 2017
Accepted 18th March 2017
DOI: 10.1039/c7ra01449j
ꢀ
rsc.li/rsc-advances
materials were found to be above 400 C.
16–18
oen have uorescence properties.
Przemyslaw Ledwon et al.
Introduction
synthesized a compound with a D–p–A–p–D structure employ-
Carbazole is an industrial raw material that is inexpensive and ing carbazole as the D unit, benzothiadiazole (BTD) as the A unit,
available in large quantities. Carbazole and its derivatives can and 2-vinyl-3-hexyl-thiophene as the p linker; the molecule has
8
be fused with many aromatic units to generate novel a unique deep red color and is infrared emitting.
1
compounds, which have applications in a variety of elds, such
As a strongly electron decient unit, BTD is frequently used
as in photoelectric materials, dyes, medicines and biosen- for the construction of D–A type conjugated molecules. Some of
sors. For carbazole, facile substituted modication on the 9 - the resultant molecules have light-emitting properties and can
nitrogen atom and optional link of the 2 - or 3 -sites enable the be used as active materials in many elds, including heavy
2,3
4,5
6
7
0
0
0
4
14,15
precise regulation of the band gaps of the resultant molecule.
The important merits of carbazole derivatives include their high probes.
metal ion assay,
biosensors and orescence imaging
Pei jian group prepared a series of D–A–D type
18–20
hole mobilities, perfect luminous performance, high molar oligomers taking BTD as the acceptor and oligo(thienylene
8
extinction coefficients and capacity to undergo polymerization.
vinylene)s with distinct lengths as the donor unit, which
Donor (D)–acceptor (A)-type compounds usually have low showed the broad absorption bands between 300–800 nm, and
21
band gaps due to the push–pull effect along the molecular their emission bands were centered at approximately 650 nm.
skeleton; these molecules have become an increasingly impor- Sanjio S. Zade et al. reported three BTD derivatives with the
9–15
tant class of conjugated materials.
Carbazole and its deriva- D–A–D conguration and phenyl, thiophene and selenophene
tives are frequently used as end-capping donor units due to their as the donor units. Respectively, the emissive colours of the
strong electron-donating abilities and the obtained molecules three molecules were green (phenyl), orange (thiophene) and
22
red (selenophene). In brief, the properties of D–A type mate-
rials depend on the proper combination of the A unit and D
unit, as well as the substitution sites.
a
b
c
23
Dongguan University of Technology, Dongguan, 523808, P. R. China
Liaocheng Pepole's Hospital, Liaocheng, 252000, P. R. China
Benzoselenadiazole (BSe) is an analogue of BTD, that is ob-
Dongchang College, Liaocheng University, 252059, P. R. China
24
tained by replacing an S atom with an Se. The selenium atom
d
Department of Chemistry, Liaocheng University, Liaocheng, 252059, P. R. China.
E-mail: zhaojinsheng@lcu.edu.cn; Fax: +86-635-8539607; Tel: +86-635-8539607
25
has a larger size and is less electronegative than sulphur.
Therefore, BSe-containing compounds usually have red shis in
both absorption spectra and uorescence-emitting spectra
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra01449j
1
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24,25
relative to the corresponding BTD-containing compounds.
A
tetramethylsilane was used as the internal standard substance.
series of dyes were designed and synthesized using 5-thienyl- Elemental analysis was performed on a Thermo Finnigan Flash
BTD or 5-thienyl-BDS as the electron-decient cores the uo- EA 1112 analyser. MALDI-TOF MS spectra were recorded using
rene derivatives as the D units. The resultant compounds had Bruker Daltonics Flexanalysis.
2
0
valuable NIR emission properties with high quantum yields.
UV-vis absorption spectra were collected by Varian Cary 5000
serials of solvents including n-hexane,
dichloromethane, trichloromethane, methylbenzene, tetrahy-
The replacement of the BTD unit with a BDS unit also shied spectrometer.
A
2
0
the absorption of the molecules toward longer wavelengths.
There are no reports concerning the direct combination of N- drofuran, acetonitrile, DMF, methanol and dimethyl sulfoxide
phenylcarbazole with BTD for the construction of D–A–D-type were used as the solvent to prepare solutions. The D–A–D
materials; novel uorescent dyes with a high performance compounds, including 4,7-bis(9-phenyl-9H-carbazole-2-yl)
could be anticipated for such a combination. The effects of the benzo[c][1,2,5] thiadiazole (BPC-2BTD), 4,7-bis(9-phenyl-9H-
replacement of BTD by BSe on the uorescence properties of the carbazole-2-yl)benzo[c][1,2,5]selenadiazole (BPC-2BSe), 4,7-
obtained D–A–D-type molecules also deserve study. Changes in bis(9-phenyl-9H-carbazole-3-yl)thiadiazole (BPC-3BTD) and 4,7-
the substitution positions on the carbazole derivative units bis (9-phenyl-9H-carbazole-3-yl)selenadiazole (BPC-3BSe), were
would also have a direct inuence on the properties of the dissolved in the above solvents to a respectively, with the
ꢂ5
ꢂ1
resultant compounds and still need to be studied. Based on determined concentration of 1 ꢁ 10 mol L . The prepared
these considerations, we present in this study the synthesis of solutions were undertaken for UV-vis measurement with a scan
four D–A–D-type analogous molecules employing N-phenyl- range of 200–800 nm.
carbazole and benzothiadiazole (or benzoselenadiazole) as the
Fluorescence spectra were measured on a LS55 uorescence
D and A units, respectively, with the aim of exploring the effects photometer (Perkin Elmer). The uorescence emission spectra
of the substitution positions and heteroatom replacement of the compounds were measured using a 1 cm standard quartz
within the acceptor units on the properties of the obtained cuvette with a slit width of 6.0 nm, appropriate excitation
26
substance. Their specic properties, including optical, redox wavelength, a voltage of 700 V and a scanning speed of 300 nm
ꢂ1
and thermal stabilities, were studied in detail. Interestingly, the min . The redox property of the compounds was studied using

uorescent dyes obtained emitted yellow, green and red colours cyclic voltammetry method in a dry DCM solution containing
upon irradiation, that is, the primary colours. Any colour can be 0.2 M TBAPF . A three-electrode system was employed with a Pt
6
made from these primary colours, which makes them impor- wire (F 0.5 mm) as the working electrode, a Pt ring (F 0.5 mm)
tant and leads them to have wide application prospects in the as the counter electrode, and a Ag/AgCl electrode as the refer-
organic electroluminescent eld. All of the compounds showed ence electrode. The thermal stabilities of compounds were
the solvatochromism effect in different polar solvents and had tested using a NETZSCH STA 499C TG-DSC tester under an N
2
ꢀ
simultaneous high thermal stability and redox activities. As atmosphere, temperature range of 40–800 C, and heating rate
ꢀ
ꢂ1
anticipated, the replacement of BTD by BSe as the acceptor unit of 10 C min . A Canon Power Shot A3000 IS was used to take
could also reduce the band gap of the resultant molecules.
pictures of the four compounds and its solutions and their
solutions.
Experimental
Materials
Synthesis of acceptor unit
The acceptor unit 4,7-dibromo-2,1,3-benzoselenadiazole was
synthesized using the methods reported previously in the
2
-Boric acid-N-phenylcarbazole (98%), 3-boric acid-N-phenyl-
carbazole (98%), 4,7-bibromine-2,1,3-benzothiadiazole (98%),
sodium borohydride (NaBH , 98%), sodium carbonate (Na CO ,
24–28
literature
(Scheme 1(a)): 5.75 g (19.56 mM) of 4,7-dibromo-
4
2
3
2,1,3-benzothiadiazole and 300 mL of ethanol were added into
98%), selenium dioxide (SeO , 99%), tetra (triphenylphosphine)
2
a 500 mL three-necked ask in a thermostat water bath. Under
palladium (Pd(PPh ) ), tetra-n-butylammonium uoride
3
4
ꢀ
electromagnetic stirring at 30 C, an excessive of 16 g (422 mM)
(
(
TBAF), ethanol (EtOH, 99.9%), methylbenzene (AR), n-hexane
AR), trichloromethane (AR), N,N-dimethylformamide (DMF,
4
NaBH was added to the above suspension in three portions
over an interval of 8 h. Forty-eight hours later, the reaction was
terminated by pouring the mixture directly into 500 mL of
distilled water; abundant white occules precipitated. The
mixture was stirred mechanically for 5 min and ltered by
decompression suction to afford white solid 3,6-dibromo-1,2-
AR), methanol (AR), dimethyl sulfoxide (AR) and anhydrous
magnesium sulphate (AR) were all purchased from the Aldrich
Chemical Reagent Company and used directly without further
treatment.
Tetrabutylammonium
hexauorophosphate
(
TBAPF , 98%) was purchased from Alfa Aesar. Thin layer
6
phenylenediamine. The solid was dried in a vacuum oven for
chromatography (TLC) silica gel (GF254) and column layer
chromatography silica gel (200–300 meshes) were provided by
Qingdao Marine Chemical Factory.
1
2
4
0 hours and reserved for further application. H NMR (CDCl
3
,
1
3
00 MHz, ppm): d ¼ 6.84 (s, 2H, Ar H), 3.89 (s, 4H). C NMR
(CDCl
3
, d, ppm): d ¼ 133.70, 123.24, 109.68.
To prepare 4,7-dibromo-2,1,3-benzoselenadiazole, 9.47 g
36.91 mM) 3,6-dibromo-1,2-phenylenediamine was dissolved
Characterization
(
1
13
H and C NMR were recorded using a Varian AMX 400 nuclear in 200 mL ethyl alcohol in a ask, and the solution was heated
ꢀ
magnetic resonance spectrometer (CDCl
3
, 400 MHz), and to 90 C in a water bath. At the same time, 4.2 g of SeO
2
(37.85
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Scheme 1 Synthetic routes of acceptor and four D–A–D compounds.
1
mM) was dissolved into 50 mL hot water, and the solution was 2BTD. H NMR (CDCl , 400 MHz, ppm): d ¼ 8.27 (d, 2H, ArH),
3
added to the reaction ask slowly. Yellow insoluble substances 8.19 (d, 2H), 7.97 (d, 2H), 7.88 (d, 2H), 7.78 (s, 2H), 7.60 (m, 8H),
1
3
were produced. The mixture was reuxed for 2 h, and 4,7- 7.44 (m, 6H), 7.31 (m, 2H) (see Fig. S1a in ESI†). C NMR
dibromo-2,1,3-benzoselenadiazole was afforded from vacuum (CDCl
3
, d, ppm): d ¼ 158.90, 146.13, 145.69, 142.16, 139.97,
28

ltration. The yellow solid was then washed with ethanol three 138.42, 134.54, 132.96, 132.12, 131.77, 130.84, 128.00, 127.74,
ꢀ
times, with water three times and dried under vacuum at 50 C. 126.07, 125.15, 124.96, 124.75, 115.27, 114.48 (see Fig. S1b in
1
13
H NMR (CDCl , 400 M Hz, ppm): d ¼ 7.64 (s, 2H, Ar H).
C
ESI†). MS (MALDI-TOF): m/z: calcd for 618.19; found 618.75.
Elemental analysis calcd (%): C 81.53, H 4.24, N 9.05, S 5.18;
found: C 81.35, H 4.27, N 9.13, S 5.25.
3
NMR (CDCl , d, ppm): d ¼ 158.16, 144.00, 139.30.
3
The synthesis of BPC-2BSe was the same as for BPC-2BTD,
which was obtained as a uffy yellow solid with a yield of
Synthesis of D–A–D compounds
1
The synthetic routes are outlined in Scheme 1(b). Four mole- 71%. H NMR (CDCl
3
, 400 MHz, ppm): d ¼ 8.26 (d, 2H, ArH),
cules including BPC-2BTD, BPC-2BSe, BPC-3BTD and BPC-3BSe 8.18 (d, 2H), 7.86 (d, 2H), 7.80 (d, 2H), 7.62 (s, 2H), 7.58 (m, 8H),
29
13
were synthesized using Suzuki coupling reactions. The specic 7.43 (m, 6H), 7.30 (m, 2H) (see Fig. S2a in ESI†). C NMR
synthetic process for BPC-2BTD was as follows: 3 g of (10.45 (CDCl
, d, ppm): d ¼ 159.88, 141.43, 140.96, 137.53, 135.95,
3
mM) 2-boric acid-N-phenylcarbazole, 1.53 g of (5.22 mM) 4,7- 135.60, 129.86, 128.68, 127.43, 127.12, 126.11, 123.26, 123.12,
ꢂ1
dibromo-2,1,3-benzoselenadiazole, 40 mL of (2 mol L ) NaCO
3
121.73, 120.47, 120.12, 120.06, 110.76, 109.80 (see Fig. S2b in
solution and 60 mL of methylbenzene (volume ratio of meth- ESI†). MS (MALDI-TOF): m/z: calcd for 666.13; found 666.17.
ylbenzene and NaCO
was 3 : 2) were added into a round- Elemental analysis calcd (%): C 75.78, H 3.94, N 8.42, Se 11.86;
bottom ask. Then, 0.6 g of Pd(PPh ) (molar mass of reactant found: C 75.56, H 4.00, N 8.48, Se 11.96.
3
3
4
was 5%) and 0.1 g of phase transfer agent TBAF were added to
BPC-3BTD was obtained as a the yellow solid with a yield of
the ask. The ask was subjected to gas displacement until the 72%, and the procedure used was the same as the above two
1
atmosphere in the ask was replaced by argon gas. The ask compounds. H NMR (CDCl
, 400 MHz, ppm): d ¼ 8.75 (d, 2H,
3
ꢀ
was stirred magnetically under argon, heated to 120 C and ArH), 8.25 (d, 2H), 8.04 (d, 2H), 7.85 (s, 2H), 7.61 (m, 8H), 7.55
1
3
maintained for 48 h. Aer the reaction was completed, the (d, 2H), 7.46 (m, 6H), 7.33 (m, 2H) (see Fig. S3a in ESI†).
solvent was removed under vacuum. The residue was dissolved NMR (CDCl
C
3
, d, ppm): d ¼ 154.52, 141.31, 140.75, 137.51,
in DCM, washed three times with water and dried over 133.21, 129.91, 129.58, 127.96, 127.55, 127.43, 127.06, 126.17,
magnesium sulphate. The resulting crude product was obtained 123.70, 123.53, 121.20, 120.56, 120.16, 109.94, 109.83 (see
by distillation of the solvent, and puried using column chro- Fig. S3b in ESI†). MS (MALDI-TOF): m/z: calcd for 618.19; found
matography (3 : 1, n-hexane, dichloromethane). Finally, 2.2 g of 618.33. Elemental analysis calcd (%): C 81.53, H 4.24, N 9.05, S
yellow powder was collected (yield of 68%), and termed BPC- 5.18; found: C 81.41, H 4.24, N 9.15, S 5.20.
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Similarly, the fourth compound BPC-3BSe was produced as characteristic absorption bands of BPC-2BSe were redshied by
1
a reddish-orange solid in a yield of 68.7%. H NMR (CDCl , 400 9 nm and 24 nm compared with those of BPC-2BTD, respec-
3
MHz, ppm): d ¼ 8.68 (d, 2H, ArH), 8.25 (d, 2H), 8.00 (q, 2H), 7.76 tively. The same phenomenon was observed between BPC-3BTD
(
s, 2H), 7.64 (m, 8H), 7.57 (d, 2H), 7.48 (m, 6H), 7.34 (m, 2H) (see and BPC-3BSe. The Se atom is larger in size than the S atom and
1
3
Fig. S4a in ESI†). C NMR (CDCl
3
, d, ppm): d ¼ 160.26, 141.30, less electronegative than the S atom. Thus, the Se atom exerts
24–26
1
1
1
6
3
40.71, 137.53, 135.14, 130.19, 129.90, 128.40, 127.71, 127.55, a weaker binding force on external electrons.
Therefore, the
27.09, 126.11, 123.58, 123.50, 121.38, 120.53, 120.09, 109.90, molecular polarization was intensied when the heteroatom S
09.67 (see Fig. S4b in ESI†). MS (MALDI-TOF): m/z: calcd for in the acceptor unit was replaced by Se. The excited state
66.13; found 666.25. Elemental analysis calcd (%): C 75.78, H energies of BPC-2BSe and BPC-3BSe in a polar solvent were
.94, N 8.42, Se 11.86; found: C 75.59, H 3.98, N 8.53, Se 11.90. lower than those of BPC-2BTD and BPC-3BTD, respectively,
leading to a redshi in the both high and low energy absorption
bands.
Results and discussion
The changes of the substitution positions may also have an
effect on the absorption properties of the compounds. It was
observed that the short wavelength absorption maximum of
BPC-3BTD was blueshied (20 nm) compared to that of BPC-
Synthesis and characterization
The general synthetic method of acceptor unit 4,7-dibromo-
2
,1,3-benzoselenadiazole and four D–A–D compounds is
shown in Scheme 1. In the ethanolic solution, 4,7-dibromo-
,1,3-benzothiadiazole reacted with NaBH4 to produce 3,6-
dibromo-1,2-phenylenediamine at a yield of 75%. SeO was
2BTD, while the long wavelength absorption maximum was
simultaneously redshied (19 nm). The same phenomenon was
observed between BPC-2BSe and BPC-3BSe. Two characteristic
absorption bands moved in different directions, caused by their
different transition mechanisms. The short wavelength
absorption bands were mainly attributed to the p–p* transition
of the N-phenylcarbazole unit in the D–A–D type molecule,
whereas the low energy absorption bands were caused by the
2
2
dissolved in hot water and added to an ethanol solution of 3,6-
dibromo-1,2-phenylenediamine to produce the acceptor unit
4
,7-dibromo-2,1,3-benzoselenadiazole (yield of 62%). Four
analogous D–A–D compounds (BPC-2BTD, BPC-2BSe, BPC-
BTD and BPC-3BSe) were synthesized using Suzuki coupling
3
26
ICT between the donor and acceptor units.
reactions between 2-boric acid-N-phenylcarbazole (as electron
donor) and 4,7-bibromine-2,1,3-benzothiadiazole or 4,7-
dibromo-2,1,3-benzoselenadiazole (as electron acceptor) at
a high yield. The four compounds exhibited high solubility in
common solvents, including hexane, dichloromethane, chlo-
roform, toluene and tetrahydrofuran, etc. The obtained
The 3-position substitution molecules including BPC-3BTD
and BPC-3BSe, appear to have better coplanarity than their
corresponding analogues BPC-2BTD and BPC-2BSe, as further
conrmed by the quantum chemistry calculation method in
the following section. The less twisted conjugationed struc-
ture of BPC-3BTD or BPC-3BSe as compared to their respective
isomers BPC-2BTD or BPC-2BSe, resulted in more effective
conjugation and stronger ICT interactions. Consequently, the
electron cloud densities of the N-phenylcarbazole units
substituted at the 3-position were reduced moderately more
1
13
compounds were conrmed by H NMR, C NMR spectroscopy,
mass spectroscopy and elemental analysis.
UV-vis absorption spectra
The UV-vis absorption spectra of four compounds in various than the N-phenylcarbazole unit substituted at the 2-position.
solvents were analyzed; the results are shown in Fig. 1. This led to a blue shi of the high energy absorption bands
In Fig. 1, all of the compounds showed two characteristic of BPC-3BTD or BPC-3BSe compared with their respective
absorption bands in all of the solvents. There was one intense analogues. As mentioned above, the low-energy absorption
absorption at short wavelength and a second weak absorption at bands were caused by ICT, and the stronger ICT within the
long wavelengths. The short wavelength absorption bands were 3-position-substituted molecules than their respective
attributed to p–p* transitions in the N-phenylcarbazole moie- analogues (2-position) caused
a redshi of low-energy
ties, and the long wavelength absorption bands were assigned absorption bands. Based on the low energy absorption edges
to the intramolecular charge-transfer (ICT) from the electron- (lonset) in the UV-vis absorption spectra, the optical band gaps
donating N-phenylcarbazole moiety to the central electron- (E
g
, E
g
¼ 1241/lonset) of BPC-2BTD, BPC-2BSe, BPC-3BTD and
24,25
withdrawing group, that is BTD or BSe.
The UV-vis absorp- BPC-3BSe were calculated to be 2.59 eV, 2.45 eV, 2.48 eV
tion spectra of each compound in different solutions are and 2.33 eV, respectively. Furthermore, it was observed
similar, but slightly solvent-dependent. With increased solvent that the band gaps of the 3-position substitution products
polarity, the absorption bands of four compounds showed were slightly lower than those of the 2-position substitution
slight redshiing (less than 10 nm), indicating a slight sol- products. According to the literature, the band gap of
3
0
vatochromic effect in the ground state of four compounds. The carbazole is 3.20 eV, and the D–A–D compounds reported
positions of the characteristic absorption peaks in Fig. 1 are in the present study achieved far lower band gaps than this
given in Table 1.
value. This proves that building a D–A–D conjugation system
The UV-vis absorption spectra of four compounds in by introducing acceptor units between Bi–N-phenylcarbazole
dichloromethane are discussed as an example. The absorption is benecial for reducing the band gap of carbazole
maxima of BPC-2BTD were located at 318 nm and 414 nm, while derivatives.
those of BPC-2BSe were located at 327 nm and 438 nm. The two
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Fig. 1 UV-vis absorption spectra of BPC-2BTD (a), BPC-2BSe (b), BPC-3BTD (c) and BPC-3BSe (d) in different solvents.
Table 1 The max absorption wavelengths of BPC-2BTD, BPC-2BSe, emission and showed a strong solvent dependence. With the
BPC-3BTD and BPC-3BSe in different solvents
increase in solvent polarity, the emission peaks exhibited large
redshis (ranging from 31 to 58 nm), displaying a signicant
Solvent lmax/nm
BPC-2BTD BPC-2BSe BPC-3BTD BPC-3BSe
31–34
solvatochromic effect.
This phenomenon might be related
Hexane
317, 414
318, 414
318, 415
321, 417
318, 416
317, 409
319, 416
326, 438
327, 438
327, 437
329, 442
326, 436
319, 432
325, 436
297, 432
298, 433
299, 434
299, 439
298, 431
296, 429
298, 436
298, 459
299, 460
300, 461
300, 462
299, 460
297, 453
299, 462
to the inherent ICT in the D–A molecules. Many studies have
Dichloromethane
Chloroform
Toluene
Tetrahydrofuran
Acetonitrile
Dimethyl
formamide
Methanol
Dimethyl sulfoxide
shown that ICT was strengthened upon excitation, which
30,31
increases the molecular dipole moment.
The dipole moment
in the excited state is greater than that of the molecule in the
ground state. The increase in solvent polarity is benecial for
electron delocalization and the stabilization of the molecule in
the excited state. The stability of the molecule in the excited
state is enhanced by the increase in solvent polarity, thereby
reducing the energy level of the excited state and making the
energy gap between the excited state and ground state smaller.
Thus, the uorescence emission wavelength has a large
317, 413
321, 416
319, 435
328, 441
297, 432
300, 436
300, 460
300, 463
Fluorescence emission spectra
The uorescence emission spectra of BPC-2BTD, the uores-
35
redshi.
Additionally, the uorescence emission spectra of four
cence emission spectra of BPC-2BTD, BPC-2BSe, BPC-3BTD and compounds in methanol solvent were approximately straight
BPC-3BSe in different solvents are shown in Fig. 2.
lines without distinct emission peaks. This is because the
The uorescence emission spectra of four compounds are hydrogen-bond interaction between the solvent and molecules
36
similar with each other in different solvents. Unlike the UV-vis quenched the uorescence completely. The maxima emission
absorption spectra, all of the uorescence emission curves of wavelength data of the uorescence emission spectra are listed
the D–A–D compounds demonstrate only one resolved ICT in Table 2.
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Fig. 2 Fluorescence spectra of BPC-2BTD (a), BPC-2BSe (b), BPC-3BTD (c) and BPC-3BSe (d) in different solvents. Photos show DCM solutions
of four D–A–D compounds illuminated by ultraviolet light at 360 nm.
Table 2 The max fluorescence emission wavelengths of BPC-2BTD, absorption spectra. Replacing the heteroatom S with Se was
BPC-2BSe, BPC-3BTD and BPC-3BSe in different solvents
benecial for molecular polarization and resulted in a lower
excited state energy in the polar solvent, leading to a lower band
Solvent lmax/nm
BPC-2BTD BPC-2BSe BPC-3BTD BPC-3BSe
gap and a redshi of the emission wavelength. With the same
acceptor unit but a different position of the substituent, the
emission peak of BPC-3BTD was 565 nm, which had a redshi
by 40 nm compared to BPC-2BTD. The same tendency was
observed between BPC-3BSe and BPC-2BSe. This is consistent
with the trend in their absorption spectra and originates from
the much stronger ICT interactions in compounds BPC-3BTD
and BPC-3BSe compared to their isomers, BPC-2BTD and
BPC-2BSe.
Hexane
509
525
525
520
524
532
536
532
565
565
545
556
572
571
522
565
562
540
555
575
578
556
595
596
577
588
597
597
Dichloromethane
Chloroform
Toluene
Tetrahydrofuran
Acetonitrile
Dimethyl
formamide
Methanol
Dimethyl sulfoxide
—
540
—
570
—
580
—
598
Fluorescence yield
The uorescence emission spectra of four compounds in
dichloromethane are discussed in detail for the sake of
contrast. The emission maxima of BPC-2BTD and BPC-2BSe
were located at 525 nm and 565 nm, respectively. An apparent
redshi of 40 nm was observed for BPC-2BSe relative to that of
BPC-2BTD. Similarly, the emission band of BPC-3BSe had
a redshi of 30 nm compared to that of BPC-3BTD. The cause of
this phenomenon was similar to the redshi of the UV-vis
The compound 4-methylamino-7-nitro-2,1,3-benzodioxazole
(
NBD) was used as a reference and has uorescence quantum
37,38
yield (FQE) of 0.38 in acetonitrile.
Based on this criterion,
the FQE values of the four molecules were measured in
dichloromethane. Under the same conditions (uorescence
scanning range: 300–700 nm; slit width: 6.0 nm; voltage: 700 V;
ꢂ1
scanning speed: 300 nm min ), the uorescence emission
spectra of the test compounds and the reference were scanned
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and their integral uorescence intensities were calculated.
Then, the FQE values of different compounds were calculated
from eqn (1). The results are shown in Table 3.
ꢀ
ꢁꢀ ꢁꢀ
ꢁ
2
F
x
A
s
n
x
F
x
¼ F
s
(1)
F
s
A
x
n
s
where F
x
and F
s
are the FQE values of the testing compounds
and F are the integral uorescence intensities
and A are absor-
bance of the testing compounds and reference at exciting
wavelength; and n and n are refractive indexes of the solvents
and reference; F
x
s
of the testing compounds and reference; A
x
s
x
s
of the testing materials and reference, respectively.
Table 3 gives the FQE values of four compounds. BPC-2BSe
had a lower uorescence efficiency than BPC-2BTD. Similarly,
the uorescence efficiency of BPC-3BSe was lower than that of
BPC-3BTD. The Se atom has a larger molecular weight than the
S atom, which might exert a heavy atom effect on the energy
24,26
level transitions.
For example, when the singlet state in the
excited state transforms into the triplet state, it cannot expedi-
ently return to the ground state through intersystem crossing
due to the presence of the large Se atom. Therefore, the uo-
rescence efficiencies of the compounds using BSe as the
acceptor were lower than for compounds using BTD as acceptor.
By comparing the same acceptor pattern (BPC-3BTD and BPC-
Fig. 3 CVcurves of BPC-2BTD (a), BPC-2BSe (b), BPC-3BTD (c) and
BPC-3BSe (d).
2
BTD or BPC-3Be and BPC-2Be), substitution at the 2-position
that the substitution position and the type of acceptor have
greatly inuence on the properties of the compounds. The 3-
position substitution has much lower oxidation potentials
than that of the 2-position analogues, suggesting a higher
degree of conjugation in the former compounds than the
latter. The onset oxidation potentials of the compounds were
gained from the corresponding tangent lines of the CV curves
shown in Fig. 3. The values were 1.22 V, 1.20 V, 1.06 V and
of N-phenylcarbazole had higher uorescence luminous effi-
ciencies than the molecules with a 3-position substitution
pattern. This was caused by the large steric hindrance of
substitution at the 2-position. Higher molecular rigidity is
23–27
benecial for increasing uorescence efficiency.
Electrochemical properties
1
.05 V for BPC-2BTD, BPC-2BSe, BPC-3BTD and BPC-3BSe,
The electrochemical properties of the compounds were tested
by Cyclic Voltammetry (CV). CV curves of four compounds are
presented in Fig. 3.
respectively. BPC-2BSe had a lower onset oxidation poten-
tial than that of BPC-2BTD. The same phenomenon was
observed between BPC-3BTD and BPC-3BSe. BPC-3BSe had
a lower onset oxidation potential than that of BPC-3BTD,
which suggested a higher HOMO level for the former
compound than the latter. The HOMO levels of the
compounds could be drawn from the following formula,
For BPC-2BTD, there are two irreversible redox peaks
located at 1.63 V/1.04 V and an additional weak reduction
peak was observed at 0.12 V. There are no any apparent redox
peaks observed in the negative scan up to ꢂ1.60 V. For BPC-
2
BSe, only one oxidation peak was observed at a potential
3
8
HOMO ¼ ꢂe(Eonset + 0.02 + 4.4), and the HOMO energy
levels of BPC-2BTD, BPC-2BSe, BPC-3BTD, and BPC-3BSe
were calculated as ꢂ5.64 eV, ꢂ5.62 eV, ꢂ5.48 eV and
ꢂ5.47 eV, respectively. It was suggested that the HOMO levels
of the D–A type molecules could be reected by the HOMO
levels of the donor unit, as conrmed by the comparable
HOMO values of the four compounds, since they have the
same donor unit. According to the formula: LUMO ¼ HOMO +
of 1.62 V, and no obvious redox peaks were found in the
negative scan. Interestingly, there are two groups of redox
peaks for both BPC-3BTD and BPC-3BSe, located at 1.44 V/
1
.25 V, 1.18 V/1.02 V and 1.41 V/1.26 V and 1.18 V/1.01 V,
respectively. It is worth mentioning that two redox peaks at
ꢂ1.26 V/ꢂ1.39 V were found in the negative scan of BPC-3BSe.
The different redox behaviours of four compounds suggested
3
9,40
E
g
,
the LUMO energy levels of the four compounds were
ꢂ3.05 eV, ꢂ3.17 eV, ꢂ3.00 eV and ꢂ3.14 eV, respectively, for
BPC-2BTD, BPC-2BSe, BPC-3BTD and BPC-3BSe. Comparing
the HOMO and LUMO values of four compounds, it was
found that using BSe as the acceptor increased the energy
levels of HOMO and decrease the energy levels of LUMO
synchronously. Thus, the band gaps of the BSe-containing
molecules are smaller than the BDT-containing analogues.
Table 3 The fluorescence quantum yields of four compounds
Compounds
F
s
F
x
F
s
A
x
A
s
n
x
n
s
F
x
BPC-2BTD
BPC-2BSe
BPC-3BTD
BPC-3BSe
0.38 50 893 38 253 0.038 0.038 1.42 1.34 0.57
0.38 23 196 51 861 0.040 0.040 1.42 1.34 0.19
0.38 33 083 51 861 0.039 0.039 1.42 1.34 0.27
0.38
8263 50 541 0.037 0.037 1.42 1.34 0.07
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Moreover, substitution of the acceptor at the 3-position of the
donor increased HOMO values and decrease LUMO values
synchronously, also leading to the reduction of the band gap
of the accordingly obtained molecule. The data for the elec-
trochemical and optical properties of four compounds were
summarized in Table 4.
Theoretical calculation
To obtain more accurate structural information for the
molecules, quantum chemical computations of the
d
compounds were undertaken. The theoretical values of E
,
g
d
d
HOMO and LUMO of four compounds are given in Table 4,
which were obtained at the B3LYP/6-31G level employing the
2
3,26,35–38
Gaussian 03 program.
The quantum chemical calcu-
lations were used to compare the theoretical values with the
experimental results and to further understanding of the
relationship between the molecule structure and its proper- Fig. 4 Molecular orbital diagrams of HOMO and LUMO of four
compounds.
ties, as well as to gain a more intuitive display of the frontier
orbital state of molecules. The HOMO and LUMO of the
optimized molecular structures are shown in Fig. 4 (cartesian
coordinates in ESI†). For optimized molecular structures,
different substitution positions led to different planarities of
the compounds with different HOMO and LUMO energy
levels. Dihedral angles between the donor and acceptor are
further validated the correctness of the predicted molecular
conguration as shown in Fig. 4.
Thermostability analysis
3
3
2
1.78 and 32.95 for BPC-2BTD, 28.57 and 28.02 for BPC-3BTD,
4.92 and 33.87 for BPC-2BSe, and 31.23 and 30.65 for BPC- The thermal stabilities of four compounds were evaluated
BSe (Fig. 4). The introduction of the acceptor units (BTD or by thermogravimetric analysis (TGA). The TGA curves of
BSe) between the 3-position of two N-phenylcarbazole mole- BPC-2BTD is shown in Fig. 5, and the TGA curves of BPC-2BSe,
cules generates a molecular system that has better copla- BPC-3BTD and BPC-3BSe are presented in Fig. S5a–c in ESI.†
narity than the isomer obtained at the 2-position These four compounds showed only a one-stage decomposi-
substitution. The distributions of p-electrons in the HOMO tion pattern, and the corresponding on-set degradation
ꢀ ꢀ ꢀ ꢀ
orbit of each of four molecules are mainly distributed on the temperatures (T ) reached 428 C, 415 C, 438 C, and 418 C,
d
N-phenylcarbazole donor unit, while the excited electron in respectively. Differential thermal gravity (DTG) curves are also
the LUMO orbit of the four molecules was mainly distributed shown in Fig. 5 and S5a–c,† from which the maximum
ꢀ
on the acceptor moieties, including benzothiadiazole and degradation rates of four compounds occurred at 525 C,
ꢀ
ꢀ
ꢀ
benzoselenadiazole. This indicates that ICT occurred from 469 C, 511 C and 464 C, respectively. The data revealed the
the ground state to the excited state.
2
2,26
The substitution good thermal stability and high temperature resistance
position inuenced the band gap signicantly, but the of these four compounds. Weight loss in the case of BPC-2BTD
changes from BTD to BSe as the acceptor unit had only and BPC-3BTD was nearly 90%, whereas in the case of
a slight inuence on the band gaps. Additionally, the 3- BPC-2BSe and BPC-3BSe, the weight losses were approximately
ꢀ
position substitution increased the HOMO value of the ob- 70% at 800 C. The residues of BPC-2BSe and BPC-3BSe
tained compound. The replacement of BTD by BSe increased remained at approximately 30% of their original
the HOMO and LUMO levels simultaneously. The HOMO and weight, higher than that of BPC-2BTD and BPC-3BTD. This
LUMO values obtained from the theoretical calculation are might be due to the formation of SeO during the decompo-
2
2
comparable with the data from experiments (Table 1), which sition of BPC-2BSe and BPC-3BSe. SeO does not evaporate,
g
Table 4 The electrochemical and optical property data of compounds, include Eonset, lmax, lonset, E , HOMO and LUMO
E
onset
,
a
b
c
d
g
d
d
Compounds
vs. (Ag-wire) (V)
l
max (nm)/lonset (nm)
E
g
(eV)
HOMO (eV)
LUMO (eV)
E
(eV)
HOMO (eV)
LUMO (eV)
BPC-2BTD
BPC-2BSe
BPC-3BTD
BPC-3BSe
1.22
1.20
1.06
1.05
318, 414/480
327, 438/507
298, 433/500
299, 460/533
2.59
2.45
2.48
2.33
ꢂ5.64
ꢂ5.62
ꢂ5.48
ꢂ5.47
ꢂ3.05
ꢂ3.17
ꢂ3.00
ꢂ3.14
2.63
2.63
2.42
2.44
ꢂ5.51
ꢂ5.46
ꢂ5.21
ꢂ5.18
ꢂ2.88
ꢂ2.83
ꢂ2.79
ꢂ2.74
a
b
Band gap calculated from lonset,
E
g
¼ 1241/lonset
.
Calculated from onset oxidation potentials of the compounds, HOMO ¼ ꢂe(Eonset + 0.02 + 4.4).
d
g g
c
Estimated using empirical equations LUMO ¼ HOMO + E
.
E
, HOMO and LUMO are calculated based on density functional theory (DFT).
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4
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results demonstrate that substitution of the acceptor at the 3-
position of N-phenylcarbazole is benecial to the co-planarity of
the obtained molecule reduced the band gaps of the molecules
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replacement of BTD by BSe as the acceptor unit could also
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and red lights upon irradiation, which are valuable primary
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