Journal of Luminescence 33(1985)175—185
North-Holland, Amsterdam
175
HYDROGEN BONDING, POLARITY AND MATRIX EFFECTS ON
THE SPECTROSCOPIC AND PHOTOPHYSICAL PROPERTIES OF
SOME N- AND 3-SUBSTITUTED DERIVATIVES OF CARBAZOLES
Pierre D. HARVEY and Gilles DUROCHER
Département de Chimie, Université de Montréal, C.P. 6210, Succ. A. MONTREAL, Québec,
H3C 3V1
Original manuscript received 30 July 1984
Revised manuscript received 14 January 1985
Fluorescence spectra and photophysical properties of N-cyano and N-carboethoxy
carbazole derivatives have been compared to the equivalent 3-substituted compounds in
fluid polar and unpolar solutions and in a low temperature rigid matrix at 77 K. The
3-carboethoxy derivatives have been shown to form an excited state complex (exciplex) with
ethanol in fluid solutions. On the other hand the photophysical properties of the N-substituted derivatives are much affected by the solvent polarity in fluid solutions in that the
radiationless decay rate constants show a marked increase in these media. A tentative
explanation has been given in terms of the relative energy gap existing between the Lb and
the ‘La states of these molecules.
Introduction
Comparison of the fluorescence spectra of carbazole (C), N-ethylcarbazole
(NEC) and N,N’-dicarbazyl (NNDC) along with the temperature dependence
of the fluorescence spectrum of NNDC have recently been interpreted in terms
of intra-molecular geometric relaxation in the NNDC bichromophore [~1].We
have shown that the perpendicular geometry in the excited Franck—Condon
state is transformed to an oblique geometry in the emitting state without any
involvement of solvent induced charge transfer relaxation processes [2]. On the
other hand we have found that the fluorescence behavior of some N-substituted and 3-substituted derivatives of carbazole is much more affected by
the polarity of the solvent and this will be the object of the present paper.
Indolic compounds are well known to interact with polar solvents causing
pronounced red-shifted fluorescence emissions. These emissions have been
interpreted by the formation of excited complexes (exciplexes) between the
indolic compounds and the polar solvents [3] giving rise to solvated electrons
in water and ethanol [4]. One important characteristic of the indolic compounds is that their lowest electronic absorption band takes its origin in the
0022-2313/85/$03.30 © Elsevier Science Publishers B.V.
(North-Holland Physics Publishing Division)
176
P.Harvey, G. Durocher/ Hydrogen bonding, polarity and matrix effects
overlapping potential surfaces of the ‘La and tLh states [5]. It is then probable
that the exciplex forms preferentially in the ‘La state, which is a much more
polar state, before ejecting an electron upon relaxation to the ground state.
This assumption seems to be corroborated in the carbazole derivatives where
the energy gap between the ‘Lb and the 1L.~band is much more pronounced.
Carbazole with a gap of 4230 cmt [6] is primarily involved in hydrogen
bonding interaction when dissolved in ethanol. The 1-substituted derivatives
are mainly involved in intramolecular hydrogen bonding complexes or in cyclic
dimeric structure [7]. The absorption and photoelectron spectra of some Nand 3-substituted derivatives of carbazole in various media have already been
published [8] and it is the purpose of this paper to study their fluorescence
spectra along with their photophysical properties in the same media. It will be
shown that as the S
2—S1 energy gap is lowered in the carbazole derivatives
containing electron-withdrawing substituents, specific interactions are taking
place in the first excited singlet electronic states of these compounds dissolved
in fluid ethanol solutions.
Experimental
Materials
3-(Carboethoxy) carbazole (3-COOEC) and 3-cyanocarbazole (3-CNC) were
obtained by photochemical reaction and were purified as described in previous
papers [9]. 3-(Carboethoxy)—N-ethylcarbazole (3-COOENEC) and 3-cyano—N-ethylcarbazole (3-CNNEC) were synthesized by N-ethylation of the
corresponding N—H derivatives using the phase transfer catalytic reaction
described in the literature [10]. These compounds were purified by column
chromatography with silica gel and by using a mixture of petroleum ether-ethyl
ether as the developing system. The purity was checked by TLC. Carbazole
(C), N-ethyl-carbazole (NEC) and all the solvents used were purified as already
described [6—8].
Apparatus
Fluorescence and absorption spectra were recorded on a Spex Fluorolog
model 1902 [11]. Fluorescence lifetimes (TF) have been obtained by the
correlated single-photon counting technique. The full instrumental description,
experimental procedure and data processes are described elsewhere [12].
Experimental procedure
All fluorescence quantum yields (4F) and lifetimes have been obtained on
oxygen-free solutions by bubbling argon into the solutions. The rate and time
P. Harvey, G.Durocher/ Hudrogen bonding, polarity and matrix effects
177
of bubbling have been calibrated using the fluorescence lifetime of carbazole as
a standard and comparing with the results obtained from the freeze
—pump—thaw method. The fluorescence quantum yield of carbazole has been
measured using 9,10-diphenylanthracene as a standard [13]. Further on,
carbazole served itself as a standard for all the compounds studied in this
work.
Theoretical radiative deactivation rate constants (k~P)have been calculated
by the method of Birks and Dyson [14].The refractive index of the media as a
function of wavelengths and temperatures have been obtained as described
previously [2]. The radiative rate constants (kF) along with the radiationless
decay rate constants (knr) have been obtained through the following usual
equations:
kF=4F/TF and
kfl~=(1—q~)/T~.
The solute concentrations in all solvents were at about 10~ mol ë’~.
Results and discussion
(1) N-Substituted derivatives
The fluorescence spectra of N-carboethoxycarbazole and N-cyanocarbazole
in 3MP and ethanol are compared to those of carbazole and N-ethylcarbazole
in the same solvents in fig. 1. The spectroscopic properties of these molecules
have been shown in table 1. The vibrational fine structures obtained in 3MP is
preserved in ethanol in all cases even for carbazole which shows the most
important red-shifted emission (840 cm~)due to the well-known intermolecular hydrogen bonding interaction between the N—H proton and the hydroxyl
electronic lone pair of ethanol, Generally, the fluorescence spectra obtained in
both solvents exhibit mirror-image symmetry with the corresponding absorption band. The Stokes shift is small in 3MP and increases a little in the polar
ethanol solvent, being more pronounced for NEC. From table 1, it is clear that
NEC in its first excited electronic state is more affected than the other
N-substituted compounds, by the polarity of the solvent as judged by the ~
and ~
values. The ground states of all the N-substituted derivatives of
carbazole are practically not affected by the solvent polarity as judged by the
~
values. This is also confirmed by the theoretical radiative decay rate
constants listed in table 2 where the media, even the low temperature rigid
matrix (EPA), have practically no influence on the k~Pvalues.
The photophysical properties of the N-substituted derivatives of carbazole
are on the other hand very much dependent upon the nature of the substituent
groups as shown in table 2. N-ethyl substitution does not affect the fluorescence quantum yield nor the fluorescence lifetime compared to carbazole and
178
P. Harvey, G. Durocher/ Hydrogen bonding, polarity and matrix effects
~
Z
L~
L~
[\x~
3-
2
1
24
30
~
(10~
cm1)
Fig. 1. Top: fluorescence spectra of N-carboethoxy carbazole in 3MP(
(._ . —) along with the fluorescence spectra of N-ethylcarbazole in 3MP (
). Bottom: fluorescence spectra of N-cyano carbazole in 3MP
along with the fluorescence spectra of carbazole in 3MP (
)at23°C.
(—
) and in
) and in
—) and
) and in
ethanol
ethanol
ethanol
ethanol
these parameters are not either affected by the polarity of the solvent. The
electron-withdrawing substituents on the other hand decrease both the quantum yields and the fluorescence lifetimes of carbazole in nonpolar and polar
fluid solvents. Moreover, the solvent polarity decreases again ~F and TF in the
NCNC molecule. It is worth noting that these 4F and TF lowering phenomena
in NCOOEC and NCNC are viscosity or temperature dependent since a
value of about 0.4 is obtained in an EPA glass at 77 K. Since the kF values are
approximately constant for the various compounds in all media, this means
that the radiationless decay rate constants (knr) increase considerably in
179
P. Harvey, G.Durocher/ Hudrogen bonding, polarity and matrix effects
Table 1.
Spectroscopic properties of some N-substituted derivatives of carbazole in various media
Molecules
Experimental
conditions
1A(O,O)
(I Lb ~—‘A)
(cm~)
0’O)
~F(
(cm
I)
~°
~~12
~
~
(cm’)
(cm—I)
(cm~)
(cm
C
3MP,23°C 30240±60 29960±60 280
EtOH,23°C 29670±60 29120±50 550
EPA, 77K
29500±60 29210±50 290
2300
2460
1950
570
840
NEC
3MP,23°C
EtOH,23°C
29130±50 28900±60 230
29090±50 28570±60 520
2000
2280
40
330
NCOOEC
3MP,23°C
EtOH,23°C
EPA,77K
32150±60 31930±60 220
32130±60 31780±60 350
32280±60 31910±60 370
2000
2300
2100
20
150
NCNC
3MP,23°C
EtOH,23°C
EPA, 77K
32150±60 31760±60 400
32140±60 31760±60 380
32180±60 31890±60 290
2000
2100
2050
10
0
(a)
(h(
1)
Stokes shift defined as ~A(O.O)—
~F(°’°)•
Fluorescence band width measured at half-height.
“~ Spectral
(d( Spectral
red shift of the absorption (0,0) band between 3MP and ethanol.
red shift of the fluorescence (0,0) band between 3MP and ethanol.
NCOOEC and NCNC as shown in table 2. The reason for this behavior is not
yet known but it might be that for the electron-withdrawing substituent
containing molecules the percentage of charge transfer (1 L~character) in the S,
(1 Lh) wavefunction increases and contribute to the quenching of the pure ir~
state. Indeed the energy gap between the ‘Lb and the ‘La electronic states is
lower for the carboethoxy (3050 cm’) and for the cyano (3200 cm’)
derivatives compared to those of NEC (5000 cm ‘) and carbazole (4230 cm
as measured in the absorption spectra of these molecules in 3MP [6].
(2) 3-substituted derivatives
The fluorescence spectra of the 3-substituted derivatives are shown in fig. 2.
A nice vibrational structure is always obtained in 3MP and all these spectra
exhibit mirror—image symmetry with the corresponding absorption ‘Lb ‘A
band [7]. On the other hand, all spectra recorded in ethanol at room temperature show a loss in the vibrational structure resolution and moreover the
3-carboethoxy derivatives show a long tail to the red in the fluorescence
spectra and consequently a marked increase in the fluorescence band width
measured at half-height (see table 3) and no mirror—image relationship between the absorption and the fluorescence spectra. The fluorescence excitation
spectra have been recorded for different fluorescence wavelengths and in all
—
5)
.1
_______
0.24 ±0.01
0.093±0.004
0.40 ±0.01
—
0.29 ±0.02
0.24 + 0.01
—
+
0.5
7.7+0.5
4.1±0.5
9.7 ±0.5
7.5
7.7±0.5
15.5±0.5
31±3
23±4
41 ±3
43 ±5
44±4
42±4
38±4
62 ± 3
99± 7
220±30
45± 5
—
47±5
55 + 6
—
92± 8
100 + 10
32 + 3
38±5
28±2
-
25 ±2
26±2
-
32 ± 2
36± 3
32±3
37±4
37+4
k~~><
10
(s 1)
-
36±2
39± 2
37-4- 5
k~rX i0~
(s ‘ )
30 ±2
27±2
29±2
28+4
k, x 10~
(s I)
roughly
roughly
yes
roughly
roughly
yes
yes
yes
yes
yes
relationship
Mirror—image
All individual measurements were always included in the error bracket.
Obtained from an average of two measurements. The associated experimental error is the time scale used in the single-photon counting apparatus.
experimental measurements.
__________
EPA, 77K
3MP. 23°C
EtOH, 23°C
EPA, 77 K
3MP, 23°C
EtOH. 23°C
0.42 ±0.02
EtOH. 23°C
16.0±0.5
-
15.8 ±0.5
15.3±0.5
15.4+0.5
(ns)
0.36 ±0.03
14.1±0.5
26±3
yes
___________________________________________
___________
_______
______
Obtained from an average of three measurements. The associated experimental error is the half difference between the maximum and minimum
NCNC
NCOOEC
0.49 ±0.01
-
0.42 ±0.01
0.44 ±0.01
0.43 +0.01
~,-
3MP. 23°C
3MP. 23°C
EtOH. 23°C
EPA.77 K
C
NEC
Experimental
conditions
Molecules
Photophysical properties of some N-substituted derivatives of carhazole
Table 2
~
‘a-
‘a
a
“a
P. Harvey, G. Durocher/ Hudrogen bonding, polarity and matrix effects
181
A mm)
520
500
450
1
21
~
420
360
340
I
—
.,
I
~
OEt 2~4~~8~0
3~m~1)
0 (~)Q
Fig. 2. Fluorescence spectra of 3-CNNEC, 3-CNC, 3-COOENEC and 3-COOEC in ethanol
) and in 3MP (—)
at 23°C.
cases they were identical in wavelength and in relative intensity to the
corresponding absorption spectra. The 3-COOEC molecule shows a very small
distortion between the excitation and the absorption spectra but is definitely
182
P.Harvey, G. Durocher/ Hydrogen bonding, polarity and matrix effects
Table 3
Spectroscopic properties of some 3-substituted derivatives of carbazole in various media
Molecules
C
Experimental
~
0)
conditions
(‘L5 ~— ‘A)
(cm’)
~
5(0,
0)
(cm
I)
~
~
-
(cm —1) (cmi)
(cm
°
~
(d)
1)
(cm
-1)
570840
3MP,23°C
302040±60 29960±60 280
2230
EtOH,23°C
EPA, 77 K
29670±60 29120±50 550
29500±60 29210±50 290
2460
1950
NEC
3MP,23°C 29130±50 28900±60 230
EtOH,23°C 29090±50 28570±60 520
2000
2280
40
330
3COOEC
3MP,23°C 30320±60 30110±60 210
EtOl-l,23°C 29820±60 29100±60 720
EPA,77K
29590±60 29370±60 220
2350
5120
2100
500
1010
3COOENEC
3PM, 23C
EtOH,23°C
29330±50 29080±60 250
29160±50 28600±50 560
2190
3950
170
480
30000±60
660
1000
60
490
3CNC
3CNEEC
29760±60 240
2440
EtOl-l,23°C
EPA, 77 K
3MP,23°C
29340±50 28760±50 580
29400±50 29030±60 370
3520
2170
3MP,23°C
EtOH,23°C
28880±50
28820±50
2100
3240
28730±50 150
28240±50 580
Stokes shift defined as ~
0)— i~(O,0).
~ Fluorescence band width measured at half-height.
Spectral red shift of the absorption (0, 0) band between 3MP and ethanol.
Id)
Spectral red shift of the fluorescence (0,0) hand between 3MP and ethanol.
much to weak in order to be able to conclude that two species in their ground
states are responsible for the emissions observed.
It has been shown recently that 3-CNC and 3-COOEC are involved in
intermolecular hydrogen bonding complexes in EPA at 77K [7]. This is also
true in ethanol at 23°Cas exemplified in table 3 where the ~éA and the ~
for C, 3-COOEC and 3-CNC are about the same. When the NH proton is
replaced by an ethyl group, a marked decrease in these parameters is observed
showing the strong influence of the intermolecular hydrogen bonding interaction on the stabilization energies of the electronic ground and first excited
singlet state of these carbazole derivatives. It is, on the other hand, quite
obvious that a new species, probably an exciplex, is responsible for the long
tail observed in the fluorescence spectra of the 3-carboethoxy derivatives. The
Stokes shifts and the fluorescence band widths are both abnormally high when
compared to those of carbazole and N.ethylcarbazole. The cyano derivatives
seem also to show some interaction with ethanol in the singlet excited state but
to a lesser extend than for the carboethoxy derivatives. We have also found
that formation of low temperature glasses at 77 K in EPA results in a decrease
3MP,23°C
EtOH,23°C
EPA, 77 K
3MP,23°C
EtOH,23°C
3MP,23°C
EtOH, 23°C
EPA, 77K
3MP, 23°C
EtOH,23°C
3MP,23°C
EtOH, 23°C
EPA, 77K
3MP,23°C
EtOH,23°C
C
NEC
3COOEC
3COOENEC
3CNC
3CNNEC
0.28±0.03
0.24±0.01
0.35±0.01
0.31 ±0.01
0.21±0.02
0.26 ±0.01
(0.27±0.01)
0.15±0.01
(0.45 ±0.01)
0.18±0.02
0.49±0.01
0.42±0.02
0.42±0.01
0.44±0.01
0.43±0.01
~l~F
~
11.9±0.5
11.8±0.5
12.1±0.5
11.8±0.05
13.0±0.5
8.5 ±0.5
8.4±0.5
8.9±0.5
8.2 ±0.5
14.0±0.5
16.0±0.5
15.5±0.5
15.8±0.5
15.3±0.5
15.4±0.5
(ns)
1’F ~
24±3
20±2
29±2
26 ±1
16±2
31 ±3
(32±3)
17±2
(55 ±5)
13±2
30±2
28±2
27±2
29±2
28±4
k F X 10
(s1)
61± 9
64± 5
54± 3
58 ± 2
61± 8
87 ± 8
(87± 8)
95±15
(67 ± 6)
58± 8
32± 2
36± 3
36± 2
39± 2
37± 5
k nr X iO~
(s’)
27±3
26±3
23±6
15 ±4
13±4
36 ±3
(24±2)
25±5
(25 ±5)
21±4
25±2
26±2
32±3
37±4
37±4
k ~ x 10~
(s1)
yes
yes
yes
yes
yes
yes
no
yes
no
yes
yes
yes
yes
yes
yes
Mirror—image
relationship
All individual measurements were always included in the error bracket.
~ Obtained from an average of three measurements. The associated experimental error is the half difference between the maximum and minimum
experimental measurements.
(b) Obtained from an average of two measurements. The associated experimental error is the time scale used in the single photon counting apparatus.
Experimental
conditions
Molecules
Table 4
Photophysical properties of some 3-substituted derivatives of carbazole
‘a-
“a
I
1 84
P. Har,’ev, G.Durocher/ Hydrogen bonding. polarity and matr,.s effect.v
in the fluorescence intensity and a large decrease in the fluorescence band
width. This observation further supports the proposal of the existence of
exciplexes between these molecules and ethanol.
The photophysical properties of the 3-substituted derivatives of carbazole
have been included in table 4. As for the N-substituted derivatives, substitution
of an electron-withdrawing substituents in position 3 decreases the fluorescence quantum yields and lifetimes in 3MP. Ethanol does not affect the
fluorescence lifetime in all the molecules studied. Moreover these decays are
single exponentials and do not vary with the observation fluorescence wavelengths even for the 3-COOEC—ethanol system. This proves that the fluorescence of the exciplex is short-lived ( < 0.5 ns, the time resolution limit of our
single-photon counting apparatus) and that the only emission measured in the
decay comes from the 3-COOEC uncomplexed molecule in this system. The
fluorescence quantum yields of 3-COOEC and 3-COOENEC in ethanol do not
mean anything since the emission is a mixture of complexed and uncomplexed
species. Consequently the k,~and the k~r values are also meaningless. In this
case of exciplex interaction (3-COOEC), the exciplex fluorescence has a higher
quantum yield than the normal fluorescence lifetime. Some examples of these
kind of exciplexes have already been described in the literature [151. In all
other compounds the k~ and k~1 values are not much affected by the solvent.
NCNC was definitely not in this category as already described, the radiationless pathway being very much amplified in ethanol.
Conclusion
The existence of an excited state complex (exciplex) has been suggested in
order to explain the fluorescence characterictics of 3-carboethoxy carbazole
and 3-(carboethoxy)—N-ethylcarbazole in ethanol in fluid solutions. On the
other hand, such a complex does not appear in the fluorescence spectra of the
3-cyanocarbazoles and all the N-substituted derivatives studied in this paper.
Nevertheless, the N-substituted compounds (containing electron-withdrawing
substituents) show strong excited state interactions with ethanol, the radiationless transition probability being much increased in this polar solvent. This
study seems to show the importance played by the S2—S1 energy gap on the
ability of the solute carbazole molecules to interact with polar solvents in their
first excited electronic states (S,). Picosecond laser kinetic studies is now
carried out on these systems and will be reported in a later communication.
Acknowledgement
The authors would like to thank the Natural Sciences and Engineering
Research Council of Canada and the “Ministère de l’Education du Québec”
P. Harvey, G.Durocher/ Hudrogen bonding, polarity and matrix effects
185
for financial assistance. Thanks are also due to Dr. Bogumil Zelent of our
laboratory for many fruitful discussions on this paper.
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[2]PD. Harvey and G. Durocher, J. Photochem. 27 (1984) 29.
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[9] B. Zelent and G. Durocher, J. Org. Chem. 46 (1981) 1496; Can. J. Chem. 60 (1982) 945; Can.
J. Chem. 60 (1982) 2442.
[10] M. Niski, K. Hisao and T. Kano, Bull. Chem. Soc. Japan 54 (1981) 1897.
[11]M. Belletéte and G. Durocher, J. Photochem. 21(1983) 251; Can. J. Chem. 60 (1982) 2332.
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[13] E.C. Lim, J.D. Laposa and J.M.H. Yu, J. Mol. Spectrosc. 19 (1966) 412.
[14] J.B. Birks and Di. Dyson, Proc. Roy. Soc. A275 (1963) 135.
[15] N. Mataga and T. Kubota, Molecular Interactions and Electronic Spectra (Marcel Dekker,
New York, 1970) pp. 437—442.