Journal of Luminescence 93 (2001) 51–74
`
`Electronic spectroscopy of carbazole and N- and C-substituted
`carbazoles in homogeneous media and in solid matrix
`
`Sergio M. Bonesi, Rosa Erra-Balsells*
`
`Departamento de Quı´mica Orga´nica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, c.c.74-suc. 30,
`1430 Buenos Aires, Argentina
`
`Received 3 August 1999; received in revised form 28 June 2000; accepted 15 November 2000
`
`Abstract
`
`The dynamic properties of the lowest excited singlet and triplet states in terms of fluorescence and phosphorescence
`lifetime, tf and tp, fluorescence and phosphorescence quantum yield, ff and fp, for carbazole and N- and C-substituted
`carbazoles have been measured in organic solutions of different polarity and in solid matrix, at 298 and at 77 K,
`
`respectively. From these data, the radiative and the radiationless rate constants (k0f ; kisc; k0f ð77Þ; kiscð77Þ; k0
`
`
`p and k0nr)
`and the intersystem crossing quantum yield, fisc, were easily derived. Electronic spectra (absorption, fluorescence and
`phosphorescence emission spectra) of carbazole and carbazole derivatives have been recorded at 298 and at 77 K and
`the solvent and substituent effects on the spectroscopic data and on the photophysical rate constant have also been
`analyzed and good linear correlations have been obtained. The values of the HOMO and LUMO energy, the oscillator
`strength ( f ), the transition dipole (Dm) and the wavelengths associated to the electronic transitions, the heat of
`formation of the carbazoles and the corresponding radical cations (DHf and DHf (RC)) and the adiabatic ionization
`potential (Ia) were calculated by using the semiempirical PM3 and ZINDO/S methods and were compared with the
`spectroscopic and photophysical parameters obtained as well as with the one-electron oxidation potential data (Ep/2)
`reported for the carbazole series. # 2001 Elsevier Science B.V. All rights reserved.
`
`Keywords: Carbazoles; Electronic spectra at 298 K; Electronic spectra at 77 K; ZINDO/S carbazole electronic spectra calculations;
`ZINDO/S carbazole molecular orbital calculations
`
`1. Introduction
`
`The electronic absorption spectra and photo-
`physics of carbazole and some carbazole deri-
`vatives have been mainly studied by using
`steady-state
`spectroscopic methods
`[1–16].
`
`*Corresponding author. Tel.: +54-11-45763346; fax: +54-
`11-45763346.
`E-mail address: erra@qo.fcen.uba.ar (R. Erra-Balsells).
`
`Quenching of the first electronic excited singlet
`state of carbazole and some N-substituted deriva-
`tives by haloalkanes has also attracted the atten-
`tion of
`several
`researchers,
`the quenching
`mechanism being explained by an intermolecular
`electron transfer process [10–17]. These electron
`transfer interactions have also been invoked as a
`primary step in the photochemical reactivity of
`carbazole in the presence of CCl4 [18–25] and
`CH2Cl2 [26–28].
`
`0022-2313/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved.
`PII: S 0 0 2 2 - 2 3 1 3 ( 0 1 ) 0 0 1 7 3 - 9
`
`RHEMK-1009.001
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`

`52
`
`S.M. Bonesi, R. Erra-Balsells / Journal of Luminescence 93 (2001) 51–74
`
`Owing to their special photophysical properties,
`polymers containing as pendant group the carba-
`zolyl chromophore are good electron donors and
`possess outstanding electrical and photoelectrical
`properties [27–31].
`Besides, in order to monitor the kinetics and the
`extent of
`thermotropic and ionotropic lateral
`phase separations in vesicles in the presence of
`Ca2+, fluorescence studies using carbazole-labeled
`and brominated phospholipids have been con-
`ducted [32]. These methods have been used to
`measure the partition coefficient and diffusional
`rate of chlorinated hydrocarbons in synthetic
`phospholipid vesicles [33,34] and in cell mem-
`branes [34]; as application of the latter, bioaccu-
`mulation of synthetic chlorinated pesticides into
`membranes has been estimated [35–38].
`Thus, the study of the photochemical reactivity
`(photostability) of carbazoles and of their photo-
`physics including time-resolved fluorescence and
`
`phosphorescence spectroscopy is essential to prop-
`erly select carbazole derivatives in order to design
`new organic polymers and new bio-organic photo-
`sensors.
`As part of an on going program related to the
`study of the photochemistry of azacarbazoles [39–
`43] and carbazoles [44–46] we decided to study in
`detail the electronic spectra of several carbazoles
`whose photochemistry we are interested in. To
`begin with we studied the absorption, fluorescence
`and phosphorescence spectra of carbazole (1),
`N-methyl- (2), N-phenyl- (3), N-vinyl- (4), N-
`acetyl- (5), N-benzoyl- (6), 2-methoxy-N-methyl- (7),
`2-hydroxy-
`(8), 2-acetoxy-
`(9), 3-chloro-
`(10),
`3-bromo- (11), 3-nitro- (12), 3,6-dibromo- (13),
`3-benzoyl- (14), 3,6-dibenzoyl- (15), 3,6-dichloro-
`N-benzoyl- (16) and 1-nitrocarbazole (17) (see
`Scheme 1). These studies have been performed in
`different organic solvents, under different atmo-
`spheres at 298 and 77 K by using time correlated
`
`Scheme 1. Structure of the carbazoles studied.
`
`RHEMK-1009.002
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`

`

`S.M. Bonesi, R. Erra-Balsells / Journal of Luminescence 93 (2001) 51–74
`
`53
`
`single photon counting technique and phosphor-
`escence lifetimes spectroscopy.
`The dynamic properties of the lowest excited
`singlet and triplet states in terms of fluorescence
`and phosphorescence lifetime, tf and tp, fluores-
`cence and phosphorescence quantum yield, ff and
`fp, have been measured at 298 and at 77 K. From
`these data, the radiative and the radiationless rate
`
`(k0f ; kisc; k0fð77Þ; kiscð77Þ; k0
`
`
`p and k0nr)
`constants
`and the intersystem crossing quantum yield, fisc,
`were derived. Also, the solvent and the substituent
`effects on the different spectroscopical and photo-
`physical parameters were analysed. The HOMO
`and LUMO energies and the ionization potential
`(Ia) of carbazoles were calculated (PM3 method)
`and related to the one electron oxidation potential
`(Ep/2) of each carbazole. Theoretical absorption
`spectra have been calculated (ZINDO/S) and
`compared with the experimental ones.
`
`2. Experimental
`
`2.1. Materials
`
`Carbazole, N-methylcarbazole, N-phenylcarba-
`zole, N-vinylcarbazole, 2-hydroxycarbazole and
`3,6-dibromocarbazole were
`purchased
`from
`Aldrich Chemicals Co. and were purified by re-
`peated recrystallizations from appropriate solvents.
`The other carbazole derivatives N-acetylcarbazole
`[44], N-benzoylcarbazole [45–47], 2-methoxy-N-
`methylcarbazole
`[48,49],
`2-acetoxycarbazole
`[50,51] 1-nitrocarbazole
`[27], 3-nitrocarbazole
`[27], 3-bromocarbazole [51,52], 3,6-dichloro-N-
`benzoylcarbazole [49], 3-benzoylcarbazole [47], 3-
`chlorocarbazole [51] and 3,6-dibenzoylcarbazole
`[47] were synthesized according to literature
`procedures.
`Acetonitrile and cyclohexane (J.T. Baker Spec-
`trograde), ethanol, iso-propanol, dichloromethane,
`ethyl ether and ethyl acetate (Merck HPLC grade)
`were used as purchased without any further
`purification. Water of MiliQ grade, perchloric
`acid and sulphuric acid of analytical grade were used.
`Rhodamine B, quinine sulphate and p-terphenyl
`(Aldrich Chemicals Co. and Sigma) were used as
`purchased.
`
`2.2. Equipment
`
`The absorption measurements were performed
`with a spectrophotometer Hewlett-Packard HP5.
`The spectrofluorimeter employed in this study
`was a Hitachi F-500. Steady-state spectra were
`obtained on freshly prepared nitrogen-degassed
`solutions (NS) in spectral grade solvents. Mea-
`surements of air-saturated solutions (AS) were
`also conducted. The quantum yields at room
`temperature were determined relative to the
`quantum yield of quinine sulphate in HClO4
`0.1 N (QS). Freshly prepared solutions of QS were
`used as references by adjusting absorbance values
`to 0.20–0.30 au (arbitrary units) at 312 nm, where
`all compounds showed significant absorption. The
`absorption of each compound in solution was
`adjusted to equal that of the standard QS sample,
`and the fluorescence emission steady-state spec-
`trum was obtained between 300 and 450 nm.
`Fluorescence emission spectra were integrated
`and the quantum yields were determined by the
`ratio of the integrated areas of the fluorescence
`spectrum with those of the QS spectrum (ff=0.55)
`[53,54].
`The quantum yield of carbazole at intervals
`between room temperature and 77 K was estab-
`lished relative to the room temperature quantum
`yield with corrections
`for changes
`in solute
`concentration, index of refraction, and absorptiv-
`ity with temperature according to the procedure
`described by Mantulin and Huber
`[55]; we
`obtained a fluorescence quantum yield value for
`carbazole similar to that reported by Huber et al.
`[56]. The quantum yields of
`the substituted
`carbazoles at 77 K were determined relative to
`that of carbazole, (ff(77 K)), by comparing the
`corresponding integrated fluorescence signals. At
`these low temperatures, phosphorescence is ob-
`served, and a simple comparison of the integrated
`areas under the fluorescence and phosphorescence
`spectra yields the phosphorescence quantum effi-
`ciency (fp) according to the recommended proce-
`dure [53,55,57].
`The fluorescence lifetimes were measured on the
`same solutions used in the steady-state measure-
`ments, by the single photon counting technique.
`The Edinburgh OB 900 nanosecond fluorescence
`
`RHEMK-1009.003
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`

`54
`
`S.M. Bonesi, R. Erra-Balsells / Journal of Luminescence 93 (2001) 51–74
`
`spectrometer employed uses a nanosecond flash
`lamp filled with hydrogen (0.42 bar). The resulting
`decay curves were analyzed by convoluting a single
`exponential with the Lamp function. The statis-
`tical parameters for lifetime analysis are, on
`average, w2=1.037 and the Durbin–Watson
`parameter=1.824. The excitation and emission
`wavelengths were set as lmax(abs) and lmax(fluo),
`respectively, the values selected depending on the
`compounds studied. Provision is made for tem-
`perature control of
`the samples
`from room
`temperature to 77 K using a quartz dewar vessel
`equipped with a special block sample compart-
`ment for round quartz cells (0.5 mm diameter). In
`the Edinburgh OB 900 fluorometer the emission
`and excitation slits are arranged in horizontal
`display. Phosphorescence decay curves were de-
`termined with the Hitachi F-500 spectrofluori-
`meter by using the phosphorescence mode, at
`77 K. The phosphorescence lifetimes were deter-
`mined by fitting the phosphorescence decay curves
`with a mono-exponential function using the Origin
`3.5 program.
`All spectra were corrected by measuring the
`instrumental response on excitation side (rhoda-
`mine B) and on emission side (cell diffuser). The
`integrated fluorescence spectra of a solution of p-
`terphenyl [53] in cyclohexane with an absorbance
`of 0.521 au at 276 nm was recorded in order
`to check the instrumental response of the Hitachi
`F-500 spectrofluorimeter after measuring the
`fluorescence emission spectra of each compound
`under study.
`
`2.3. Theoretical calculations
`
`The ground state geometries were optimized by
`ab initio calculations (HF level; 3-21G basis set;
`Gaussian 98W,
`[58]). Heat of
`formation of
`carbazoles was calculated by using the semiempi-
`rical parametrized PM3 method as implemented in
`the HyperChem Suite program [58]. The geome-
`tries of the radical cations were optimized using
`the unrestricted Hartree–Fock (UHF) formalism.
`(Ia) were
`The adiabatic ionization potentials
`calculated as the difference in DHf of the radical
`cation, calculated by using RHF formalism from
`the optimized structures using the UHF formalism
`
`and DHf of the neutral form with the optimized
`geometry. Qualitative structure–activity relation-
`ships
`(QSAR) properties were
`calculated as
`implemented
`in ChemPlus: Extensions
`for
`Hyperchem Suite [58]. UV visible spectroscopic
`transitions
`and the
`corresponding oscillator
`strength ( f ) and transition dipole (Dm) were
`calculated by using the ZINDO/S method as it is
`parametrized in HyperChem Suite [58].
`
`3. Results
`
`The absorption and fluorescence emission spec-
`tra of carbazole and carbazole derivatives were
`analyzed in non-polar and polar solvents at 298 K.
`The absorption spectra were recorded in the 200–
`800 nm region and generally three bands were
`observed. The position and oscillator strength of
`the bands ( f ) at 330 and 290 nm of the carbazole
`derivatives were compared with those of carbazole
`[59] and were assigned as 1Lb (1S1 1S0) and 1La
`(1S2 1S0)
`electronic
`transitions,
`respectively.
`Furthermore, the positions and oscillator strength
`of 1La and 1Lb were also verified by quantum
`chemical calculations (ZINDO/S) which are fairly
`close to the experimental values. Besides, a more
`accurate analysis of the oscillator strength of 1La
`and 1Lb bands of carbazole derivatives with
`respect to those of carbazole may give additional
`information about the geometry as well as the
`electronic
`characteristic of
`these
`compounds.
`Table 1 shows the oscillator strength ( f ) and the
`transition dipole (Dm) associated with the electro-
`nic transition bands, 1La and 1Lb, together with
`the lmax of absorption (lmax(abs)) and lmax of
`fluorescence emission (lmax(fluo)).
`The substituent effect on the 0,0 electronic
`transition energy (E0,0) was also analyzed. For
`this analysis Eq. (1) was used
`ð1Þ
`½E0;0ðXÞE0;0ðCAފ=2:303RT ¼ rAs;
`where X is the substituted carbazole and CA is
`carbazole. The rA value is defined as the absorp-
`tion constant and s are the Hammett parameters
`[60] (sl, sp and sm). Kosower et al. [61] had earlier
`developed Eq. (1) and later, Reichardt et al. [62,63]
`successfully used this correlation for the study of
`
`RHEMK-1009.004
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`

`

`S.M. Bonesi, R. Erra-Balsells / Journal of Luminescence 93 (2001) 51–74
`
`55
`
`Table 1
`Spectroscopic data of carbazoles in different organic media at 298 K under inert atmosphere (N2) together with calculated spectroscopic data by ZINDO/S after
`HF/3-21G geometrical optimization
`
`Carbazoles MeCN
`
`fðS1S0Þa
`
`lmax
`(abs)
`(nm)
`
`lmax
`(fluo)
`(nm)
`
`EtOH
`
`lmax
`(abs)
`(nm)
`
`lmax
`(fluo)
`(nm)
`
`fðS1S0Þa
`
`Cyclohexane
`
`lmax
`(abs)
`(nm)
`
`lmax
`(fluo)
`(nm)
`
`fðS1S0Þa Dmb
`(D)
`
`fðS1S0Þc Dmc
`(D)
`
`fðS2S0Þc Dmc
`(D)
`
`lmax
`(abs)c
`(nm)
`
`lmax
`(abs)c
`(nm)
`
`1
`334
`355
`0.071
`338
`358
`2
`346
`366
`0.059
`344
`363
`3
`340
`360
`0.050
`340
`358
`4
`340
`362
`0.041
`340
`360
`5
`314
`329
`0.010
`312
`329
`6
`314
`355
`0.007
`314
`356
`7
`336
`358
`0.040
`332
`356
`8
`318
`345
`0.048
`320
`347
`9
`332
`353
`0.059
`332
`355
`10
`344
`356
`0.057
`346
`357
`11
`346
`357
`0.057
`346
`357
`13
`350
`361
`0.091
`350
`360
`a Calculated according to fðS1S0Þ ¼ 1:5k0
`f xn2.
`b Dipole moment of the 1Lb transition derived from Lippert–Mataga relationship (see Eq. (2)).
`c Calculated using the semiempirical ZINDO/S method after HF/3-21G geometrical optimization.
`d N-phenyl substituent/carbazole moiety diedral angle ð fðS1S0Þ, Dm, lmax(abs), fðS2S0Þ, Dm, lmax(abs)), 548 (0.462, 5.15 D, 270 nm, 0.598, 5.82 D, 265 nm); 908 (0.809,
`6.21 D, 268 nm).
`
`5.63
`5.36
`3.81
`5.30
`6.06
`12.06
`4.71
`6.26
`5.88
`5.02
`5.04
`5.90
`
`0.009
`0.028
`0.082d
`0.064
`0.027
`0.859
`0.021
`0.045
`0.016
`–
`–
`–
`
`0.75
`1.31
`2.05d
`1.99
`1.27
`7.25
`1.13
`1.65
`0.97
`–
`–
`–
`
`281
`293
`290d
`291
`282
`266
`288
`286
`283
`–
`–
`–
`
`0.535
`0.350
`0.483d
`0.363
`0.542
`0.618
`0.555
`0.576
`0.754
`–
`–
`–
`
`5.54
`4.50
`5.26d
`4.58
`5.57
`5.77
`5.72
`5.82
`6.64
`–
`–
`–
`
`270
`272
`269d
`271
`269
`253
`277
`277
`275
`–
`–
`–
`
`0.076
`0.073
`0.162
`0.186
`0.007
`–
`0.117
`0.134
`0.190
`0.055
`0.064
`0.013
`
`332
`344
`340
`340
`310
`316
`334
`314
`330
`340
`340
`348
`
`347
`360
`356
`357
`328
`347
`353
`339
`355
`354
`355
`351
`
`0.077
`0.067
`0.118
`0.093
`0.015
`–
`0.082
`–
`0.059
`0.061
`0.031
`0.018
`
`RHEMK-1009.005
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`

`

`56
`
`S.M. Bonesi, R. Erra-Balsells / Journal of Luminescence 93 (2001) 51–74
`
`the substituent effect on the E0;0 of the betaine
`chromophores. The results obtained in the present
`work for carbazole and carbazole derivatives are
`shown in Fig. 1.
`Carbazole and carbazole derivatives exhibit
`structured fluorescence spectra in different organic
`media at 298 K with excitation into the S1 level.
`The fluorescence and absorption spectra at room
`temperature are found to display excellent mirror
`symmetry with an exact overlap of the 0,0 bands
`(Fig. 2). Also, these carbazoles show structured
`fluorescence and phosphorescence spectra in iso-
`propanol-ethyl ether (1 : 1; v : v) glass matrix at
`77 K with electronic excitation into the S1 level
`(Fig. 2). It is noteworthy to mention that there is
`no noticeable change of lmax(fluo) on going from
`298 to 77 K. Also, the C- and N-substitution
`effects on lmax(fluo) at both temperatures show a
`is observed when E0;0
`is
`similar trend as it
`correlated to the Hammett parameters. All these
`spectroscopic data of carbazoles are shown in
`Tables 1 and 2.
`
`Fig. 1. Hammett correlation between sI- values and the
`modified transition energies of the 0,0 absorption band of
`substituted carbazoles in acetonitrile at 298 K: (*) N-sub-
`stituted carbazoles; (m) C-3 substituted carbazoles; (&) C-2
`substituted carbazoles. Numbers are the carbazole derivatives
`(see Scheme 1). Lines show the best linear regressions obtained
`according to Eq. (1) (see text).
`
`Fig. 2. Electronic absorption spectra and fluorescence and
`phosphorescence emission spectra of carbazole: (Solid line)
`absorption spectra of carbazole in acetonitrile at 298 K; (bold
`solid line) fluorescence emission spectra of carbazole in
`acetonitrile at 298 K under nitrogen atmosphere; (dashed line)
`fluorescence and phosphorescence emission spectra of carbazole
`in solid matrix at 77 K.
`
`the solvent on the
`the effect of
`Besides,
`absorption and emission fluorescence spectra of
`carbazoles in aerated solutions (AS) of acetoni-
`trile,
`ethanol,
`iso-propanol, dichloromethane,
`ethyl acetate and cyclohexane was analyzed. On
`changing the solvent polarity, there was no drastic
`modification of the shape of the absorption and
`fluorescent emission spectra. Qualitatively,
`this
`analysis was carried out relating the absorption
`and fluorescence
`emission spectroscopic data
`(lmax(abs), lmax(fluo) and l0,0) of the carbazoles
`to the solvent polarity parameters ET(30) and Z
`[60]. The spectroscopic data in Table 3 show the
`solvent effect on both lmax (abs) and lmax(fluo). A
`mere bathochromic shift of only 2–6 nm was
`observed which suggests that the nature of the
`lowest singlet excited state is mostly likely to be
`p, p* [64,65].
`
`RHEMK-1009.006
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`S.M. Bonesi, R. Erra-Balsells / Journal of Luminescence 93 (2001) 51–74
`
`57
`
`Table 2
`Spectroscopic data of carbazoles in solid matrix (iso-propanol:ethyl ether; (1 : 1; v : v)) at 77 K together with calculated spectroscopic
`data by ZINDO/S after HF/3-21G geometrical optimization
`fðS1S0Þa
`fðT12S0Þb
`
`Carbazoles
`
`lmax(fluo)
`(nm)
`
`lmax(phosp)
`(nm)
`
`E(S1S0)
`(eV)
`
`E(T1S0)
`(eV)
`
`1
`356
`406
`0.058
`2
`364
`408
`0.061
`3
`358
`407
`0.060
`4
`359
`414
`0.131
`5
`326
`414
`0.111
`NFe
`410
`–
`6
`7
`350
`412
`0.058
`8
`346
`419
`0.067
`9
`353
`409
`0.091
`10
`358
`410
`0.061
`11
`360
`412
`0.049
`12
`NF
`472
`–
`13
`365
`412
`0.061
`14
`NF
`414
`–
`15
`NF
`415
`–
`16
`NF
`419
`–
`17
`NF
`474
`–
`a Calculated according to fðS1S0Þ ¼ 1:5k0
`f xn2.
`b Calculated according to fðT1S0Þ ¼ 1:5k0
`pxn2.
`c DE(S1T1)=E(S1S0)E(T1S0).
`d Calculated using the semiempirical ZINDO/S method after HF/3-21G geometrical optimization.
`e No fluorescence was detected.
`
`1.55 1010
`1.32 10
`10
`1.74 1010
`7.12 1010
`1.74 10
`10
`5.39 10
`9
`2.49 1010
`2.55 1010
`2.11 10
`10
`2.54 10
`9
`7.04 107
`8.32 109
`8.15 10
`8
`1.51 10
`7
`7.23 108
`3.34 10
`9
`2.28 10
`7
`
`3.63
`3.55
`3.60
`3.59
`3.94
`–
`3.59
`3.73
`3.66
`3.47
`3.48
`–
`3.32
`–
`–
`–
`–
`
`3.04
`3.03
`3.04
`2.98
`2.98
`3.01
`3.00
`2.95
`3.02
`2.97
`2.98
`2.62
`2.98
`2.99
`3.00
`2.95
`2.61
`
`DE(S1T1)c
`(eV)
`0.58
`0.52
`0.56
`0.61
`0.96
`0.95
`0.59
`0.78
`0.64
`0.50
`0.48
`–
`0.34
`–
`–
`–
`–
`
`fðS1SoÞd
`
`fðS2S0Þd
`
`0.009
`0.028
`0.462
`0.064
`0.027
`0.859
`0.021
`0.045
`0.016
`–
`–
`0.081
`–
`0.742
`0.035
`–
`0.158
`
`0.535
`0.350
`0.598
`0.363
`0.542
`0.618
`0.555
`0.576
`0.754
`–
`–
`0.312
`–
`0.167
`0.676
`–
`0.127
`
`In a more quantitative way, the Lippert–Mataga
`relationship shown in the following equation [66]:
`nðabsÞnðfluoÞ ¼ ðDmÞ2D f =2phce0a3;
`ð2Þ
`
`where D f
`is the polarity–polarizability function
`ðD f ¼ fðeÞ fðnÞÞ and a denotes the spherical
`approximation radii of the carbazoles. The Dm
`value, which is the difference of the dipole moment
`between the excited and ground states of the
`carbazoles, was calculated from the slope of the
`linear correlation obtained according to Eq. (2)
`and is shown in Table 1. The spherical approx-
`imation radii of the carbazoles (a) were calculated
`by the QSAR method (QSAR properties
`in
`ChemPlus, HyperChem Suite [58]).
`The dynamic properties of the lowest excited
`singlet and triplet states,
`including fluorescence
`lifetime tf, phosphorescence lifetime tp, fluores-
`cence quantum yield ff and phosphorescence
`quantum yield fp for carbazole and carbazole
`derivatives in acetonitrile, ethanol and cyclohex-
`
`ane at 298 K and in iso-propanol : ethyl ether (1:1;
`v : v) at 77 K are shown in Tables 4 and 5.
`From these spectroscopic data different photo-
`f ; kisc and fisc
`physical rate constants such as kf ; k0
`p; knr and fisc at
`
`at 298 K and kf ; k0f ; kisc; kp; k0
`77 K for carbazoles 1–17 were easily calculated;
`these parameters are shown in Tables 4 and 5,
`respectively. These photophysical rate constants
`were calculated under the assumption that the
`non-radiative internal conversion (kic) is neglected
`owing to the planarity and the rigidity of the
`carbazole structures. Recently, Chakravorti et al.
`[67] have analyzed the photophysical properties of
`9-phenylfluorene, triphenylamine, 9-phenylcarba-
`zole and carbazole, showing that the intersystem
`crossing channel,
`is also favoured when the
`planarity of
`the heteroaromatic compound is
`increased. Thus, increased radiative singlet decay
`at the expense of suppressed radiationless internal
`conversion on going from triphenylamine to 9-
`phenylcarbazole is another evidence of increased
`rigidity in carbazole derivatives. Taking into
`
`RHEMK-1009.007
`
`

`

`58
`
`S.M. Bonesi, R. Erra-Balsells / Journal of Luminescence 93 (2001) 51–74
`
`Table 3
`Solvent effect on the absorption and fluorescence emission spectrum of carbazoles at 298 K
`
`Solvent Carbazole
`
`N-methylcarbazole
`
`N-phenylcarbazole
`
`N-vinylcarbazole
`
`lmax(abs)
`(nm)
`
`lmax(fluo)
`(nm)
`
`lmax(abs)
`(nm)
`
`lmax(fluo)
`(nm)
`
`lmax(abs)
`(nm)
`
`lmax(fluo)
`(nm)
`
`lmax(abs)
`(nm)
`
`292
`Cy
`292
`AcOEt
`292
`CH2Cl2
`MeCN 292
`PriOH
`294
`EtOH
`294
`
`332
`334
`334
`334
`338
`338
`
`335
`340
`338
`341
`343
`344
`
`347
`352
`354
`358
`357
`358
`
`294
`294
`296
`294
`294
`294
`
`344
`344
`346
`346
`344
`344
`
`346
`350
`351
`352
`350
`350
`
`360
`364
`365
`366
`363
`363
`
`294
`296
`294
`294
`292
`292
`
`340
`342
`340
`340
`340
`340
`
`343
`348
`346
`345
`345
`347
`
`356
`362
`359
`358
`358
`360
`
`292
`294
`292
`292
`292
`292
`
`340
`342
`340
`340
`340
`340
`
`lmax
`(fluo)
`(nm)
`
`343
`350
`348
`348
`348
`352
`
`Solvent N-acetylcarbazole
`
`N-benzoylcarbazole
`
`2-methoxy-N-methylcarbazole
`
`2-hydroxylcarbazole
`
`lmax(abs)
`(nm)
`
`lmax(fluo)
`(nm)
`
`288
`Cy
`288
`AcOEt
`286
`CH2Cl2
`MeCN 286
`PriOH
`286
`EtOH
`288
`
`310
`314
`314
`312
`312
`314
`
`316
`316
`317
`317
`316
`318
`
`328
`328
`328
`328
`329
`329
`
`lmax
`(abs)
`(nm)
`
`280
`280
`276
`278
`278
`276
`
`316
`316
`316
`314
`314
`314
`
`lmax
`(fluo)
`(nm)
`
`331
`337
`341
`343
`343
`340
`
`347
`348
`355
`356
`356
`355
`
`lmax(abs)
`(nm)
`
`lmax(fluo)
`(nm)
`
`lmax(abs)
`(nm)
`
`302
`304
`302
`302
`302
`304
`
`334
`334
`334
`334
`332
`336
`
`338
`343
`342
`341
`341
`343
`
`353
`358
`358
`356
`356
`358
`
`292
`300
`302
`304
`302
`302
`
`314
`326
`320
`320
`320
`318
`
`lmax
`(fluo)
`(nm)
`
`325
`331
`331
`334
`334
`332
`
`357
`366
`360
`360
`360
`362
`
`339
`343
`345
`348
`347
`345
`
`Solvent
`
`3-bromocarbazole
`
`3,6-dibromocarbazole
`
`lmax(abs)
`(nm)
`
`lmax(fluo)
`(nm)
`
`lmax(abs)
`(nm)
`
`lmax(fluo)
`(nm)
`
`296
`Cy
`298
`CH2Cl2
`298
`AcOEt
`MeCN 300
`PriOH
`298
`EtOH
`298
`
`340
`342
`344
`346
`346
`346
`
`342
`336
`339
`340
`341
`340
`
`355
`351
`355
`357
`357
`357
`
`302
`302
`302
`304
`304
`302
`
`348
`350
`352
`354
`354
`350
`
`332
`341
`343
`344
`344
`344
`
`351
`355
`360
`359
`360
`361
`
`the
`the above-mentioned assumption,
`account
`intersystem crossing quantum yield (fisc) can be
`easily calculated according to the
`following
`equation:
`fisc ¼ 1 ff ¼ kisc=kf :
`In order to study the effect of oxygen on the
`fluorescence emission of carbazoles, fluorescence
`quantum yields (ff) were measured in acetonitrile,
`ethanol and cyclohexane air-saturated (AS) and
`nitrogen-saturated (NS) solutions (Tables 6 and 4).
`As shown in both the tables, the fluorescence
`emission quantum yield diminishes upon going
`from inert (NS) to aerated atmosphere (AS). This
`
`ð3Þ
`
`result suggests that there is an oxygen physical
`quenching of the excited state of the carbazoles
`since no oxidized photoproduct was detected and
`also the oxygen effect extent depended on the
`solubility of the oxygen in the organic media used.
`In addition, we have measured both the fluores-
`cence lifetime and the fluorescence quantum yield
`of carbazole and carbazole derivatives in AS
`solutions of acetonitrile, ethanol,
`iso-propanol,
`dichloromethane, ethyl acetate and cyclohexane at
`298 K (Table 6). The photophysical parameters
`f ; kisc and fisc) were easily calculated from
`(kf ; k0
`the experimental tf and ff values (Table 6).
`Also, the solvent effect on the above mentioned
`
`RHEMK-1009.008
`
`

`

`S.M. Bonesi, R. Erra-Balsells / Journal of Luminescence 93 (2001) 51–74
`
`59
`
`Table 4
`Fluorescence lifetime, fluorescence quantum yield, photophysical depletion rate parameters and intersystem crossing quantum yield of carbazoles in acetonitrile, ethanol
`and cyclohexane at 298 K under inert atmosphere (N2)
`
`Carbazoles MeCN
`
`ff
`
`tf
`(ns)
`
`kf
`( 107)
`s1a
`
`0
`kf
`( 107)
`s1b
`
`kisc
`( 107)
`s1c
`
`EtOH
`
`
`fiscd tf
`(ns)
`
`ff
`
`Cyclohexane
`
`kf
`( 107)
`s1a
`
`0
`kf
`( 107)
`s1b
`
`kisc
`( 107)
`s1c
`
`
`fiscd tf
`(ns)
`
`ff
`
`kf
`( 107)
`s1a
`
`0
`kf
`( 107)
`s1b
`
`kisc
`( 107)
`s1c
`
`d
`fisc
`
`1
`2
`3
`4
`5
`6
`7
`8
`9
`10
`11
`13
`
`6.62
`0.62
`15.1
`6.80
`0.47
`14.7
`8.77
`0.31
`11.4
`9.01
`0.25
`11.1
`0.38 0.0025 263
`0.25 0.004
`400
`14.3
`0.34
`6.99
`12.3
`0.36
`8.13
`13.2
`0.45
`7.58
`2.7
`0.060
`96
`1.2
`0.038
`83.3
`0.47 0.0021 212
`
`4.10
`3.19
`2.79
`2.25
`0.66
`0.72
`2.27
`2.89
`3.41
`3.17
`3.17
`0.45
`
`2.52
`3.61
`5.48
`6.82
`262
`399
`4.72
`5.24
`4.17
`93.0
`80.3
`212
`
`0.38
`0.53
`0.62
`0.76
`0.99
`0.99
`0.68
`0.64
`0.55
`0.97
`0.96
`0.99
`
`6.94
`0.52
`14.4
`7.19
`0.48
`13.9
`9.17
`0.39
`10.9
`10.4
`0.33
`9.6
`0.43 0.0019 232
`0.26 0.0020 384
`14.3
`0.45
`6.99
`12.3
`0.42
`8.13
`9.0
`0.33
`9.0
`2.3
`0.054
`101
`1.1
`0.033
`90.9
`0.35 0.0025 285
`
`4.20
`3.45
`4.11
`3.98
`0.44
`0.77
`3.15
`4.04
`11.1
`3.00
`3.45
`0.71
`
`2.74
`3.74
`5.06
`6.44
`232
`383
`3.25
`4.09
`3.12
`8.0
`87.5
`285
`
`0.39
`0.52
`0.55
`0.62
`0.99
`0.99
`0.46
`0.50
`0.35
`0.97
`0.96
`0.99
`
`7.04
`0.53
`14.2
`6.49
`0.48
`15.4
`9.71
`0.58
`10.3
`11.2
`0.40
`8.9
`243
`0.41 0.004
`0.31 0.0023 322
`14.1
`0.57
`7.09
`–
`–
`10.0
`0.34
`3.1
`0.072
`0.70 0.012
`0.29 0.005
`
`–
`10.0
`115
`142
`344
`
`4.36
`3.66
`6.57
`5.21
`0.98
`0.74
`4.72
`–
`3.40
`3.45
`1.71
`0.97
`
`2.68
`2.83
`3.14
`6.03
`242
`321
`2.37
`
`–
`
`6.60
`112
`14.8
`343
`
`0.38
`0.44
`0.32
`0.54
`0.99
`0.99
`0.33
`–
`0.66
`0.97
`0.99
`0.99
`
`1.
`a kf=tf
`b k0
`f =kf ff.
`c kisc=kfkf
`0.
`d Calculated according to Eq. (3) (see text).
`
`RHEMK-1009.009
`
`

`

`60
`
`S.M. Bonesi, R. Erra-Balsells / Journal of Luminescence 93 (2001) 51–74
`
`Table 5
`Fluorescence lifetime, fluorescence quantum yield, phosphorescence lifetime, phosphorescence quantum yield and photophysical
`depletion rate parameters of carbazoles in solid matrix (iso-propanol-ethyl ether; (1 : 1; v : v)) at 77 K
`kf( 107) s1a
`0( 107) s1b
`kisc( 107) s1c fisc
`kf
`
`d
`
`kp s1e
`
`0 s1f
`kp
`
`knr s1g
`
`Carbazoles
`
`tf(ns) ff
`
`tf(ns) ff
`
`1
`2
`3
`4
`5
`6
`7
`8
`9
`10
`11
`12
`13
`14
`15
`16
`17
`
`14.4
`13.9
`10.9
`9.60
`0.43
`0.25
`14.3
`12.3
`9.00
`1.04
`1.20
`NFh
`0.47
`NF
`NF
`NF
`NF
`
`0.44
`0.43
`0.34
`0.65
`0.030
`0.010
`0.45
`0.46
`0.44
`0.033
`0.030
`–
`0.001
`–
`–
`–
`–
`
`7.73
`7.88
`7.57
`4.44
`6.93
`5.50
`4.32
`4.19
`5.13
`0.57
`0.34
`0.159
`0.018
`0.020
`0.030
`0.009
`0.41
`
`0.24
`0.24
`0.35
`0.004
`0.40
`0.99
`0.23
`0.22
`0.24
`0.55
`0.54
`0.35
`0.57
`0.80
`0.84
`0.61
`0.90
`
`6.94
`7.19
`9.17
`10.4
`232
`400
`6.99
`8.13
`11.1
`96.2
`83.3
`NFh
`212
`NF
`NF
`NF
`NF
`
`3.07
`3.09
`3.12
`6.77
`6.98
`4.00
`3.15
`3.74
`4.89
`3.17
`2.50
`–
`0.21
`–
`–
`–
`–
`
`3.87
`4.10
`6.05
`3.65
`225
`396
`3.84
`4.39
`6.22
`93.0
`80.8
`–
`212
`–
`–
`–
`–
`
`0.56
`0.57
`0.66
`0.35
`0.97
`0.99
`0.55
`0.54
`0.56
`0.97
`0.97
`1
`0.99
`1
`1
`1
`1
`
`0.129
`0.127
`0.132
`0.225
`0.144
`0.182
`0.231
`0.239
`0.195
`1.75
`2.94
`6.29
`55.6
`50.0
`33.3
`111
`0.709
`
`0.055
`0.053
`0.070
`0.003
`0.059
`0.184
`0.097
`0.097
`0.084
`0.995
`1.64
`2.20
`31.9
`40.0
`27.9
`67.8
`0.638
`
`0.074
`0.074
`0.062
`0.22
`0.085
`0
`0.134
`0.142
`0.111
`0.759
`1.30
`4.09
`23.6
`10.0
`5.33
`43.3
`0.071
`
`1.
`a kf=tf
`
`b kf0=kfff.
`c kisc=kfkf
`0.
`d Calculated according to Eq. (3) (see text).
`1.
`e kp=tp
`1.
`
`f kp0=fiscfptp
`g knr=kpkp
`0.
`h No fluorescence was detected.
`
`photophysical parameters was analyzed by using
`the solvent polarity parameters, ET(30) and Z
`[60,67]. Figs. 3 and 4 show the correlation of the
`photophysical parameters k0
`f and kisc of carbazole
`and N-methylcarbazole with ETð30Þ. Similar
`results were obtained for the other carbazole
`derivatives studied.
`The effect of the substituents on the fluorescence
`emission and on the singlet deactivation of
`carbazoles was analyzed in cyclohexane, acetoni-
`trile and ethanol at 298 K, using Hammett
`
`correlations between log kf0 or log kisc and sI; sm
`or sp substituent constants. The statistical treat-
`ment of these correlations is shown in Table 7. As
`can be seen, in all the cases studied, good linear
`correlations were obtained.
`Finally, the heat of formation of carbazoles and
`radical-cation (DHf and DHf(RC),
`carbazoles
`respectively) were calculated by using the semi-
`empirical PM3 method after the optimization of
`
`the ground state geometry at HF/3-21G level. The
`adiabatic ionization potentials (Ia) of the carbazole
`derivatives were calculated according to the
`following equation [68]:
`ð4Þ
`Ia¼ DHfðRCÞ DHf :
`Also, the HOMO and LUMO energies (EHOMO
`and ELUMO) and the LUMO–HOMO energy
`difference (DE) of the carbazole derivatives were
`calculated by using the same semiempirical meth-
`od (PM3). These thermodynamic parameters are
`listed in Table 8. The EHOMO and DE values are
`related to the 1Lb(1S1 1S0) energy transition
`which is denoted as E(S0) and they are obtained
`from the absorption spectra recorded in acetoni-
`trile (see Table 3). Figs. 5 and 6 show the
`correlations obtained between both EHOMO and
`DE, and E(S0).
`Since
`carbazole and carbazole derivatives
`are heteroatomic compounds easily oxidizable
`
`RHEMK-1009.010
`
`

`

`S.M. Bonesi, R. Erra-Balsells / Journal of Luminescence 93 (2001) 51–74
`
`61
`
`Table 6
`Fluorescence quantum yield, fluorescence lifetime and photophysical rate constants of carbazoles in different organic media at 298 K under air saturated atmosphere
`
`Cyclohexane (31.2)a
`
`Carba-
`zoles
`
`Ethyl acetate (32.1)a
`
`CH2Cl2 (41.1)a
`
`ff
`
`tf
`(ns)
`
`kf
`( 107) s
`1b
`
`0
`kf
`( 107) s
`1c
`
`kisc
`( 107) s
`1d
`
`tf
`(ns)
`
`ff
`
`kf
`( 107) s
`1b
`
`0
`kf
`( 107) s
`1c
`
`kisc
`( 107) s
`1d
`
`tf
`(ns)
`
`ff
`
`kf
`( 107) s
`1b
`
`0
`kf
`( 107) s
`1c
`
`kisc
`( 107) s
`1d
`
`1
`2
`3
`4
`5
`6
`7
`8
`9
`10
`11
`13
`
`8.70 0.484
`9.25 0.371
`7.06 0.343
`6.21 0.253
`0.41 0.009
`0.37 0.003
`8.20 0.315
`7.12 0.085
`7.84 0.380
`1.10 0.048
`0.50 0.025
`0.30 0.003
`
`11.5
`10.8
`14.3
`16.1
`243
`270
`12.2
`14.0
`12.8
`90.9
`200
`333
`
`MeCN (46.0)a
`
`Carba-
`zoles
`
`5.56
`3.34
`4.86
`4.07
`0.98
`0.81
`3.84
`1.19
`4.85
`5.3
`5.0
`1.0
`
`5.93
`7.46
`9.30
`12.0
`242
`269
`8.36
`12.9
`7.91
`85.6
`195
`332
`
`13.65
`7.38 0.276
`13.5
`7.43 0.288
`15.7
`6.38 0.276
`17.1
`5.84 0.224
`241
`0.41 0.007
`285
`0.35 0.003
`14.3
`7.01 0.220
`14.9
`6.69 0.304
`14.5
`6.89 0.324
`73.3
`1.37 0.043
`175
`0.51 0.025
`0.27 0.0026 370
`
`3.62
`3.88
`4.33
`3.84
`0.90
`0.86
`3.14
`4.54
`4.70
`3.15
`2.81
`0.96
`
`9.93
`9.57
`11.3
`13.3
`240
`284
`11.1
`10.4
`9.81
`70.1
`172
`369
`
`13.6
`7.33 0.367
`15.0
`6.66 0.261
`17.3
`5.79 0.284
`20.2
`4.95 0.194
`237
`0.42 0.003
`0.32 0.0005 312
`6.29 0.328
`15.9
`6.57 0.310
`15.2
`6.58 0.394
`15.2
`1.25 0.047
`79.8
`0.35 0.013
`285
`0.25 0.0016 400
`
`5.01
`3.92
`4.91
`3.92
`0.81
`0.16
`5.21
`4.72
`5.99
`3.75
`3.71
`0.64
`
`8.63
`11.1
`12.4
`16.3
`236
`312
`10.7
`10.5
`9.21
`76.0
`282
`399
`
`PriOH (48.6)a
`
`EtOH (51.9)a
`
`ff
`
`tf
`(ns)
`
`kf
`( 107) s1b
`
`0
`kf
`( 107) s1c
`
`kisc
`( 107) s1d
`
`tf
`(ns)
`
`ff
`
`kf
`( 107) s1b
`
`0
`kf
`( 107) s1c
`
`kisc
`( 107) s1d
`
`tf
`(ns)
`
`ff
`
`kf
`( 107) s1b
`
`0
`kf
`( 107) s1c
`
`kisc
`( 107) s1d
`
`1
`2
`3
`4
`5
`6
`7
`8
`9
`10
`11
`13
`
`12.8
`7.80 0.328
`12.9
`7.70 0.309
`14.9
`6.69 0.273
`13.1
`7.62 0.323
`0.38 0.0018 263
`0.35 0.0028 285
`7.67 0.339
`13.0
`7.06 0.344
`14.2
`7.47 0.285
`13.4
`1.15 0.040
`86.9
`0.47 0.014
`125
`0.47 0.0013 212
`
`4.20
`4.01
`4.08
`4.24
`0.66
`0.80
`4.42
`4.87
`3.82
`3.47
`1.75
`0.28
`
`8.62
`8.98
`10.9
`8.88
`262
`284
`8.62
`9.29
`9.57
`83.4
`123
`121
`
`11.3
`8.82 0.349
`11.5
`8.69 0.373
`13.6
`7.35 0.406
`15.6
`6.40 0.378
`250
`0.40 0.003
`285
`0.35 0.002
`12.1
`8.30 0.338
`12.9
`7.72 0.390
`15.2
`6.57 0.361
`84.0
`1.19 0.025
`128
`0.78 0.022
`0.47 0.0020 212
`
`3.96
`4.29
`5.53
`5.91
`0.75
`0.57
`4.07
`5.05
`5.49
`3.28
`2.82
`0.43
`
`7.38
`7.22
`8.08
`9.72
`249
`285
`7.98
`7.90
`9.73
`80.8
`125
`212
`
`11.3
`8.83 0.349
`11.6
`8.65 0.322
`13.7
`7.30 0.308
`15.6
`6.43 –
`0.43 0.0010 232
`0.38 0.0015 263
`8.38 0.291
`11.9
`7.74 0.320
`12.8
`6.32 0.226
`15.8
`1.20 0.038
`83.3
`0.79 0.021
`126
`0.45 0.0010 222
`
`3.95
`3.72
`4.22
`5.88
`0.44
`0.39
`3.47
`4.11
`3.58
`3.17
`2.66
`2.2
`
`7.38
`7.84
`9.48
`9.67
`232
`262
`8.46
`8.73
`12.2
`80.1
`123
`220
`
`a Reichardt solvent polarity parameter (ET(30)) [62,63].
`1.
`b kf=tf
`
`c kf0=kf ff.
`d kisc=kfkf
`0.
`
`RHEMK-1009.011
`
`

`

`62
`
`S.M. Bonesi, R. Erra-Balsells / Journal of Luminescence 93 (2001) 51–74
`
`[25,69,70], we attempted to correlate the one
`e