`
`22 November 1996
`
`Chemical Physics Letters 262 (1996) 633—642
`
`
`
`CHEMICAL
`
`PHYSICS
`LETTERS
`
`Phosphorescent intramolecular charge transfer triplet states
`
`Jerzy Herbich a, Andrzej Kapturkiewicz a, Jacek Nowacki b
`a Institute of Physical Chemistry. Polish Academy of Sciences, Kasprzaka 44/52. 01-224 Warsaw. Poland
`b Department ofChemistry. Warsaw University. Pasteura 1. 02-093 Warsaw, Poland
`
`Received 4 September 1996
`
`Abstract
`
`A comparative study of the electronic structure of the lowest excited triplet state T1 is presented for a series of N-bonded
`donor—acceptor derivatives of 3,6-di-tert-butylcarbazole containing benzonitrile, nicotinonitrile or various dicyanobenzenes
`as electron acceptor. Solvent, temperature and concentration effects on phosphorescence, measurements of luminescence
`anisotropy and lifetimes, and ESR investigations of selected compounds show the dependence of the electronic structure of
`their T] states on the electron affinity of the acceptor moiety and point to the 3 CT character of the emitting triplet states in
`3,6—di-tert—butylcarbazol-9—yl dicyanobenzenes.
`
`1. Introduction
`
`The concept of excited-state intramolecular elec-
`tron transfer (IET) in donor (D)—acceptor (A) com—
`pounds formally linked by a single bond has played
`a central
`role over the last
`three decades in the
`
`discussion of their singlet state photophysical proper—
`ties [1—12]. The nature of dual fluorescence (short-
`wave band corresponding to an excited state of
`relatively low polarity and a low-energy band emit—
`ted from a highly polar state) of 4-dialkylamino
`derivatives of benzonitrile [1—5], benzaldehyde [6],
`benzoic acid esters [2], pyrimidines and pyridine [7]
`in a sufficiently polar and mobile environment is still
`a subject of controversy, especially concerning the
`role of the solvent in the charge transfer (CT) state
`formation and the changes in the solute geometry
`[1—5].
`large
`Contrary to 4-dialkylamino compounds,
`conjugate n-systems like aryl derivatives of aromatic
`amines [8,9], various derivatives of biphenyl [10] and
`
`N—aryl carbazoles [11,12] show a single fluorescence
`band at room temperature. The CT character of their
`fluorescent state in polar solvents seems to be well
`proven. In particular, a bandshape analysis of the
`stationary CT emission spectra of numerous aryl
`derivatives of dimethylanilines [9] and of the CT
`absorption and CT emission bands of a series of
`N-bonded D-A carbazole derivatives [12] allowed us
`to estimate the quantities relevant
`to optical and
`non—radiative IET reactions (e.g.
`the free energy
`changes, the solvent and intramolecular reorganiza-
`tion energies,
`the average energy of the vibronic
`states coupled to the ET [13,14]) as well as to obtain
`approximate data for the excited-state dipole mo-
`ments and structural changes in the molecules under
`study.
`The lowest excited triplet state T], in all the D—A
`compounds studied until now, seems to be a locally
`excited 3(n,rr*) state of low polarity [1,15]. This
`conclusion has been drawn from phosphorescence,
`transient triplet-triplet absorption and ESR investi—
`
`0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved.
`PII SOOO9—2614(96)01122—0
`
`RHEMK-1007.001
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`634
`
`.I. Herbich er al. / Chemical Physics Letters 262 ( I996) 633—642
`
`gations. The formation of non-emissive CT triplet
`states, however, has been reported for a few flexible
`polymethylene bridged D—(CHZ),,—A compounds
`[16—18] and a piperidine bridged D—A system [19].
`A precondition for studying phosphorescent CT
`triplet states is to find a particular system with the
`energy levels of the 307,17“) states being higher
`than those of the lowest L3CT states, in the approxi—
`mation of a weakly interacting radical
`ion
`air
`A’—D+ (as forming the CT state) the ICT and ‘CT
`state should be nearly degenerate. An acceptable
`strategy can involve a lowering of the CT state
`energies with respect to those of the locally excited
`(17,17") states (e.g., by a change in the donor or
`acceptor subunits with proper redox potentials).
`It
`should be noted that
`the notation L3(”i-rm") and
`"3 CT corresponds here to the excited states resulting
`either from the electronic transition mainly localized
`in the donor (or acceptor) subunit or from the trans—
`fer of an electron between molecular orbitals of the
`
`donor and acceptor moiety. respectively.
`In this Letter we present a study of the electronic
`structure of the lowest excited triplet states for a
`series of N-bonded D—A derivatives of 3,6—di-tert-
`
`butylcarbazole (being an electron donor) containing
`benzonitrile, nicotinonitrile or various dicyanoben—
`zenes as an electron acceptor (see Scheme 1). The
`electron affinity of the acceptor moiety. increasing in
`the order: benzonitrile—nicotinonitrile—dicyanoben—
`zenes, is the main variable in our comparative inves—
`tigations. We focus on the solvent, concentration and
`
`temperature effects on the long-lived emission (phos-
`phorescence and delayed fluorescence), measure—
`ments of low-temperature luminescence polarization
`and lifetimes, and preliminary ESR investigations.
`
`2. Experimental
`
`The synthesis and purification of 3,6-di-tert—
`butylcarbazole (CAR) and its electron donor—accep-
`tor derivatives: 4-(3,6-di-tert-butylcarbazol-9—yl)ben-
`zonitrile
`(CBP),
`3—(3,6—di-tert-butylcarbazol-9—
`yl)benzonitrile (CBM), 2-(3,6-di—tert—butylcarbazol-
`9—yl)benzonitrile (CBO), 6-(3,6-di-tert-butylcarbazol-
`9—yl)nicotinonitrile (CNP), 2-(3,6-di-tert—butylcarba—
`zol-9—yl)nicotinonitrile (CNO), 3—(3,6-di-tert—butyl~
`carbazol-9-yl)phthalonitrile
`(CPO),
`4-(3,6—di-tert-
`butylcarbazol-9-yl)isophthalonitrile
`(CIP), 3,6-di-
`tert-butylcarbazol-9-yl-terephthalonitrile (CTO), 4-
`(3,6-di—tert—butylcarbazol-9-yl)phthalonitrile
`(CPM)
`and 5—(3,6—di-tert—butylcarbazol-9-yl)isophthalonitrile
`(CIM) will be described elsewhere [20].
`Solvents:
`hexane
`(HEX), methylcyclohexane
`(MCH), 3-methylpentane (3-MP), 2-methylbutane
`(2-MB), ethyl ether
`(EE),
`isopropyl ether
`(IPE),
`acetonitrile (ACN),
`tetramethylenesulfone (CH2)4
`SO2 (TMS), methanol, ethanol and n-propanol were
`of spectroscopic grade. Butyronitrile (BN) (Merck,
`for synthesis) was triply distilled over KMnO4+
`KZCO3, P205 and CaHZ, respectively.
`(+)-Cam—
`phor CioHieO (Fluka, cryometry grade, mp. 176—
`
`(CH ) c
`C(CH3)
`(CH ) c
`cicns)
`(CH ) c
`cicns)
`(cu ) c
`cicua)
`(CH) c
`C(C )
`33“ 3 as“ 3 as“ 3 as“ a as“ "33
`©
`N©Am
`©°”
`CL...
`CN
`
`CNP
`
`CNO
`
`CBP
`
`CBO
`
`CBM
`
`(CH3)361©I
`
`I©[C(CH3)3
`°“
`
`CN
`
`(ctt3)3c1(¢D (lg[ctcnfi
`CLNC
`CN
`
`,C(CH3)3
`(CH3)35“C(CH3)3 (CH3)JCI@~I
`gen
`CECN
`on
`NC
`
`(cu3y3t21@|)CQQCHB)3
`cw
`
`CN
`
`CIP
`
`CIM
`CTO
`Scheme l. Chart with molecular structures.
`
`CPO
`
`CPM
`
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`

`J. Herbieh e! til/Chemical Physics Letters 262 (1996) 633—642
`
`635
`
`180°C) was used without further purification. Low—
`temperature experiments were performed in various
`glasses like MCH, 3-MP, BN, n-propanol, EME (a
`mixture of ethanol—methanol—EE, 822:1, v/v) and
`EPA (a mixture of EE—2-MB—ethanol, 5:5:2, v/v).
`Absorption spectra were run on a Shimadzu UV
`3100 spectrophotometer. Corrected luminescence and
`excitation spectra at various temperatures as well as
`their polarization were
`recorded with a
`Jasny
`spectrofluorimeter and phosphorimeter [21]. A chop-
`per system of the apparatus (modulation frequency
`of about 4 kHz) served for
`the separation of a
`long-lived luminescence (phosphorescence and / or
`delayed fluorescence) from the total emission spec-
`tra. The phosphorescence lifetimes were measured
`by means of a TDS420A digitizing oscilloscope
`(Tektronix) and the ESR spectra with a JEOLME-3X
`spectrometer using a T130” cavity and 100 kHz
`magnetic field modulation.
`The standard potentials of the one—electron oxida-
`tion ngD’ and reduction E131) of the compounds
`studied in ACN (containing 0.1 M tetra-n-butylam-
`monium hexafluorophosphate as the supporting elec-
`trolyte) were determined by cyclic voltammetry [12].
`
`3. Results and discussion
`
`3.1. Absorption and luminescence spectra
`
`The room-temperature absorption spectra of the
`selected compounds in n-hexane are presented in
`Fig. l. The spectra show a superposition of the bands
`corresponding to the donor and acceptor subunits
`[12]. Similarly to carbazole [22], the first two absorp-
`tion bands of CAR, being centred in n-hexane at 337
`and 296 nm, have been assigned to the final l(17,1r *)
`states of lAl and 1B2 symmetry (corresponding in
`Platt’s notation to the 1LI) and 1La excited states of
`phenanthrene), respectively. The detailed inspection
`of the low—energy absorption region of the D—A
`carbazole derivatives clearly indicates an appearance
`of additional charge transfer singlet states. Whereas
`two transitions l(11,“rr*)<— S0 and lCT<— SO are
`superimposed in the first absorption band of CBP
`[11,12] and CNP (Fig. l), in CEO and CNO [12] a
`long-wave shoulder attributed to the 1CT <— S0 tran—
`sition is observed. The red shift of the CT absorption
`band in the latter molecules may be explained by an
`increase in the coulombic stabilization energy in a
`
`Table l
`
`Spectral positions (in cm‘ ') of the room-temperature absorption bands (13abs , corresponding to two transitions 'CT <— S0 and lLb <— So)
`and fluorescence maxima (17“,) of the carbazole derivatives in methylcyclohexane as well as the fluorescence (17,“) a and phosphorescence
`(17pm) maxima in n-propanol glass at 77 K
`Compound
`Room temperature
`
`77 K
`
`i’abs ILI: ‘” So
`f’abs ICT ‘— S0
`fipho
`ijflu a
`iflu
`29650
`—
`CAR
`24200 D
`28650 c
`29050 b
`23850 b
`27400 1‘
`27600 C
`’?
`= 29100
`CBP
`= 29600
`°
`CBO
`24150 b
`26300 d
`26400 C
`= 29400
`?
`CBM
`24150 "
`26850 d
`27550 C
`?
`z 28600
`CNP
`23550 h
`26350 C
`25200 C
`= 30100
`°
`CNO
`24100 b
`‘7
`: 24750 d
`21900 G
`'2
`= 23050 d
`= 29700
`e
`CIM
`29950
`26450
`CIP
`21750 d
`= 23200 ‘1
`23500 d
`29950
`= 26050
`CPO
`21500 d
`7
`22500 d
`29600
`26800
`CPM
`21000 d
`= 23200 d
`23800 d
`
`
`= 22250 c22200 c 20850 c
`30150
`25000
`CTO
`
`l
`a Low—temperature fluorescence spectra for CPM, CIP and CT0 were obtained by means of a computational procedure (see text).
`b Spectral position of the 0,0 transition, accuracy of the results: i 50 cm‘ .
`l
`6 Spectral position of the highest energy maximum (as a possible 0,0 transition). accuracy of the results: i 200 cm’ '.
`d Spectral position of the broad luminescence band maximum, accuracy of the results: i 200 cm’ .
`e CBO, CNO and CIM show the CT absorption band as a long<wave shoulder [12].
`
`RHEMK-1007.003
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`

`636
`
`J. Herbich er a1. / Chemical Physics Letters 262 (1996) 633~642
`
`
`
`(a
`
`N
`
`coefficient8I105M“cm‘1
`Molarextinction
`
`
`0
`50000
`
`40000
`
`t
`30000
`
`20000
`
`Wavenumber [cm1
`l. Room-temperature absorption spectra of CAR, CBO, CNP
`Fig.
`and CIP in n-hexane. Spectra of CEO, CNP and CIP are shifted
`along me Y-axis by a factor of l>< 105. Low-energy parts of the
`absorption spectra are expanded by a factor of 5.
`
`ion pair A‘ and D+ with
`corresponding radical
`respect to that in CBP and CNP (as arising from the
`position of the negatively charged CN group versus
`the positively charged donor moiety). Due to the
`increasing electron affinity of the acceptor subunit
`and the corresponding lowering of the energy of the
`CT state, 3.6-di—tert-butylcarbazol—9-yl dicyanoben-
`zenes (except CIM) show a well separated low-en-
`ergy CT absorption band (Table 1, Figs.
`1 and 4).
`The analysis of the solvatochromic effects on the
`spectral position and bandshape for the stationary
`fluorescence spectra at room temperature proves the
`CT character of the emitting singlet states of the
`D—A carbazole derivatives in a polar as well as
`non-polar environment
`[12]. On the other hand.
`molecules containing benzonitrile or nicotinonitrile
`as the electron acceptor show structured phospho—
`rescence spectra in various glassy solvents (Fig. 2);
`the vibrational structure and the spectral positions of
`the 0,0 band are similar to those observed for CAR
`(Table 1);
`the T, state of carbazole is of 3La type
`[23]. These results indicate the dominant 3('rrxrr“)
`
`the
`character of their phosphorescent triplet states,
`excitation being mainly localized in the carbazole
`moiety. On the contrary, the phosphorescence spec-
`tra of the dicyanobenzene derivatives are structure-
`less and considerably shifted to lower energies (Fig.
`3). which suggests a different electronic structure of
`their Tl
`states than that of CAR. The
`hospho-
`rescence can hardly be attributed to the
`(1T.Tl'*)
`state localized in the acceptor subunit — the energy
`levels of the T, states of various dicyanobenzenes
`are higher
`than that of carbazole (the electronic
`origins of the structured phosphorescence spectra are
`centred at 24700 cm‘l for p—dicyanobenzene, 25500
`cm‘l
`for o—dicyanobenzene and 26400 cm"I
`for
`m-dicyanobenzene) [24].
`The spectral position and shape of the fluores-
`cence spectra of the benzonitrile derivatives in glasses
`at 77 K are nearly the same as those observed at
`room temperature in non-polar solvents (Fig. 2, Table
`l);
`the spectra are unambiguously assigned to the
`1CT —-> S0 emission. The positions of the correspond-
`
`
`
`lau.
` Phosphorescence&fluorescenceintensity
`
`
`
`30000
`
`25000
`
`20000
`
`15000
`
`Wavenumber /cm’1
`
`Fig. 2, Luminescence spectra of D«A derivatives of 3,6-di-tert-
`butylcarbazole containing benzonitrile or nicotinonitn‘le as elec-
`tron acceptor. Fluorescence spectra (dashed lines) in n—hexane at
`room temperature and phosphorescence spectra (solid lines) in
`n-propanol glass at 77 K.
`
`RHEMK-1007.004
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`J. Herhit'h ct LIL/Chemical Physics Letters 262 ([996) 633—642
`
`637
`
`
`
`/a.u.
` Phosphorescenee&fluorescenceintensity
`
`
`30000
`
`25000
`
`20000
`
`15000
`
`Wavenumber mm1
`
`Fig. 3. Luminescence spectra of D—A derivatives of 3,6-di—tert-
`butylcarbazole containing terephthalonitrile,
`isophthalonitrile or
`phthalonitrile as electron acceptor. Fluorescence spectra (dashed
`lines) in methylcyclohexane at room temperature and phospho‘
`rescence spectra (solid lines) in n-propanol glass at 77 K.
`
`ing spectra of the molecules containing nicotinoni—
`trile are also similar (a trace of the structure appears
`in the fluorescence spectrum of CNP with a 0, 0 band
`at about 26350 cm”; a weak fluorescence band of
`CNO is strongly overlapped by an efficient phospho-
`rescence).
`
`The fluorescence spectra of the dicyanobenzene
`derivatives at liquid nitrogen temperature could not
`be directly detected due to a strong overlap of their
`fluorescence and phosphorescence. Measurements of
`the total luminescence and phosphorescence (as sep—
`arated from the total emission by the chopper system
`of the spectrofluorimeter) as well as an estimation of
`their
`relative intensities allowed us. however,
`to
`
`obtain the low-temperature fluorescence spectra for
`CPM, CTO and CIP by means of a simple subtrac—
`tion of the phosphorescence spectrum. appropriately
`weighted. from the total luminescence spectrum. The
`extracted fluorescence spectra of the latter com-
`pounds are similar to those observed in non-polar
`solvents at room temperature; the separation between
`the fluorescence and phosphorescence maxima is
`
`about 1400 cm”l (Fig. 3, Table 1). The singlet—tri-
`plet energy gap is somewhat larger for CPM (about
`2000 cm‘ ' ). CPO and CIM in various glasses at 77
`K mainly phosphoresce.
`It should be pointed out that the described changes
`in the shape and spectral position of the phospho-
`rescence of the dicyanobenzene derivatives with re-
`spect to those containing a benzonitrile or nicotinoni-
`trile subunit cannot be explained by the formation of
`aggregates of the former compounds. The low~tem-
`perature emission spectra of CIP, CTO and CFO in
`various glasses like MCH, 3-MP, EME and n—pro—
`panol do not depend on the concentration in the
`range oz 5 ><10‘6 to 2X10'4 M and the corre-
`sponding excitation spectra monitored in the fluores-
`cence and phosphorescence regions match closely
`the respective bands of the room-temperature absorp-
`tion. CPM and CIM. however. seem to show aggre—
`gation effects at liquid nitrogen temperature but only
`in pure hydrocarbon glasses:
`('1)
`the luminescence
`excitation spectrum of CPM in MCH reveals the
`low-energy shoulder with respect to the CT absorp-
`tion band and (ii) the emission spectra of CPM in
`3-MP and MCH (cz 5 ><10_‘J—10'4 M) and CIM
`in 3-MP are considerably red—shifted with respect to
`those observed in BN and n-propanol glasses (6 = 5
`X 1076—5 X 10‘5 M). For example.
`the fluores—
`cence and phosphorescence maxima of CPM in MCH
`at 77 K lie at about 22000 and 19950 cm”. respec—
`tively;
`in n—propanol
`the low-temperature fluores-
`cence centred at 23200 cm" is similar to that
`
`recorded in MCH at room temperature (Fig. 3. Table
`1). Thus. the phosphorescence spectra of CPM and
`CIM in glasses formed by polar solvents are also
`assigned to monomeric forms.
`
`3.2. Charge transfer phosphorescence
`
`In order to gain more insight into the nature of the
`states
`in
`3.6-di-tert—butylcarbazol—9—yl
`di~
`T1
`cyanobenzenes we attempted to observe the phos—
`phorescence in rigid polar media. For our investiga—
`tions we have at first chosen (+)-camphor, one of
`the best known ‘solid rotators’ [25]. Its mesotropic
`phase is characterized by a high static dielectric
`constant value sz 12 at 295 K; the value increases
`
`monotonously upon cooling whereas at about 238 K
`
`RHEMK-1007.005
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`

`638
`
`J. Herbie}! er al. / Chemical Physicx Letters 262 (1996) 6334542
`
`it decreases rapidly from 14 to 2.7. The spectral
`position and shape of the fluorescence of CTO and
`CPO in camphor at various temperatures roughly
`reflect the changes in its polarity: the cooling down
`from 295 to 240 K results in a red shift of the spectra
`of about 150 cm“, below 233 K a blue shift of
`about 700 cm“ is observed. Phosphorescence. how-
`ever, has not been observed between 295 and 220 K
`
`— the shape and spectral position of the long—lived
`luminescence (as selected out by the chopper sys-
`tem) match closely those of the total emission spec-
`trum (as recorded without choppers). Most probably
`due to the relatively high temperature and small
`singlet—triplet energy gap the rate of the delayed
`(intramolecular) fluorescence is larger than that of
`the spin-forbidden radiative transition. Therefore. to
`examine the solvent polarity effect on the phospho—
`rescence at 77 K. we have chosen tetramethylenesul-
`fone [26], which is a more polar ‘rigid rotator’ than
`camphor. The a value of TMS is 44 at 295 K and
`
`Nonpolar envtronment (MC H)
`
`Polar envtronment (IPE & TMS)
`
`/a.u
`Absorbanceandemissionintensity
`
`
`35000
`
`30000
`
`25000
`
`20000
`
`15000
`
`Wavenumber /cm'1
`
`Fig. 4. Profiles of absorption and luminescence spectra of CTO in
`non-polar and polar environments. Solid lines: room-temperature
`absorption (the first three bands corresponding to ICT *— S0 and
`l(‘lT.Tl" )e— S0 transitions, see text), and fluorescence in methyl-
`cyclohexane (MCH) and isopropyl ether (IPE). Low-temperature
`(77 K) fluorescence (dotted lines) and phosphorescence (dashed
`lines) spectra in MCH and tetramethylenesulfone (TMS).
`
`35
`
`F"0
`
`
`
`Transitionenergy/eV
`
`CAR Singlet
`
`
`
`CAR Triplet
`
`
`
`
`
`FluorescencemACN
`El
`0 Fluorescence in NCH
`0 Phosphoresoence in PrOH
`3.5
`4 0
`3 0
`2.5
`Difference in the Standard Redox Potentials
`
`2 0
`
`
`
`/ V
`
`Fig. 5. Correlation between energies of luminescence maxima and
`the difference in the standard redox potentials (Eg3)— 5:3,), as
`measured in acetonitrile) for CTO, CPM, CPO, CIP, CIM. CNO,
`CNP‘ CBM. CEO and CBP (from left to right, correspondingly).
`The diamonds and squares refer to the room temperature CT
`fluorescence in methylcyclohexane (MCH) and acetonitrile (ACN).
`respectively; the circles to the phosphorescence detected in n—pro—
`panol (PrOH) at 77 K.
`
`below the temperature of phase transition (for T<
`288 K, a =5 3.8 > n2 z 2.2. where n is the refractive
`
`index) is similar to that of IPE (a== 3.9). Fig. 4
`presents the results obtained for CTO at liquid nitro-
`gen and room temperature. The low-temperature flu-
`orescence spectra in MCH and TMS (as obtained by
`a subtraction of the phosphorescence from the total
`emission spectrum) are similar to those detected at
`room temperature in a medium of respective polarity
`(MCH and IPE). The phosphorescence spectra in
`non—polar solvents at 77 K also show a trace of the
`structure characteristic of the fluorescence 'CT —+ S0.
`The fluorescence and phosphorescence spectra of
`CTO in TMS are found to be markedly red—shifted
`with respect to those in MCH. Similar effects have
`been observed for CIP. It strongly suggests a consid-
`erable CT character of the emitting triplet states of
`the dicyanobenzene derivatives;
`the |CT—3CT en-
`ergy gap in a polar surrounding seems to be lower
`than 1000 cm“.
`
`This hypothesis agrees well with the finding of a
`linear relationship between the phosphorescence en-
`ergies and the difference in oxidation E3?) and re-
`duction Egg/:1) potentials of the donor and acceptor
`
`RHEMK-1007.006
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`

`J. Herbich er al. / Chemical Physics Letters 262 (I996) 633—642
`
`639
`
`Table 2
`
`Anisotropy values of the lowtemperature fluorescence (Rm) and phosphorescence (Rpm) of selected ID—A carbazole derivatives1n EME
`and EPA glasses at 86 K upon excitation to the first three absorption bands corresponding to transitions ICT <— S0, Lb «— So and L <— S0
`(see text and Figs. 1 and 4)
`
`
`Compound
`
`Medium
`
`Rflu “
`
`Rplno a
`
`CAR
`
`c130
`CBM
`CBP
`
`cro
`
`CPO
`ClM
`
`‘CT<—s0
`-
`EME
`_
`EPA
`~ 0.00
`z 0.00 d
`EME
`~ 0.00
`z 0.00 ‘1
`EME
`~ 0.03
`0.05 ‘1
`EME
`~ 0.03
`= 0.05 d
`EPA
`011
`0.18
`EME
`0.13
`0.22
`EPA
`0.11
`0.19
`EPA
`—0.07
`~ 0.16
`0.16 ‘1
`EME
`
`EPA —0.08 0,17 d ~ 0.17
`
`
`
`lth—s0
`— 0.05 C
`
`lLaws0
`
`~ 0.00
`
`~ 0.02
`~ 0.01
`
`CTr—SO
`'
`—
`0.30 d
`0.26 d
`0.31 d
`0.24 d
`0.19
`0.29
`0.31
`
`1Lb<—So
`0.26 b
`0.23 b
`0.18
`0.15
`0.23
`0.12
`0.09
`0.16
`0.14
`
`‘1.‘.,<—s0
`—0.13 b
`-0.12 b
`
`—0.07
`—0.04
`= 0.02
`z 0.01
`—0.10
`
`a Typical accuracy of the results: $0.02.
`b As measured at the 0,0 fluorescence band upon excitation into the 0.0 transition of the first (‘Lb <— 80) or second (’La <- So) excited
`singlet state. At lower fluorescence energies the absolute R values decrease as a result of strong vibronic coupling between the first two
`1(1r,1r " ) states of 1Al and 1B2 symmetry [22.29.30].
`C Measured at 77 K.
`
`d Upon excitation to the red edge of the absorption spectrum (Am in the range 350 to 360 nm).
`
`di‘
`3, 6-di- tert--butylcarbazol9--yl
`in
`subunits
`cyanobenzenes (Fig. 5). The difference E(D)— EEQ,’
`can be used as a measure of the energy difference
`
`between the donor HOMO and the acceptor LUMO
`orbitals [9.27]. The correlation is similar to those
`observed for the room temperature 1CT -> SO fluo~
`
`Table 3
`
`Zero-field splitting parameters D ‘ = (D2 + 3E2)”2 (in cm' 1) and phosphorescence lifetimes (in s) as determined in n-propanol glass at
`77 K (see text)
` Compound Medium D * 7,, Ref.
`
`
`
`
`carbazole
`EPA
`0.1024
`7.6
`[23]
`benzonitn'le
`ethanol
`0.1389
`[32]
`3.7
`o—dicyanobenzene
`ethanol
`0.1450
`[24]
`5.]
`m—dicyanobenzene
`ethanol
`0.1476
`[24]
`2.1
`p—dicyanobenzene
`ethanol
`0.1301
`[24]
`7 2 + 0.5
`CAR
`n—propanol
`0.1026 “
`this work
`5.1 i 0.3
`CBO
`n-propanol
`0.1059 “
`this work
`‘1
`CBP
`n-propanol
`0.1030 “'C
`this work
`5.9 i 0.3
`CBM
`n—propanol
`0.1055 a
`this work
`d
`CNP
`n-propanol
`0.1037 at
`this work
`4.0 i 0.3
`CNO
`n-propanol
`0.1058 "‘
`this work
`0.41 i 0.03
`CPO
`n-propanol
`0.0500 b
`this work
`0.44 i 0.03
`CIP
`n—propanol
`0.0800 3
`this work
`0.65 i 0.05
`ClM
`n—propanol
`0.0560 3
`this work
`0.34 i 0.03
`CTO
`n—propanol
`0.0600 3
`this work
`
`CPM this work n-propanol 0.0810 “‘ 0.79 i 0.05
`
`
`
`
`3 Accuracy of the results: i000]. b Accuracy of the results: i0.005. c CBP and CNP show two ESR signals with 0’ parameters being
`about 0.103 and 0.096cm7 ' , the presented values correspond to the dominant ones. The relative intensity of both signals does not depend on
`concentration. d Phosphorescence decay of CNP and CBP cannot be satisfactorily fitted by a monoexponential approximation. The decay
`curves. however, indicate a presence of a long component being about 3.2 and 5.1 s. respectively.
`
`RHEMK-1007.007
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`

`640
`
`J. Herbie/1 er al. / Chemical Physics Letters 262 ( 1996) 633—642
`
`rescence of the D—A carbazole derivatives in polar
`(ACN) and non--polar (MCH) solvents.
`The CT charlacter of the lowest excited triplet
`states and small CT— 3CT energy gap should favour
`intersystem crossing (ISC) as a probable radiation-
`less deactivation path and should increase the phos-
`phorescence probability due to a significant direct
`spin—orbit coupling between the 3CT and ICT states
`[23]. Thus. the increase in the electron affinity of the
`acceptor moiety in the series of D—A molecules and
`the corresponding growth in the CT character of the
`T1 states is expected to shorten the phosphorescence
`lifetimes and significantly change the phospho-
`rescence polarization. Similarly to carbazole. the life-
`time 'rp of the emitting triplet state of CAR in
`n-propanol glass at 77 K is as long as 7.2 5 (Table
`3). The phosphorescence lifetime for CBM and C B0
`is about 5—6 s, and for CNO it is somewhat shorter.
`
`being about 4 s. The phosphorescence decay for
`CNP and CE? is equally long, but the kinetic curves
`show an additional shorter component (about 0.5—1
`5). The considerable decrease in T},
`to values shorter
`than 1 s is observed for the dicyanobenzene deriva-
`tives.
`
`3.3. Low-temperature luminescence anisotropy
`
`The hypothesis of the CT character of the phos-
`phorescent states of 3.6-di-tert—butylcarbazol-9—yl di-
`cyanobenzenes is also supported by the results of the
`polarization studies (Table 2). The anisotropy of the
`emission R is given by [28]:
`
`RE(IN—IL)/(I”+2Ii)=0.2(3coszar— 1)
`
`(1)
`
`where I“ and Il are the intensities of luminescence
`polarized parallel and perpendicular to the electric
`vector of the exciting radiation. respectively, and a
`is the angle between the electric dipole transition
`moment in absorption and emission.
`The first
`two pure electronic transitions to the
`](1T.1T*) states of 1A] and ‘82 symmetry in car-
`bazole are polarized in—plane along the short C2
`symmetry axis and the long axis oriented perpendic-
`ularly to it. respectively [22,29]. Similarly to car—
`bazole [30], the phosphorescence of CAR in several
`glasses (MCH. 3—MP and EME) at 77 K is nega-
`tively polarized with respect to the excitation to both
`
`the anisotropy values are R:
`absorption bands:
`—0.05 (Table 2). It reflects the out-of—plane polar-
`ization of the T —-> SO radiative electronic transition
`being the result of the first-order spin—o—rbit coupling
`between the lowest triplet(17 'n' ) state and singlet
`(n.1r )and ',(o 11' )states [23].
`The R values of the phosphorescence of CBM
`and CEO in 3-MP, EME and EPA at 77 and 86 K
`
`are found to be zero, and the phosphorescence of
`CBP is slightly positively polarized (R = 0.05) upon
`excitation to the first absorption band. The phospho-
`rescence of dicyanobenzene derivatives is strongly
`positively polarized upon excitation t0 the first two
`excited states. The R values are relatively large upon
`CT<— S0 excitation and tend to decrease with in-
`creasing excitation energy for the molecules without
`a C2 symmetry axis (Table 2). For example, for
`CTO in EPA at 86 K the values of the phospho—
`rescence anisotropy are: R = 0.22 ()1exc = 400 nm),
`CXC
`R = 0.13 (Am. = 325 nm) and R = 0.01 ()1
`= 297
`nm). The corresponding values of the fluorescence
`anisotropy are: 0.29 (400 nm). 0.16 (325 nm) and
`002 (297 nm). For CIM however,
`the phospho-
`rescence R values are the same upon excitation to
`the lowest CT state and to the 'bL state (R: 0. 17):
`the phosphorescence is negatively polarized upon
`excitation to the 13L state (R = —008) These re—
`sults prove the dominant CT character of the T
`states of 3 ,6-di- tert--butyl-carbazol-9— yl dicyanoben—
`zenes; most probably the direct spin——orbit coupling
`between the CT and CT states is responsible for
`the high phosphorescence anisotropy values. The
`directions of the electric dipole transition moments
`corresponding to the CT fluorescence and phospho-
`rescence are found to be nearly the same.
`
`3.4. ESR investigations
`
`ESR spectra of the studied compounds have been
`measured in n-propanol at 77 K. In our preliminary
`investigations we have recorded transitions in the
`Am=2 region corresponding to the lowest reso-
`nance fields Bmm. In Table 3 are collected zero-field
`splitting (ZFS) parameters DX as calculated from
`the expression [31]:
`
`D’ =(D3+3E2)
`
`”2 { (X +Y +zz)}'/2
`=:{‘ (M —3(gBBmm) }'”
`
`(2)
`
`RHEMK-1007.008
`
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`
`

`

`J. Herbich er ul./Chemical Physics Letters 262 (1996) 633—642
`
`641
`
`where D and E are ZFS parameters related to the
`principal values X, Y and Z of the spin—spin cou-
`pling tensor and hv is a resonance microwave en—
`ergy. The isotropic value of g= 2.0023 has been
`assumed in calculations.
`
`The values of the parameter D* for the benzoni—
`trile and nicotinonitrile derivatives are similar to that
`
`of CAR (CBP and CNP show an additional weak
`
`ESR signal). It indicates that the spin density in the
`T1 state is distributed over the carbazole Tr-system.
`On the contrary, dicyanobenzene derivatives show
`markedly smaller D“ values. The increasing dis-
`tance between two unpaired electrons localized in
`the donor and acceptor subunits in the 3CT state of
`the latter compounds seems to be the most probable
`explanation of this finding. For all the studied com—
`pounds the decay of the ESR signals agrees well
`with the obtained phosphorescence lifetimes (for CNP
`and CBP the dominant ESR signal is connected with
`a long-lived phosphorescence component).
`
`On the contrary, the results point to a consider—
`able CT character of the emitting T] states of 3.6-di~
`tert-butylcarbazol—9-yl dicyanobenzenes. The phos-
`phorescence
`spectra
`are:
`(i)
`structureless,
`(ii)
`markedly shifted to the longer wavelengths with
`respect to those of the electron donor (carbazole) and
`acceptors (dicyanobenzenes), and (iii) Positively po—
`larized upon excitation to the CT absorption band.
`Similarly to the CT fluorescence. the energies of the
`phosphorescence maxima are correlated with the dif-
`ference in the standard redox potentials. The life-
`times of the 3CT states are considerably shorter and
`the values of the ZFS parameter D‘ are markedly
`smaller than those corresponding to the lowest car-
`bazole 3(1T. 17 ‘) state.
`We are now attempting to determine more accu-
`rately the structure and photophysical properties of
`the CT triplet states as well as the energy transfer
`routes by means of the polarization. ESR and tran-
`sient absorption and emission investigations.
`
`4. Conclusions
`
`Acknowledgements
`
`Comparative investigations of the lowest excited
`triplet state T1 in the series of D—A carbazole deriva-
`tives undoubtedly show a change in their electronic
`structure with increasing electron affinity of the ac
`ceptor moiety. 3,6-Di—tert—butylcarbazol-9-yl ben~
`zonitriles and 3,6-di—tert—butylcarbazol-9—yl nicoti~
`nonitriles exhibit a number of phosphorescence fea~
`tures characteristic for carbazole: (i) the vibrational
`structure or the emissmn spectrum, in) the spectral
`
`position of the 0,0 band, and (iii) the decay being as
`long as several seconds. Moreover, ESR measure—
`ments yield values of the zero—field splitting parame-
`ter D‘ =(D2 + 3E2)”2 similar to that of 3.6-di-
`ten—butylcarbazole indicating that the spin density in
`the emitting Tl state is distributed over the carbazole
`int-system. These results unambiguously imply the
`dominant 3(”n-,Frr") character of their lowest triplet
`states.
`the excitation being mainly localized in the
`carbazole moiety. It should be noted, however, that
`the behaviour of CNP and CBP is more complicated.
`ESR and phosphorescence investigations indicate the
`presence of an additional
`long-lived species, most
`probably an intermediate in the lowest T1 state popu-
`lation.
`
`This work was sponsored by grant 3T09A12708
`from the Committee of Scientific Research. Techni-
`
`cal assistance from Mrs. A. Zielir’iska is deeply ap-
`preciated.
`
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`
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`RHEMK-1007.009
`
`RHEMK-1007.009
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`J. Herbich er al. / Chemicul Physics Letters 262 (1996) 633—642
`
`[10] W. Van Damme, J. Hot