Phosphorescent intramolecular charge transfer triplet states

22 November 1996 :2

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CHEMICAL PHYSICS LETTERS

ELSEVIER

Chemical PhysicsLetters 262 (1996) 633-642

Phosphorescent intramolecular charge transfer triplet states Jerzy Herbich a, Andrzej Kapturkiewicz a, Jacek Nowacki h a Institute of Physical Chemistry. Polish Academy of Sciences, Kasprzaka 44/52.01-224 Warsaw. Poland b Department of Chemistry. 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 T~ 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 3CT character of the emitting triplet states in 3,6-di-tert-butylcarbazol-9-yl dicyanobenzenes.

1. Introduction

The concept of excited-state intramolecular electron transfer (lET) in donor (D)-acceptor (A) compounds formally linked by a single bond has played a central role over the last three decades in the discussion of their singlet state photophysical properties [1-12]. The nature of dual fluorescence (shortwave band corresponding to an excited state of relatively low polarity and a low-energy band emitted 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]. Contrary to 4-dialkylamino compounds, large conjugate "rr-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 lET reactions (e.g. the free energy changes, the solvent and intramolecular reorganization 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 moments 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(Tr,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© 1996Elsevier Science B.V. All rights reserved. PII S0009-2614(96)01122-0

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J. Herbich et al. / Chemical Physics Letters 262 (1996) 633-642

gations. The formation of non-emissive CT triplet states, however, has been reported for a few flexible polymethylene bridged D-(CH2)n-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 3 ( ~ , r r . ) states being higher than those of the lowest t'3CT states, in the approximation of a weakly interacting radical ion pair A - - D + (as forming the CT state) the ~CT and -'(ST 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 ('rr,'rr *) states (e.g., by a change in the donor or acceptor subunits with proper redox potentials). It should be noted that the notation t.3(n,n~) and t'3CT corresponds here to the excited states resulting either from the electronic transition mainly localized in the donor (or acceptor) subunit or from the transfer 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-tertbutylcarbazole (being an electron donor) containing benzonitrile, nicotinonitrile or various dicyanobenzenes as an electron acceptor (see Scheme 1). The electron affinity of the acceptor moiety, increasing in the order: benzonitrile-nicotinonitrile-dicyanobenzenes, is the main variable in our comparative investigations. We focus on the solvent, concentration and

(c.,),c~=~
(~.3),o~c(~)%

CN

CNP

temperature effects on the long-lived emission (phosphorescence and delayed fluorescence), measurements of low-temperature luminescence polarization and lifetimes, and preliminary ESR investigations.

2. Experimental The synthesis and purification of 3,6-di-tertbutylcarbazole (CAR) and its electron donor-acceptor derivatives: 4-(3,6-di-tert-butylcarbazol-9-yl)benzonitrile (CBP), 3-(3,6-di-tert-butylcarbazol-9yl)benzonitrile (CBM), 2-(3,6-di-tert-butylcarbazol9-yl)benzonitrile (CBO), 6-(3,6-di-tert-butyicarbazol9-yl)nicotinonitrile (CNP), 2-(3,6-di-tert-butylcarbazol-9-yl)nicotinonitrile (CNO), 3-(3,6-di-tert-butylcarbazol-9-yl)phthalonitrile (CPO), 4-(3,6-di-tertbutylcarbazol-9-yl)isophthalonitrile (CIP), 3,6-ditert-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 SO 2 (TMS), methanol, ethanol and n-propanol were of spectroscopic grade. Butyronitrile (BN) (Merck, for synthesis) was triply distilled over KMnO 4 + K2CO 3, 1'205 and Call:, respectively. (+)-Camphor C I o H ) 6 0 (Fluka, cryometry grade, m.p. 176-

(c.3),c~c(c~),

(c.,),c~c(c)s) ~ (c,~),c~c(c.3) ~

CN

CNO

CBP

CBO

CN

CIP

CBM

CN

CIM

CTO

Scheme I. Chart with molecular structures.

CPO

CPM

J. Herbich et al. / Chemical Physics Letters 262 (1996) 633-642

180°C) was used without further purification. Lowtemperature experiments were performed in various glasses like MCH, 3-MP, BN, n-propanol, EME (a mixture of ethanol-methanol-EE, 8:2:1, 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 chopper system of the apparatus (modulation frequency of about 4 kHz) served for the separation of a long-lived luminescence (phosphorescence a n d / o r delayed fluorescence) from the total emission spectra. The phosphorescence lifetimes were measured by means of a TDS420A digitizing oscilloscope (Tektronix) and the ESR spectra with a JEOLME-3X spectrometer using a TE0, , cavity and 100 kHz magnetic field modulation. The standard potentials of the one-electron oxidag;,(A) o f the compounds t i o n --OX E (D) and reduction ~ed studied in ACN (containing 0.1 M tetra-n-butylammonium hexafluorophosphate as the supporting electrolyte) were determined by cyclic voltammetry [12].

635

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. 1. The spectra show a superposition of the bands corresponding to the donor and acceptor subunits [12], Similarly to carbazole [22], the first two absorption bands of CAR, being centred in n-hexane at 337 and 296 nm, have been assigned to the final '(w, ~r * ) states of tA t and ~B2 symmetry (corresponding in Platt's notation to the IL b and 1L a 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('rr, r r * ) ~ S 0 and ICT*--S0 are superimposed in the first absorption band of CBP [11,12] and CNP (Fig. 1), in CBO and CNO [12] a long-wave shoulder attributed to the ~CT ,,--S O transition 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 c m - t ) of the r o o m - t e m p e r a t u r e absorption b a n d s (~abs , c o r r e s p o n d i n g to t w o transitions t C T ,o- So and ~L b '-- So) and fluorescence m a x i m a (~nu) o f the carbazole derivatives in m e t h y l c y c l o h e x a n e as well as the fluorescence (~nu) a and p h o s p h o r e s c e n c e (~pho) m a x i m a in n-propanol glass at 77 K Compound

R o o m temperature Vabs JCT ~ S O

CAR CBP CBO CBM CNP CNO CIM CIP CPO CPM CTO a b c d

= 29100 e ') = 28600 e e 26450 -- 2 6 0 5 0 26800 25000

77 K ~'abs ILb '-- So

= = = =

29650 "~ 29600 29400 ? 30100 29700 29950 29950 29600 30150

Vnu 29050 27600 26400 27550 25200 = 24750 = 23050 23500 22500 23800 22200

Vnu a b c c c c d o d d ~ c

28650 27400 26300 26850 26350

c a 0 d c ? ? = 23200 a ? --- 2 3 2 0 0 d = 22250 c

Vpho 24200 23850 24150 24150 23550 24100 21900 21750 21500 2100(I 20850

b b b ~ b b ~ ~ d d c

L o w - t e m p e r a t u r e fluorescence spectra for C P M , CIP and C T O were obtained b y m e a n s o f a computational procedure (see text). Spectral position o f the 0 , 0 transition, a c c u r a c y o f the results: + 50 c m - ~. Spectral position o f the highest energy m a x i m u m (as a possible 0, 0 transition), a c c u r a c y o f the results: +_ 200 c m t. Spectral position o f the b r o a d l u m i n e s c e n c e b a n d m a x i m u m , a c c u r a c y of the results: + 200 c m ~. C B O , C N O a n d CIM show the C T absorption b a n d as a l o n g - w a v e shoulder [12].

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J. Herbieh et al. / Chemical Physics Letters 262 (1996) 633-642

,A ~'E t.) "7,

CIP 3

%

,,"!

] "~

oNP

2

'

o o

CBO

~

.

-5

"i ~ 0 50000

, 40000

CAR " 30000

! 20000

Wavenumber /cm a Fig. I. Room-temperature absorption spectra of CAR, CBO, CNP and CIP in n-hexane. Spectra of CBO, CNP and CIP are shifted along the Y-axis by a factor of 1 X 105. Low-energy parts of the absorption spectra are expanded by a factor of 5.

corresponding radical ion pair A - and D + with 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 dicyanobenzenes (except CIM) show a well separated low-energy 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 phosphorescence 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 3L, type [23]. These results indicate the dominant 3(rr,'rr*)

character of their phosphorescent triplet states, the excitation being mainly localized in the carbazole moiety. On the contrary, the phosphorescence spectra of the dicyanobenzene derivatives are structureless and considerably shifted to lower energies (Fig. 3), which suggests a different electronic structure of their Tt states than that of CAR. The ~hosphorescence can hardly be attributed to the (~, rr ) 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-~ for p-dicyanobenzene, 25500 cm-~ for o-dicyanobenzene and 26400 cm-J for m-dicyanobenzene) [24], The spectral position and shape of the fluorescence 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 1); the spectra are unambiguously assigned to the ~CT ~ S O emission. The positions of the correspond-

CNP

,c_ m

N

O

!

30000

!

25000

20000

15000

Wavenumber /cm-1 Fig. 2. Luminescence spectra of D-A derivatives of 3,6-di-tertbutylcarbazole containing benzonitrile or nicotinonitrile as electron acceptor. Fluorescence spectra (dashed lines) in n-hexane at room temperature and phosphorescence spectra (solid lines) in n-propanol glass at 77 K.

J. Herhich et a l . / Chemical Physics Letters 262 (1996) 633-642

:::J CIP

ot-

CIM

o o ,.,,_.

CPO

o CTO 0

o 12.

CPM i 30O00

25000

20000

15000

Wavenumber /cm-~ Fig. 3. Luminescence spectra of D-A derivatives of 3,6-di-tertbutylcarbazole containing terephthalonitrile, isophthalonitrile or phthalonitrile as electron acceptor. Fluorescence spectra (dashed lines) in methylcyclohexaneat room temperature and phosphorescence spectra (solid lines) in n-propanol glass at 77 K.

ing spectra of the molecules containing nicotinonitrile 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 phosphorescence). 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 separated 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 subtraction of the phosphorescence spectrum, appropriately weighted, from the total luminescence spectrum. The extracted fluorescence spectra of the latter compounds are similar to those observed in non-polar solvents at room temperature; the separation between the fluorescence and phosphorescence maxima is

637

about 1400 cm -~ (Fig. 3, Table 1). The singlet-triplet 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 phosphorescence of the dicyanobenzene derivatives with respect to those containing a benzonitrile or nicotinonitrile subunit cannot be explained by the formation of aggregates of the former compounds. The low-temperature emission spectra of CIP, CTO and CPO in various glasses like MCH, 3-MP, EME and n-propanol do not depend on the concentration in the range c ~ 5 X 10 . 6 to 2 × 10 - 4 M and the corresponding excitation spectra monitored in the fluorescence and phosphorescence regions match closely the respective bands of the room-temperature absorption. CPM and CIM, however, seem to show aggregation effects at liquid nitrogen temperature but only in pure hydrocarbon glasses: (i) the luminescence excitation spectrum of CPM in MCH reveals the low-energy shoulder with respect to the CT absorption band and (ii) the emission spectra of CPM in 3-MP and MCH ( c = 5 × 1 0 - 6 - 1 0 - 4 M) and CIM in 3-MP are considerably red-shifted with respect to those observed in BN and n-propanol glasses (c = 5 X 10-6-5 X 10 .5 M). For example, the fluorescence and phosphorescence maxima of CPM in MCH at 77 K lie at about 22000 and 19950 cm -~. respectively; in n-propanol the low-temperature fluorescence centred at 23200 cm -t 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 T t states in 3,6-di-tert-butylcarbazol-9-yl dicyanobenzenes we attempted to observe the phosphorescence in rigid polar media. For our investigations 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 e = 12 at 295 K; the value increases monotonously upon cooling whereas at about 238 K

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J. Herbich et a l . / Chemical Physics Letters 262 (1996) 633-642

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 c m - ~ is observed. Phosphorescence, however, 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 system) match closely those of the total emission spectrum (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 phosphorescence at 77 K, we have chosen tetramethylenesulfone [26], which is a more polar 'rigid rotator' than camphor. The e value of TMS is 44 at 295 K and

~.s'"g'5_.......................... /.._.~ ...... 35-

>o 3.0.

~25' F[] C)

Fluorescencem ACN Fluorescence in MCH Phosphorescencem prOH

20 2.5

3w0

315

410

Difference in the Standard Redox Potentials

/ V

Fig. 5. Correlation between energies of luminescence maxima and the difference in the standard redox potentials (E{ox °) - E~eAa~, as measured in acetonitrile) for CTO, CPM, CPO, CIP, CIM, CNO, CNP, CBM, CBO 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-propanol (PrOH) at 77 K.

Nonpolarenvironment (MCH) L

L

\,

(1) E

E

Polar envfronment(IPE & TMS)

m o c m

'\

'\

..o <

\,\

35000

30000

25000

20000

15000

Wavenurnber /cm -~

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 rCT *--S o and t(w,,n * ) , - - S O transitions, see text), and fluorescence in methylcyclohexane (MCH) and isopropyl ether (IPE). Low-temperature (77 K) fluorescence (dotted lines) and phosphorescence (dashed lines) spectra in MCH and tetramethylenesulfone (TMS).

below the temperature of phase transition (for T < 288 K, e = 3.8 > n 2 = 2.2, where n is the refractive index) is similar to that of IPE ( e = 3.9). Fig. 4 presents the results obtained for CTO at liquid nitrogen and room temperature. The low-temperature fluorescence 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 t CT ~ S 0. 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 considerable CT character of the emitting triplet states of the dicyanobenzene derivatives; the I C T - 3 C T energy gap in a polar surrounding seems to be lower than 1000 c m - 1. This hypothesis agrees well with the finding of a linear relationship between the phosphorescence energies and the difference in oxidation E(o~) and reduction E[~ ~ potentials of the donor and acceptor

J. Herhich et a l . / Chemical Physics Letters 262 (1996) 633-642

639

Table 2 A n i s o t r o p y values o f the low-temperature fluorescence ( R n u ) and p h o s p h o r e s c e n c e (Rph o) of selected D - A carbazole derivatives in E M E and E P A glasses at 86 K upon excitation to the first three absorption b a n d s c o r r e s p o n d i n g to transitions t C T ,-- S o, t L b ' - So and t L a <---S o (see text and Figs. 1 a n d 4) Compound

CAR CBO CBM CBP CTO CPO CIM

Medium

Rn u a

Rpho a

ICT ,-- S O

tL b *- S O

IL a ~ S O

ICT ,,-- S O

IL b ~ S O

EME EPA EME EME EME EPA

0.30 0.26 0.31 0.24

0.26 b 0.23 b 0.18 0.15 0.23 0.12

- 0.13 b -0.12 b

_ _ -----0.00 = 0.00 0.05 = 0.05

- 0.05 c

EME EPA EPA EME EPA

0.19 0.29 0.31

0.09 0.16 0.14

= 0.02 = 0.01 -0.10

d d d d

--0.07 -- 0 . 0 4

d d o d

0.18 0.22 0.19 0.16 o 0.17 d

~ ~ ~ ~

0.00 0,00 0,03 0.03

IL a , - S O

~ 0.00

0,11 0,13 0.11 ~ 0.16 ~ 0.17

~ 0.02 ~ 0.01 -0.07 --0.08

a Typical a c c u r a c y of the results: _+0.02. b As m e a s u r e d at the 0 , 0 fluorescence b a n d upon excitation into the 0 , 0 transition o f the first ( I L b ~ S o) or second (~L a ,-- S o) excited singlet state. At l o w e r fluorescence energies the absolute R values decrease as a result of strong vibronic coupling between the first two l(-rr, "r: *) states o f IA I and I B , s y m m e t r y [22,29,30]. c M e a s u r e d at 77 K. ,1 U p o n excitation to the red e d g e o f the absorption spectrum (A.exc in the range 350 to 360 nm).

between the donor H O M O and the acceptor L U M O orbitals [9,27]. The correlation is similar to those observed for the room temperature ~CT-~ S o fluo-

subunits in 3,6-di-tert-butylcarbazol-9-yl dicyanobenzenes (Fig. 5). The difference E w ) - L'red L'{A) --ox can be used as a measure of the energy difference

Table 3 Zero-field splitting parameters D * = ( D 2 + 3E2) I/2 (in c m - l ) a n d p h o s p h o r e s c e n c e lifetimes (in s) as determined in n-propanol glass at 77 K (see text) Compound

Medium

D *

zp

Ref.

carbazole benzonitrile o-dicyanobenzene m-dicyanobenzene p-dicyanobenzene CAR CBO CBP CBM CNP CNO CPO CIP CIM CTO CPM

EPA ethanol ethanol ethanol ethanol n-propanol n-propanol n-propanol n-propanol n-propanol n-propanol n-propanol n-propanol n-propanol n-propanol n-propanol

0.1024 0.1389 0.1450 0.1476 0.1301 0.1026 0.1059 0.1030 0.1055 0.1037 0.1058 0.0500 0.0800 0.0560 0.0600 0.0810

7.6

[23] [32] [24] [24] [24] this w o r k this w o r k this w o r k this w o r k this w o r k this w o r k this w o r k this w o r k this w o r k this w o r k this w o r k

a a a,c a a,c a b a a a a

3.7 5. I 2.1 7.2 4- 0.5 5.1 + 0.3 d 5.9 4- 0.3 a 4.0 + 0.3 0.41 4- 0.03 0.44 4- 0.03 0.65 4- 0.05 0.34 4- 0.03 0.79 4- 0.05

a A c c u r a c y o f the results: 4-0.001. ~ A c c u r a c y o f the results: _+0.005. c C B P and C N P show two E S R signals with D* parameters being about 0.103 and 0 . 0 9 6 c m - t, the presented values c o r r e s p o n d to the d o m i n a n t ones. The relative intensity o f both signals does not d e p e n d on concentration, d P h o s p h o r e s c e n c e d e c a y of C N P and C B P c a n n o t be satisfactorily fitted b y a monoexponential approximation. The d e c a y curves, however, indicate a presence o f a long c o m p o n e n t being a b o u t 3.2 a n d 5.1 s, respectively.

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J. Herbich et 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 character of the lowest excited triplet states and small JC T - 3 C T energy gap should favour intersystem crossing (ISC) as a probable radiationless deactivation path and should increase the phosphorescence probability due to a significant direct spin-orbit coupling between the 3CT and ~CT 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 T 1 states is expected to shorten the phosphorescence lifetimes and significantly change the phosphorescence polarization. Similarly to carbazole, the lifetime rp of the emitting triplet state of CAR in n-propanol glass at 77 K is as long as 7.2 s (Table 3). The phosphorescence lifetime for CBM and CBO is about 5 - 6 s, and for CNO it is somewhat shorter, being about 4 s. The phosphorescence decay for CNP and CBP is equally long, but the kinetic curves show an additional shorter component (about 0.5-1 s). The considerable decrease in rp to values shorter than 1 s is observed for the dicyanobenzene derivatives.

3.3. Low-temperature luminescence anisotropy The hypothesis of the CT character of the phosphorescent states of 3,6-di-tert-butylcarbazol-9-yl dicyanobenzenes is also supported by the results of the polarization studies (Table 2). The anisotropy of the emission R is given by [28]:

R=(lll-l,)/(l,

+ 21±)=O.2(3cos2a - 1)

(1)

where III and I± 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 ~ ( v , v * ) states of ~A~ and IB 2 symmetry in carbazole are polarized in-plane along the short C 2 symmetry axis and the long axis oriented perpendicularly to it, respectively [22,29]. Similarly to carbazole [30], the phosphorescence of CAR in several glasses (MCH, 3-MP and EME) at 77 K is negatively polarized with respect to the excitation to both

absorption bands: the anisotropy values are R = - 0 . 0 5 (Table 2). It reflects the out-of-plane polarization of the T~ ~ S o radiative electronic transition, being the result of the first-order spin-orbit coupling 3 . between the lowest t r i p l e t ( v , rr ) state and singlet I(n, rr* ) and t(cr, w* ) states [23]. The R values of the phosphorescence of CBM and CBO 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 phosphorescence of dicyanobenzene derivatives is strongly positively polarized upon excitation to the first two excited states. The R values are relatively large upon ICT*--S o excitation and tend to decrease with increasing excitation energy for the molecules without a C 2 symmetry axis (Table 2). For example, for CTO in EPA at 86 K the values of the phosphorescence anisotropy are: R = 0.22 ('~exc = 400 nm), R = 0.13 (Aex,. = 325 nm) and R = 0.01 (Aexc = 297 rim). The corresponding values of the fluorescence anisotropy are: 0.29 (400 nm), 0.16 (325 nm) and 0.02 (297 nm). For CIM, however, the phosphorescence R values are the same upon excitation to the lowest ~CT state and to the ~L b state (R = 0.17): the phosphorescence is negatively polarized upon excitation to the ~L a state (R = - 0 . 0 8 ) . These resuits prove the dominant CT character of the T~ states of 3,6-di-tert-butyl-carbazol-9-yl dicyanobenzenes: most probably the direct spin-orbit coupling between the 3CT and ICT states is responsible for the high phosphorescence anisotropy values. The directions of the electric dipole transition moments corresponding to the CT fluorescence and phosphorescence 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 resonance fields Bin,.. In Table 3 are collected zero-field splitting (ZFS) parameters D ~ as calculated from the expression [31 ]: D" = ( D 2 + 3E2) '/2 = { } ( X 2 + y2 qt_ Z2)}1/2 2 i/2 = { 3 ( h p ) 2 - 3(g~Bmin ) } •

(2)

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where D and E are ZFS parameters related to the principal values X, Y and Z of the spin-spin coupling tensor and h~, is a resonance microwave energy. The isotropic value of g = 2.0023 has been assumed in calculations. The values of the parameter D* for the benzonitrile 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 TI state is distributed over the carbazole w-system. On the contrary, dicyanobenzene derivatives show markedly smaller D* values. The increasing distance 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 compounds 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 considerable CT character of the emitting T1 states of 3,6-ditert-butylcarbazol-9-yl dicyanobenzenes. The phosphorescence 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 polarized upon excitation to the CT absorption band. Similarly to the CT fluorescence, the energies of the phosphorescence maxima are correlated with the difference in the standard redox potentials. The lifetimes of the 3CT states are considerably shorter and the values of the ZFS parameter D" are markedly smaller than those corresponding to the lowest carbazole 3(rr, ~r ~ ) state. We are now attempting to determine more accurately the structure and photophysical properties of the CT triplet states as well as the energy transfer routes by means of the polarization, ESR and transient absorption and emission investigations.

4. Conclusions

Acknowledgements

Comparative investigations of the lowest excited triplet state T~ in the series of D - A carbazole derivatives undoubtedly show a change in their electronic structure with increasing electron affinity of the acceptor moiety. 3,6-Di-tert-butylcarbazol-9-yl benzonitriles and 3,6-di-tert-butylcarbazol-9-yl nicotinonitriles exhibit a number of phosphorescence features characteristic for carbazole: (i) the vibrational structure of the emission spectrum, (ii) the spectral position of the 0, 0 band, and (iii) the decay being as long as several seconds. Moreover, ESR measurements yield values of the zero-field splitting parameter D ~ = ( D 2 + 3 E 2 ) E/2 similar to that of 3,6-ditert-butylcarbazole indicating that the spin density in the emitting T~ state is distributed over the carbazole • r-system. These results unambiguously imply the dominant 3(~r, "rr ~) 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 T~ state population.

This work was sponsored by grant 3T09A12708 from the Committee of Scientific Research. Technical assistance from Mrs. A. Zielifiska is deeply appreciated.

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