Synthesis of novel substituted methoxybenzo[2,3-b]carbazole derivatives via C-H functionalization. Experimental and theoretical characterization of their photophysical properties

Article abstract of DOI:10.1016/j.dyepig.2016.08.012

The novel, highly blue, fluorescent N-methyl-6,11-dimethoxybenzo[2,3-b]carbazole derivatives were prepared by oxidative coupling of anilines with naphthoquinone followed by palladium catalyzed oxidative [Formula presented] functionalization and a one pot

Full text of DOI:10.1016/j.dyepig.2016.08.012

Accepted Manuscript
Synthesis of novel substituted methoxybenzo[2,3-b]carbazole derivatives via C-H
functionalization. Experimental and theoretical characterization of their photophysical
properties
Cinthia da Silva Lisboa, Nanci C. de Lucas, Simon J. Garden
PII:
S0143-7208(16)30395-3
10.1016/j.dyepig.2016.08.012
DYPI 5392
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 25 May 2016
Revised Date: 1 August 2016
Accepted Date: 5 August 2016
Please cite this article as: Lisboa CdS, de Lucas NC, Garden SJ, Synthesis of novel substituted
methoxybenzo[2,3-b]carbazole derivatives via C-H functionalization. Experimental and theoretical
characterization of their photophysical properties, Dyes and Pigments (2016), doi: 10.1016/
j.dyepig.2016.08.012.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Synthesis of Novel Substituted Methoxybenzo[2,3-b]carbazole Derivatives via C-H
Functionalization. Experimental and Theoretical Characterization of their
Photophysical Properties.
Cinthia da Silva Lisboa, Nanci C. de Lucas, Simon J. Garden*
Instituto de Química, Universidade Federal do Rio de Janeiro, Cidade Universitária
CT bloco A, RJ 21941-909, Brazil
*
Contact information, email: garden@iq.ufrj.br
Keywords: palladium catalysis; oxidative C-H functionalization; benzo[2,3-
b]carbazoles; UV-vis spectroscopy; time resolved fluorescence spectroscopy; density
functional theory calculations.
Abstract
The novel, highly blue, fluorescent N-methyl-6,11-dimethoxybenzo[2,3-
b]carbazole derivatives were prepared by oxidative coupling of anilines with
naphthoquinone followed by palladium catalyzed oxidative C-H functionalization and
a one pot reductive methylation reaction. The benzocarbazole fluorophores were
photophysically characterized and the effects of the substituents investigated. (TD)-
DFT calculations faithfully reproduced experimental photophysical properties of the
benzocarbazoles and revealed how the substituents affect the photophysical properties
of these compounds.
1

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1. Introduction
Carbazole derivatives are an important class of nitrogen containing
heterocyclic compounds that are widespread in nature and have been found to have
useful applications. Since the isolation of carbazole in 1872 by Graebe and Glazer,[1]
and later the description of the isolation and antimicrobial properties of murrayanine,
a naturally occurring carbazole alkaloid from Murraya koenigii Spreng,[2] the
intriguing structural features and promising pharmacological activities of these natural
products have attracted the attention of numerous research groups. The interest in this
heterocyclic system has led to the isolation of new natural compounds and an
enormous effort to develop synthetic methodology for the synthesis of novel
derivatives.[3] The carbazole structure is found in a wide range of compounds that
possess a diverse array of pharmacological activities, including anti-tumor,
antibacterial, anti-parasitic, antifungal, anti-inflammatory, and as treatments for
neurological disorders.[3a, 3b, 4]
In addition to the interesting pharmacological properties of carbazole
derivatives, carbazole derivatives have been increasingly attracting attention in terms
of their electrical and optical properties.[5] Applications of carbazole derivatives
include for example: use in organic light emitting diodes,[5m, 6] organic
photorefractive materials,[7] optical sensors,[8] as an active semiconductor layer in
organic field effect transistors,[9] as a fluorescent switch,[10] or as components of
dye-sensitized solar cells.[6h, 11]
The present study details the synthesis and photophysical characterization of
novel 6,11-dimethoxybenzo[b]carbazole derivatives derived from benzocarbazol-
6,11-diones[12] and comparison is made with the known properties of carbazole[13]
2

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and other derivatives of the parent heterocyclic system benzo[2,3-b]carbazole.[13a,
14]
2
2
. Experimental Section
.1 General
All reagents were purchased from Sigma-Aldrich and used as received.
Spectroscopic grade solvents were used for the photophysical characterization of the
1
13
benzocarbazole derivatives. H and C NMR spectra were recorded using either a
Bruker 200 or 300 MHz spectrometer with TMS as an internal standard. Mass spectra
(ESI:HRMS) were obtained with a Mass Spectrometer Q-TOF Micromass, in positive
mode. Absorption spectra were recorded using Shimatzu UV-2450
a
spectrophotometer. Fluorescence experiments were performed using an Edinburgh
Instruments FS920 and a HITACHI F4500 spectrofluorimeter, as indicated. The
fluorescence lifetime was measured using an Edinburgh Instruments FL920 CD
pumped by an EPL 404.2 nm laser, also from Edinburgh Instruments. The excitation
and emission wavelengths ranged from 365 to 370 nm and 420 to 465 nm,
respectively.
2
2
.2 Photophysical characterization
.2.1 Measurement of Molar Absorption Coefficient. The molar absorption coefficient
(ε) was determined from a single weighed sample dissolved in a stock solution that
was subsequently diluted. The dilutions were made in such a way that the range of
-5
-6
absorbance was from 0 to 1 (~ 10 to 10 mol/L) at the maximum wavelength and
each measurement was made 5 times.
3

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2
.2.2 Fluorescence Quantum Yields (
measured by the relative standard method,[15] in air saturated solutions, using
perylene as a reference standard ( = 0.73 in cyclohexane[16]) and a FS920
f
Φ ). The fluorescence quantum yields were
Φ
f
Edinburgh Instruments spectrofluorimeter with a Xenon Arc Lamp (Xe900, 450 W)
and a TMS300 monochromator. In this method, the Φf of the sample were calculated
from equation 1.
2
2
Φf X
=
Φf ST x (G /G )(η /η )
ST
Eq. 1
X
ST
X
The subscripts ST and X denote standard and sample, respectively; Φf is the
fluorescence quantum yield, G the slope from the plot of integrated area of the
fluorescence intensity versus absorbance and η the refractive index of the solvent (for
an example see SI Figure 65). The sample and the reference were excited at the same
wavelength, the same range of emission wavelengths was analyzed and both samples
were maintained under the same experimental conditions.
For each
f
Φ determination, the fluorescence spectra of isoabsorptive solutions
(five samples in the absorbance range of 0.01 to 0.1 at the same wavelength) of the
sample in an appropriate solvent (acetonitrile, dichloromethane, methanol and
cyclohexane) relative to a standard solution of perylene in cyclohexane, were
recorded using a 10 mm quartz cuvette.
2.2.3 Fluorescence Lifetime (τ) Measurements. Solutions with an absorbance of 0.1-
0.5 at the absorption wavelengh (404.2 nm) were placed in quartz cuvettes.
Fluorescence decay profiles of air saturated and argon-deoxygenated (15 min)
solutions were recorded with the use of a fluorescence lifetime spectrometer FL920
from Edinburgh Instruments. Decays were monitored at the corresponding emission
4

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maxima of the compounds. The FAST software, Edinburgh Instruments, allowed the
fitting of the decay spectra and yielded the fluorescence lifetimes.
2.3 Theoretical calculations
The theoretical calculations were performed using Gaussian 09 rev. C.01.[17]
The density functional methods B3LYP,[18] PBE0,[19] CAM-B3LYP,[20] ωB97X-
D[21] were used in conjunction with Pople’s double-ζ basis set augmented with
diffuse functions and polarized with d orbitals.[22] Previous studies substantiate the
effectiveness of this basis set for calculation of UV-vis spectra.[23]
2.4 Synthesis and Characterization of 5. The methyl benzo[b]carbazolediol ethers 5
were prepared as shown in scheme 1. The substituents are detailed in Table 1. The
first step was an oxidative coupling of naphthoquinone with anilines to give
anilinonaphthoquinones (3), using catalytic quantities of copper acetate with acetic
acid as the solvent.[24] The benzo[b]carbazolediones 4 were prepared by oxidative
cyclization of 3 in an analogous fashion to a literature procedure.[25] The final step
involved the one pot reduction and methylation using sodium dithionite and methyl
iodide adapting a previously reported procedure.[26]
2
.4.1 Preparation of compounds 4
-Anilino-1,4-naphthoquinone 3 (0,5 mmol), Pd(OAc)
Cu(OAc)2. O (20 mol%), K CO (20 mol%) and pivalic acid (2 g) were added to a
2
2
(10 mol%),
H
2
2
3
round bottomed flask and closed with a septum to which an oxygen filled balloon was
connected. The reaction mixture was heated at 130°C under an O atmosphere for 14-
2
20 hours. The substrate consumption was monitored by TLC and after complete
5

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consumption the volatiles were removed under reduced pressure. The resulting crude
product was taken up in a minimal volume of CH
2
Cl
2
and applied to a column of
Cl and an EtOAc
chromatography silica gel. The column was eluted with CH
2
2
gradient. The fractions containing the product 4 were combined and the volatiles
removed under reduced pressure.
2.4.2 Preparation of compounds 5
To the reaction flask were added: benzo[b]carbazolequinone 4 (0.25mmol),
Na S O (2mmol), Bu NBr (0.1-1mmol) and a mixture of THF/H O (1:1V/V, 10mL).
2
2
4
4
2
The reaction flask was closed with a septum but allowed to pressure equalize with the
atmosphere via an open syringe needle. The reaction was stirred at room temperature
for about one hour or until the reaction color had faded. Subsequently, NaOH
3
(10mmol) and an excess of CH I (1-2 mL) were added and the reaction was left
stirring at room temperature for another hour. The crude product was isolated by
partitioning the reaction between dichloromethane and water. The organic extracts
2 4
were dried over Na SO , filtered and evaporated onto silica. The silica adsorbed
product was applied to a short column of chromatography silica gel and the column
eluted with dichloromethane. Fractions containing the product were identified by
TLC, combined, and evaporated to give the isolated purified product.
2
2
3
1
.5 Spectroscopic structural characterization of compounds 5
-1
.5.1 6,11-Dimethoxy-5-methylbenzo[b]carbazole 5a: 81% yield; FT-IR (cm ; KBr):
062, 3030, 2987, 2931, 2850, 2831, 1633, 1604, 1462, 1444, 1394, 1363, 1296,
1
083, 997, 765, 744; H NMR (200MHz, CDCl ): δ 8.38 (3H, m); 7.48 (5H, m); 4.21
3
1
3
(
6H, s); 4.09 (3H, s); C NMR (50MHz, CDCl ): δ 147.2; 143.6; 134.9; 132.1; 127.2;
3
6

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1
6
26.9; 125.0; 123.4; 122.4; 122.3; 122.0; 121.4; 120.7; 119.3; 118.0; 107.7; 62.9;
+
1.2; 31.1; ESI:HRMS (m/z + H ): calculated 292.1332; found 292.1329
+
(C
19
H18NO
2
).
-1
2
.5.2 6,11-Dimethoxy-3,5-dimethylbenzo[b]carbazole 5b: 57% yield. FT-IR (cm ,
KBr):. 3074, 3062, 3000, 2929, 2850, 2831, 1633, 1604, 1461, 1442, 1429, 1396,
1
9
9
=
359, 1311, 1295, 1211, 1193, 1176, 1149, 1128, 1103, 1081, 1051, 1020, 995, 968,
1
50, 906, 838, 808, 769, 736, 707. H NMR (300MHz, CDCl
3
): δ 8.34 (1H, d, J =
.0); 8.27 (2H, m); 7.56 (1H, t, J = 6.0); 7.47 (1H, t, 6.0); 7.20 (1H, s); 7.15 (1H, d, J
13
9.0); 4.19 (6H, s); 4.08 (3H, s); 2.64 (3H, s); C NMR (75MHz, CDCl ): δ 146.6;
3
1
1
3
43.9; 137.2; 134.7; 132.1; 126.8; 124.6; 124.3; 122.9; 122.2; 121.8; 120.6; 120.5;
+
18.8; 117.9; 108.0; 77.2; 62.8; 61.0; 30.8; 22.0; ESI:HRMS (m/z ): calculated
+
05.1410; found 305.1414 (C20
H19NO
2
).
-1
2
.5.3 6,11-Dimethoxy-2,5-dimethylbenzo[b]carbazole 5c: 60% yield. FT-IR (cm ,
KBr):. 3064, 3023, 2987, 2933, 2856, 2832, 1737, 1633, 1616, 1583, 1490, 1463,
1
1
444, 1407, 1390, 1361, 1338, 1294, 1241, 1230, 1218, 1189, 1166, 1157, 1145,
1
108, 1081, 1051, 1006, 970, 944, 883, 848, 798, 765, 746, 701, 692. H NMR
(
200MHz, CDCl ): δ 8.22 (2H, m); 8.11 (1H, s); 7.33 (3H, m); 7.16 (1H, d, J = 8.0);
3
1
3
4
1
1
3
.08 (3H, s); 4.06 (3H, s); 3.96 (3H, s); 2.50 (3H, s); C NMR (50MHz, CDCl ): δ
3
47.5; 142.2; 135.2; 132.8; 129.0; 128.3; 127.5; 125.2; 123.8; 122.6; 122.4; 122.4;
21.8; 120.9; 118.2; 107.7; 63.1; 61.3; 31.3; 21.5; ESI:HRMS (m/z): calculated
+
05.1410; found 305.1406 (C20
H19NO
2
).
7

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-1
2
.5.4 4,6,11-Trimethoxy-5-methylbenzo[b]carbazole 5d: 84% yield. FT-IR (cm ,
KBr): 3066, 2990, 2953, 2931, 2832, 1629, 1585, 1456, 1360, 1308, 1254, 1082, 766,
1
7
34. H NMR (200MHz, CDCl
3
): δ 8.33 (2H, t, J = 8.0); 8.09 (1H, d, J = 6.0); 7.53
(3H, m); 7.25 (1H, t, J = 8.0); 7.07 (1H, d, J = 6.0); 4.49 (3H, s); 4.18 (3H, s); 4.08
1
3
(
3H, s); 4.04 (3H, s); C NMR (50MHz, CDCl ): δ 147.3; 147.0; 135.8; 133.5; 127.6;
3
125.2; 123.9; 122.8; 122.7; 122.3; 121.1; 120.4; 118.8; 116.6; 109.5; 62.9; 61.6; 56.1;
+
+
3
3
4.6; ESI:HRMS (m/z + H ): calculated 322.1438; found 322.1423 (C20H20NO ).
-1
2
.5.5 3,6,11-Trimethoxy-5-methylbenzo[b]carbazole 5e: 81% yield. FT-IR (cm ,
1
KBr): 3061, 2997, 2926, 2849, 2833, 1605, 1460, 1359, 1296, 1231, 1080, 769. H
3
NMR (200MHz, CDCl ): δ 8.29 (2H, m); 8.27 (1H, d, J = 8.0); 7.51 (2H, m); 6.92
(1H, dd, J = 2.0; J = 8.0); 6.85 (1H, d, J = 2.0); 4.16 (3H, s); 4.15 (3H, s); 4.06 (3H,
1
3
s); 3.98 (3H, s); C NMR (50MHz, CDCl
3
): δ 160.3; 146.3; 145.5; 135.1; 132.7;
1
5
26.8; 124.9; 124.4; 122.8; 122.7; 122.1; 120.9; 118.2; 115.3; 107.1; 93.2; 63.2; 61.3;
+
5.8; 31.3; ESI:HRMS (m/z + H ): calculated 322.1438; found 322.1428
+
(C
20
H20NO
3
).
-1
2
.5.6 2,6,11-Trimethoxy-5-methylbenzo[b]carbazole 5f: 76% yield. FT-IR (cm ,
KBr): 3065, 2991, 2934, 2903, 2848, 2831, 1634, 1618, 1609, 1584, 1488, 1363,
1
1
083, 1000, 765, 744, 459. H NMR (200MHz, CDCl ): δ 8.21 (1H, d, J = 8.0); 8.13
3
(1H, d, J = 8.0); 7.84 (1H, d, J = 2.0); 7.39 (2H, m); 7.15 (1H, d, J = 2.0); 7.08 (1H,
13
dd; J = 2.0; J = 8.0); 4.06 (6H, s); 3.93 (3H, s); 3.87 (3H, s); C NMR (50MHz,
CDCl
3
): δ154.0; 147.5; 138.9; 135.2; 133.0; 127.5; 125.3; 122.5; 122.3; 122.2; 120.9;
+
1
3
18.2; 115.3; 108.6; 107.7; 63.1; 61.6; 56.4; 31.4; ESI:HRMS (m/z + H ): Calculated;
+
22.1438; found 322.1446 (C H NO ).
2
0
20
3
8

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-1
2
.5.7 1,5-Dimethyl-4,6,11-trimethoxybenzo[b]carbazole 5g: 80% yield. ; FT-IR (cm ,
1
KBr): 3067, 2991, 2933, 2832, 1360, 1291, 1259, 1087, 1062, 971, 766. H NMR
(
200MHz, CDCl ): δ 8,30 (2H, m). 7.49 (2H, m); 7.01 (1H, d, J = 8.0); 6.96 (1H, d, J
3
1
3
=
8.0); 4.44 (3H, s); 4.05 (3H; s); 4.01 (3H, s); 3.98 (3H, s); 3.05 (3H, s); C NMR
): δ 147.2; 145.4; 136.0; 135.1; 134.9; 127.8; 127.4; 125.6; 123.7;
23.3; 123.1; 123.0; 123.0; 121.1; 119.7; 110.4; 63.6; 62.6; 56.6; 35.2; 22.8
(50MHz, CDCl
3
1
+
+
3
ESI:HRMS (m/z + H ): calculated 336.1594; found 336.1579 (C21H22NO ).
2.5.8 2-Chloro-6,11-dimethoxy-5-methylbenzo[b]carbazole 5h: 41% yield. FT-IR
-1
(
cm , KBr): 3311, 3066, 2991, 2934, 2833, 1636, 1604, 1482, 1456, 1361, 1339,
1
1
297, 1083, 999, 799, 765, 698, 657. H NMR (200MHz, CDCl
3
): δ 8.33 (1H, d, J =
13
2
.0); 7.27 (1H, m); 7.52 (4H, m); 7.29 (1H, d, J = 4.0); 4.17 (6H, s); 4.07 (3H, s); C
NMR (50MHz, CDCl ): δ147.8; 142.3; 135.4; 132.6; 127.9; 127.1; 125.7; 124.9;
3
123.2; 123.0; 122.9; 122.6; 122.5; 121.0; 117.3; 108.9; 63.3; 61.7; 31.5; ESI:HRMS
+
+
(
2
m/z + H ): calculated 326.0942; found 326.0933 (C19H17ClNO ).
2.5.9 6,11-Dimethoxy-5-methyl-3-trifluoromethylbenzo[b]carbazole 5i: 15% yield.
-1
FT-IR (cm , KBr): 3068, 3035, 2993, 2939, 2850, 2835, 2721, 1641, 1614, 1481,
1
1
462, 1444, 1367, 1328, 1292, 1168, 1103, 1083, 1060, 999, 948, 759. H NMR
(200MHz, CDCl
3
): δ 8.46 (1H, d, J = 8.0); 8.33 (1H, d, J = 8.0); 8.26 (1H, d, J = 8.0);
1
3
7
1
1
3
.53 (4H, m); 4.25 (3H, s); 4.18 (3H, s); 2.99 (3H, s); C NMR (50MHz, CDCl ): δ
3
48.3; 143.3; 135.5; 132.8; 129.2; 128.3; 126.0; 124.6; 123.8; 123.2; 122.8; 122.6;
+
21.1; 117.2; 116.3; 105.1; 63.4; 61.8; 31.5; ESI:HRMS (m/z + H ): calculated
+
2
60.1206; found 360.1196 (C H FNO ).
2
0
17
9

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2.5.10 3-Fluoro-6,11-dimethoxy-5-methyl-benzo[b]carbazole 5j: 53% yield of isomer
-1
mixture from which a sample of 5j was separated. FT-IR (cm , KBr): 3066, 2987,
2
1
929, 2850, 2834, 1614, 1592, 1492, 1459, 1444, 1429, 1396, 1361, 1342, 1297,
1
228, 1191, 1168, 1097, 1081, 1052, 998, 973, 958, 919, 825, 806, 765, 707. H
NMR (200MHz, CDCl
3
): δ 8.28 (3H, m); 7.51 (2H, m); 7.02 (2H, m); 4.14 (3H, s);
13
1
4
.06 (3H, s); C NMR (50MHz, CDCl
3
): δ [165.5; 160.6 ( JC-F 240Hz)]; 147.0; 145.0
(J
C-F 10Hz); 135.3; 132.7; 127.3; 125.3; 124.5 (JC-F 15Hz); 124.4; 123.0; 122.8; 122.2;
121.0; 117.9 (JC-F 15Hz); 107.2 (JC-F 20Hz); 95.4 (JC-F 25Hz); 63.3; 61.4; ESI:HRMS
+
2
(
m/z): calculated 309.1160; found 309.1166 (C H FNO ).
1
9
16
2.5.11 1-Fluoro-6,11-dimethoxy-5-methylbenzo[b]carbazole 5j’: 53% yield of isomer
-1
mixture from which a sample of 5j’ was separated. FT-IR (cm , KBr): 3066, 2989,
2
1
929, 2852, 2834, 1616, 1494, 1463, 1444, 1361, 1322, 1295, 1241, 1147, 1083,
1
052, 1008, 983, 968, 921, 867, 848, 790, 765, 736, 705, 690. H NMR (300MHz,
CDCl
3
): δ 8.37 (1H, d, J = 6.0); 8.22 (1H, d, J = 9.0); 7.51 (3H, m); 7.15 (1H, d, J =
1
3
6
.0); 6.98 (1H, dd, J = 9.0, J = 9.0); 4.20 (3H, s); 4.13 (3H, s); 4.05 (3H, s); C NMR
1
(
3
75MHz, CDCl ): δ [159.7; 156.4 ( JC-F 248Hz)]; 147.7; 146.4(JC-F 15Hz); 135.2;
1
32.3; 128.5(JC-F 7.5Hz); 127.8; 125.8; 123.2; 123.1; 123.0; 120.8; 116.2(JC-F 7.5Hz);
09.5(JC-F 15Hz); 106.6 (JC-F 23Hz); 106.3; 104.0; 104.0; 77.2; 63.6; 63.3; 31.8;
1
+
+
2
ESI:HRMS (m/z + Na ): calculated 332.1057; found 332.1062 (C19H17FNO Na ).
2.5.12 2-Fluoro-6,11-dimethoxy-5-methylbenzo[b]carbazole 5k: 79% yield. FT-IR
-1
(
cm , KBr): 2991, 2935, 2835, 1629, 1606, 1487, 1460, 1442, 1361, 1301, 1274,
1
1
082, 997, 937, 862, 806, 767, 748. H NMR (200MHz, CDCl ): δ 8.33 (2H, m); 8.12
3
1
0

ACCEPTED MANUSCRIPT
13
(1H, m); 7.53 (3H, m); 7.32 (1H, m); 4.19 (3H, s); 4.17 (3H, s); 4.08 (3H, s);
C
1
NMR (50MHz, CDCl
3
): δ [159.9; 155.2 ( JC-F 235Hz)]; 147.7; 140.2; 135.3; 133.0;
1
27.8; 125.6; 122.8; 122.4; 122.3; 122.1(JC-F 10Hz); 121.0; 117.8(JC-F 5Hz); [114.7;
14.2 (JC-F 15Hz)]; [109.8; 109.3(JC-F 25Hz)]; 108.4 (JC-F 5Hz); 63.1; 61.7; 31.4;
1
+
+
2
ESI:HRMS (m/z + H ): calculated 310.1238; found 310.1234 (C H FNO ).
1
9
17
3
3
. Results and Discussion
.1 Synthesis
The previously unknown 6,11-dimethoxybenzocarbazoles were prepared
through the sequence of reactions given in Scheme 1. The synthesis of the anilino-
naphthoquinones (3) has been described by our group.[24] These compounds were
obtained in good to excellent yields. The compounds 3 were then submitted to a
palladium(II) catalyzed oxidative cyclization.[25, 27] The meta-substituted substrates
2
3
(R = CH 3b, OCH 3e, CF 3i, and F 3j) could result in the synthesis of two
3
3
3
regioisomeric benzocarbazoldiones (4). However, the oxidative cyclization of 3e and
i gave products that were found to be single regioisomers (4e and 4i, respectively).
3
On the other hand, the substrates 3b and 3j each gave a mixture of two regioisomeric
products as determined by GC-MS analysis (4b/b’ and 4j/j’). In the former case only
4b was isolated upon chromatographic purification, as 4b’ was a very minor
component, and in the latter case the regioisomers, 4j/j’, could not be separated.
Reductive methylation of compounds 4 was performed in a one pot procedure.[26]
There are a very limited number of examples of reduction of compounds 4 in the
literature to give benzo[2,3-b]carbazole-6,11-diols. The use of zinc in acetic
anhydride results in the obtention of the benzo[b]carbazole-6,11-diol acetate
esters,[28] whilst reduction of 4 with zinc in AcOH[29] or NaBH in MeOH[10] gives
4
1
1

ACCEPTED MANUSCRIPT
the air unstable benzo[2,3-b]carbazole-6,11-diol derivatives which are rapidly
oxidized back to 4. Therefore the one pot reductive methylation of compounds 4
allowed the synthesis and facile isolation of the novel, highly blue, fluorescent 6,11-
dimethoxybenzo[2,3-b]carbazole derivatives (5). The reductive methylation of the
regioisomeric mixture 4j/j’ allowed the separation of the regioisomeric products 5j
1
13
and 5j’. The new compounds 5 were structurally characterized by H and C NMR,
FT-IR spectroscopy and high resolution mass spectrometry.
Scheme 1: Preparation of methoxybenzocarbazole derivatives (See Table 1 for
substituent details).
(#)
Table 1: Substituent details for derivatives (Deriv.) of compounds 1, 3, 4, and 5.
1
2
3
4
1
2
3
4
Deriv.
R
R
R
R
Deriv.
R
R
R
R
a
H
H
H
H
H
Me
H
H
H
H
H
f
H
OMe
H
H
H
OMe
H
H
Me
H
b
b’
c
g
h
i
H
Me
H
H
Cl
H
Me
H
CF
3
H
H
1
2

ACCEPTED MANUSCRIPT
d
e
OMe
H
H
OMe
H
H
H
H
H
H
j
H
H
H
F
H
H
H
H
F
H
F
j’
k
e’
H
OMe
H
(
#)
For compounds 1 and 3, b = b’, e = e’, j = j’. Compounds 4 (b,b’) and (j,j’) were obtained as
mixtures of two regioisomers resulting from the oxidative cyclization of 3b and 3j respectively. The
purification of 4b,b’ resulted in the isolation of only 4b (the principal regioisomer of the mixture). The
regioisomers 4j/j’ could not be separated and the mixture was used directly in the reductive
methylation reaction. The regioisomers 5j,j’ were separated and individually characterized. In
principal, the oxidative cyclization of 3e could give rise to a regioisomeric mixture 4 (e,e’). However,
the product 4e was obtained as a single regioisomer and this was transformed into 5e. Compound 5e’
was modeled in the theoretical study but has not been synthesized. Similarly 3i could have given a
mixture of two regioisomers, but a single regioisomer corresponding to 4i was obtained which was
subsequently transformed into 5i.
3.2 UV-vis absorption spectroscopy
The UV-vis absorption and emission spectra of compounds 5 employed in this
study were recorded in dilute solution using four solvents with different polarities:
cyclohexane (Cyh), dichloromethane (DCM), acetonitrile (ACN) and methanol
(MeOH) at room temperature.
The absorption spectra of compounds 5 show strong absorption bands at short
wavelengths (270 - 300 nm), a weak band at slightly longer wavelengths (300 - 330
nm), and a weak band that extends into the visible region (350 - 430 nm). Figure 1
shows the absorption spectra for 5a in four solvents of different polarities. The
longest wavelength absorption band exhibits vibronic structure. A discrete
bathochromic effect (0-2 nm) was observed when the solvent was changed from
cyclohexane to acetonitrile (5a, figure 1). Although, in general, the UV-vis spectra
2 2
obtained in CH Cl revealed a marginally larger red shift relative to cyclohexane (3-4
1
3

ACCEPTED MANUSCRIPT
nm, Table 2, see SI tables 2-13). Table 2 details values of λmax and ε for the most
intense and the lowest energy absorption bands for compounds 5 in cyclohexane as
0 1
well as λmax for the S -S transition in the other solvents. Bonesi et al[13b] have
observed a small bathochromic shift of the absorbance spectra with increasing solvent
polarity for carbazole derivatives. Such a behavior is consistent with the π→π* nature
of the electronic transition associated with the first excited state. In comparison to the
absorption spectrum of the parent benzo[2,3-b]carbazole (6), the spectra of 5 reveal a
bathochromic shift for the longest wavelength band (aprox. 30 nm) and a considerable
reduction in intensity of the intermediate band.[13a]
The absence of a significant solvatochromic effect is not unusual for
polyaromatic hydrocarbons of intermediate polarity (possessing few heteroatoms
within a structure that is predominantely hydrocarbon).[13b, 15b] That both apolar
and polar solvents result in very similar absorption spectra must reflect differing
extents of interaction (electrostatic, hydrogen bonding and or other permanent dipole-
dipole interactions, as well as dispersion forces) of the solvent with the HOMO and
LUMO orbitals, where the net effects with respect to the HOMO-LUMO gap result in
a similar difference in energy for extremes of solvent polarity. In this context it is
perhaps therefore not surprising that a solvent of intermediate polarity could therefore
result in a larger bathochromic shift of the π→π* excitation in comparison to a more
(a)polar solvent.
1
4

ACCEPTED MANUSCRIPT
Figure 1: Normalized absorption spectra for 5a in DCM, ACN, Cyh, and MeOH.
-5
-6
-4
Concentration variation from 10 to 10 mol/L and around 10 mol/L at expansion.
The data in Table 2 reveals how both the nature (Figure 2, part A - variation of
3
2
the substituent R , part B - variation of the substituent R ) and the position (Figure 3,
fixed the nature of the substituent and varied its position) of substituents have an
3
effect on the absorption spectra. Variation of the substituent R (Figure 2A) resulted
in a bathochromic shift of λmax for 5c (Me 6 nm), 5f (OMe 16 nm), 5h (Cl 9 nm), 5k
(F 11 nm) for the first excited state relative to the unsubstituted 5a. In contrast,
2
variation of R (Figure 2B) resulted in a hypsochromic shift for 5b (Me 3 nm), 5e
(OMe 14 nm), and 5j (F 10 nm) whilst a bathochromic shift was observed for 5i (CF
3
1
1
1 nm). The introduction of a MeO group at position R gave bathochromic shifts for
1
5

ACCEPTED MANUSCRIPT
4
4
5
d (4 nm) and 5g (R =Me, 12 nm) whilst a F substituent at R gave a small
hypsochromic shift (5l 2 nm). In a general manner, substituents with positive
1
3
mesomeric or sigma electron donating effects at positions R and R result in a
2
bathochromic shift whilst these same substituents at position R resulted in a
hypsochromic shift, with the exception of 5g, whilst the strongly electron
2
withdrawing substituent of 5i (R = CF
3
) has the opposite effect.
1
6

ACCEPTED MANUSCRIPT
Figure 2: A) Normalized absorption spectra of 5a and compounds 5 with a substituent
3
at the R position in cyclohexane. B) Normalized absorption spectra of 5a and
2
compounds 5 with a substituent at the R position in cyclohexane. Concentration
-5
-6
variation from 10 to 10 mol/L.
1
7

ACCEPTED MANUSCRIPT
Table 2: Photophysical absorption and emission parameters of 5.
a
λmax Abs (nm, Cyh)
Comp.
Sub.)
λmax Em (nm),
a
S -S λ Abs (nm)
0
1
max
4
-1
-1 b
a,c
(
(ԑ
10 .L.mol cm )
(Stokes shift, nm)
2
2
2
35, 274 (5.42), 283, 297,
21, 384, 405 (0.55)
36, 274 (2.52), 284, 299,
409 (DCM)
413 (8 Cyh), 422 (13 DCM)
5
a
3
406 (ACN); 405 (MeOH) 423 (17 ACN), 420 (15 MeOH)
405 (DCM) 411 (9 Cyh), 419 (14 DCM)
404 (ACN); 401 (MeOH) 422 (18 ACN), 418 (17 MeOH)
414 (DCM) 421 (11 Cyh), 428 (14 DCM)
412 (ACN); 410 (MeOH) 431 (19 ACN), 426 (16 MeOH)
412 (DCM) 418 (9 Cyh), 431 (19 DCM)
5b
324, 339, 381, 402 (0.26)
37, 270 (8.01), 301, 323,
5
c
3
89, 410 (0.86)
82 (18.6), 322, 388,
09 (1.03)
72, 281, 302 (6.21), 327,
42, 374, 392 (0.60)
73 (4.49), 287, 309, 325,
99, 421 (0.48)
86 (7.72), 308, 324, 396,
17 (0.42)
36, 269 (4.23), 304, 322,
37, 391, 413 (0.43)
69 (6.56), 278, 299, 321,
35, 394, 416 (0.64)
32, 268 (4.72), 278, 299,
2
5d
4
409 (ACN); 407 (MeOH) 433 (24 ACN), 429 (22 MeOH)
d
2
2
2
2
2
2
2
2
396 (DCM)
402 (11 Cyh) , 411 (15 DCM)
5
e
3
393 (ACN); 392 (MeOH) 413 (20 ACN), 409 (17 MeOH)
425 (DCM) 431 (10 Cyh), 443 (18 DCM)
422 (ACN); 421 (MeOH) 445 (23 ACN), 441 (20 MeOH)
418 (DCM) 427 (10 Cyh), 443 (25 DCM)
416 (ACN); 414 (MeOH) 445 (29 ACN), 440 (26 MeOH)
417 (DCM) 421 (8 Cyh), 431 (14 DCM)
415 (ACN); 414 (MeOH) 432 (17 ACN), 428 (14 MeOH)
419 (DCM) 426 (10 Cyh), 437 (18 DCM)
417 (ACN); 416 (MeOH) 440 (23 ACN), 437 (21 MeOH)
398 (DCM) 404 (9 CH), 411 (13 DCM)
398 (ACN); 396 (MeOH) 413 (15 ACN), 410 (14 MeOH)
406 (DCM) 411 (8 Cyh), 418 (12 DCM)
405 (ACN); 404 (MeOH) 421 (16 ACN), 417 (13 MeOH)
419 (DCM) 425 (10 Cyh), 434 (15 DCM)
417 (ACN); 416 (MeOH) 435 (18 ACN), 431 (15 MeOH)
5
f
3
5
5
g
4
h
3
5
i
3
5
j
3
21, 337, 375, 395 (0.42)
75 (17.3), 305, 319, 381,
5j’
4
03 (1.23)
36, 270 (5.85), 287, 302,
21, 394, 415 (0.72)
5k
3
a
Cyclohexane (Cyh), dichloromethane (DCM), acetonitrile (ACN) or methanol (MeOH) as
solvent.
1
8

ACCEPTED MANUSCRIPT
b
Molar extinction coefficients of the most intense band and the lowest energy band,
respectively, in cyclohexane.
c
Excitation at 370 nm and emission range from 390-600 nm;
d
Excitation at 365 nm and emission range from 380-600 nm.
Figure 3: Normalized absorption spectra of 5a and compounds 5 substituted with
-5
-6
methoxy groups in cyclohexane. Concentration variation from 10 to 10 mol/L.
The large bathochromic shift of the lowest excited state of benzo[2,3-
b]carbazole (6) relative to the heteroatom free benzo[2,3-b]fluorene has been
attributed to the facile displacement of electron density from nitrogen to the naphthyl
system on excitation, thus resulting in an excited state structure for which an extended
1
9

ACCEPTED MANUSCRIPT
quinoid type canonical form was proposed as an important structural
contribution.[13a] In principal, the compounds of the current study could present
somewhat similar excited state structures (Figure 4). Using this model allows for an
1
3
interpretation of how electron donating substituents R (ortho to nitrogen) and R
(para to nitrogen) of the phenylene ring system can contribute to stabilize, and hence
1
red shift, S relative to the substituent free 5a. The hypsochromic shift when electron
2
4
donating substituents are meta to the nitrogen (R and/or R ) is better rationalized
through consideration of the resonance model (7). At the same time model 7 may
rationalize the large bathochromic shift for strongly electron withdrawing CF (5i) at
3
2
position R .
Figure 4: Resonance structure representation of a potential extended quinoid type
1
canonical form contribution to the structure of the S excited state of 5 and 6, and a
resonance model representation (7) of conjugated electron density delocalization.
3.3 Fluorescence spectroscopy
Figure 5 shows normalized absorption, excitation and emission spectra for 5a
in cyclohexane. The excitation and emission spectra display excellent mirror
symmetry with only a small Stokes shift separating the absorption and emission
2
0

ACCEPTED MANUSCRIPT
maxima. Additionally, the excitation spectrum reveals that some of the higher excited
1
states can contribute to emission through internal conversion to the S
1
state. The
fluorescence spectra of compounds 5 occur in the range of 400-500 nm and reveal
vibrational structure (see also SI Figures of normalized emission spectra of 5 in the
SI). Figure 6 shows the normalized emission spectra for 5a in four solvents of
different polarities (λmaxEm: Cyh 413nm, MeOH 420nm, DCM 422nm, ACN
423nm). A minor red shift of the emission spectra in differing solvents relative to
emission in cyclohexane is observed and it is notable that emission in the protic
solvent MeOH is blue shifted relative to the less polar CH Cl or similarly polar
2
2
ACN. The greater stability of the relaxed excited state in the polar solvents is
reflected by the larger Stokes shift (Table 2) for emission in the polar solvents (Cyh 8
nm, MeOH 15 nm, CH
Table 3, τ).[15b] The observations are consistent with the view that the excited state
has a slightly larger dipole moment than the ground state.
2 2
Cl 13 nm, ACN 17 nm) and longer fluorescence lifetimes
(
2
1

ACCEPTED MANUSCRIPT
Figure 5: Normalized absorption, excitation and emission spectra of 5a in
cyclohexane.
2
2

ACCEPTED MANUSCRIPT
Figure 6: Normalized emission spectra for 5a: cyclohexane, methanol,
dichloromethane, acetonitrile (λexc = 370 nm).
Figure 7 reveals the effect of the variation of the nature of the substituent at a
fixed position. Figure 7A compares the emission spectra, recorded in cyclohexane, of
3
5
a with the spectra recorded for derivatives with substituents at the R position
(methyl 5c, methoxy 5f, chlorine 5h and fluorine 5k) and Figure 7B compares the
2
emission spectra for compounds 5 with a substituent at the R position (methyl 5b,
methoxy 5e, trifluoromethyl 5i and fluorine 5j) with 5a. Figure 8 compares the
emission spectra of 5a with the methoxy substituted derivatives 5d-g where the
position of the substituent (MeO group) is varied.
2
3

ACCEPTED MANUSCRIPT
Figure 7: Part A. Normalized emission spectra of 5a and derivatives with a substituent
3
at R in cyclohexane (λ = 370 nm). Part B. Normalized emission spectra of 5a and
exc
2
derivatives with a substituent at R in cyclohexane (λexc = 370 nm).
2
4

ACCEPTED MANUSCRIPT
Figure 8: Normalized emission spectra of 5a and compounds with methoxy
substituent in cyclohexane. For 5a, 5d, 5g and 5f (λexc = 370 nm) and for 5e (λexc
65 nm).
=
3
The fluorescence emission spectra of the substituted benzocarbazole
derivatives 5 preserve the substituent effect observed in the absorption spectra and
values of λmax emission are linearly red shifted relative to λmax for absorption in
cyclohexane (Figure 9). The change of solvent from cyclohexane to acetonitrile
results in a greater variation of the larger Stokes shifts, although a similar tendency is
observed to cyclohexane as solvent. The subtle differences in stabilization of both S0
1
and S for each compound in the polar solvent may be responsible for the greater
variation of the observed Stokes shifts (Figure 9).
In comparison with substituted carbazole derivatives,[13b] the emission from
compounds 5 is more than 50 nm red shifted whilst comparison with the parent
2
5

ACCEPTED MANUSCRIPT
heterocycle 6[14a] reveals that the introduction of the 6,11-dimethoxy substituents
results in an approximately 30 nm red shift in the emission spectra. Further, the
emission maxima of 5 are very similar to the recently reported 6,11-
diphenylbenzo[2,3-b]carbazole derivatives.[14b]
Figure 9. Graphical analysis of emission λmax against absorption λmax for 5.
3
.4 Quantum yield and fluorescence lifetime measurements
Table 3 details the quantum yields for 5 measured in four air saturated
solvents of different polarities. The fluorescence quantum yield (
f
Φ ) for 5a is similar
to that of N-methylcarbazole ( 0.48 deoxygenated Cyh, 0.37 air saturated Cyh)[13b]
Φ
f
and to 6,11-diphenylbenzo[2,3-b]carbazole (Φf 0.50 in DCM).[14b] Further, the
fluorescence quantum yields for the derivatives of 5 vary in function of the
substituents and the solvents (Table 3). Within the error of the experiment, the
fluorescence quantum yields in MeOH are systematically smaller than in the non-
protic solvents and
Φ
f
in CH
2
Cl
2
is generally the largest or similar to
Φ
f
in
1
4
cyclohexane. The smallest values of
Φ
f
are observed with 5g (R =MeO,R =Me) and
2
6

ACCEPTED MANUSCRIPT
3
5
f
h (R =Cl). The reduced Φ of 5g and 5h maybe due to enhanced internal conversion
as a result of the increased number of substituents with rotational freedom in the
former and possibly spin-orbit coupling in the latter. On the other hand the largest
values of Φf are observed for 5a (ACN) and 5b (CH Cl ). The fluorescence lifetimes
2
2
of 5 were measured using air and argon saturated samples. As can be seen for 5a in
figure 10 and for the other compounds in table 3, the presence of oxygen decreases
the respective fluorescence lifetime of each compound. The fluorescence quenching
by oxygen maybe due to a number of mechanistic possibilities.[30] Interestingly,
under air saturated conditions the longest fluorescence lifetimes are observed in
CH Cl . In the absence of oxygen the fluorescence lifetimes are slightly longer in the
2
2
polar ACN and MeOH solvents reflecting the increased stability of the more polar
lowest singlet excited state.
Figure 10: Fluorescence lifetime determination for 5a in cyclohexane for an air
saturated sample (—) and for a de-oxygenated, argon saturated, sample (⋅ ⋅ ⋅).
2
7

ACCEPTED MANUSCRIPT
Table 3: Fluorescence emission photophysical properties: Fluorescence quantum yield, lifetime, kf e Σknr of 5a-k.
Cyh
DCM
ACN
MeOH
a
b,c
d
d
a
b,c
d
d
a
b,c
d
d
a
b,c
d
d
Sample
Φ
f
τ (ns)
4.58
10.22)
k
f
∑knr
Φ
f
τ (ns)
k
f
∑knr
Φ
f
τ (ns)
k
f
∑knr
Φ
f
τ (ns)
k
f
∑knr
2
23.19
29.77
(9.76)
25.81
(9.27)
31.06
(9.88)
26.71
28.72
(11.13)
22.88
5a
0.46
0.40
0.37
0.39
0.35
0.30
0.23
4.50
4.15
3.61
4.17
4.39
3.03
2.62
5.28
6.22
6.15
6.52
8.15
7.08
8.77
0.41
0.57
0.48
0.44
0.39
0.42
0.31
2.76
5.29
3.04
3.14
4.49
2.79
2.69
3.97
3.99
3.29
3.99
7.02
3.86
5.98
0.55
0.42
0.28
0.40
0.33
0.39
0.26
5.64
4.53
2.83
3.96
4.04
3.76
2.83
4.61
6.26
7.29
5.94
8.20
5.88
8.06
0.39
0.34
0.28
0.39
0.27
0.31
0.25
3.50
3.59
2.50
3.56
2.97
2.67
2.51
5.48
6.98
6.42
5.58
8.03
5.95
7.52
(
(14.86)
21.98
2
0.62
5b
(
9.64)
(10.78)
27.56
(9.46)
29.78
2
4.59
5c
(
10.24)
(15.80)
22.01
(11.21)
25.24
2
1.82
5d
(
9.35)
(14.02)
12.06
(10.10)
19.13
(10.94)
18.33
1
4.26
7.98)
4.09
9.89)
7.6
8.78)
5e
(
(8.69)
27.12
(8.17)
30.98
(9.09)
30.71
2
5f
(
(15.04)
17.55
(10.37)
24.21
(11.60)
21.48
1
5g
(
(11.53)
(9.18)
(9.98)
2
8