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Intramolecular electron-transfer excited state in 6-cyanobenzquinuclidine
Volume
70, number
3
CHEMICAL
INTRAMOLECULAR Krystyna Inshhttt!
ELECTRON-TRANSFER
ROTKIEWICZ
1979;
15 March 1980
LETTERS
EXCITED
STATE
IN 6-CYANOBENZQUINUCLlDlNE
and Wresrawa RUBASZEWSKA
of PhyacalChermsny,
Recervcd 23 November
PHYSICS
Polrsh Academy of Science, 01-224 Warsaw,Poland m final form
10 December
1979
6-cyanobenzquinuchdmc has a ngd structure wrth the lone-pau orbrtal of the ammo-group mtrogen atom and the norbrtals of the aromntrc rmg mutually perpcndrcular. It 1s a model for the previously postulated twrsted rnternal chargetrnnsfcr cxcrted states. The fluorcsccnt smglet state was rdentrfied as a strongly polar state with a full charge separation, observed m absorption as the t(n, rr*) ewttcd state. The results strongly support the twtsted Internal charge-transfer state hypothesrs.
1. Introduction Ln prevrous papers [l] we interpreted the double lurrunescence of 4-cyano-N,N-dimethyianilme (CDMA) in terms of two excited forms of this molecule, B* and A*.
6-cyanobenzquirmclidine (CBQ) has in its ground state a geometry close to that postulated for the TICT states (orthogonal donor and acceptor systems) and can be used as a model compound for testing the TICT hypothesrs.
2. Experimental
B’
A’
CBQ
The structural difference between B* and A* 1s due to the conformation of the dunethylamino group (electron donor) with respect to the benzene ring (acceptor): m the case of B* the lone electron pau of the donor is perpendrcular to the ring (resembhng the geometry of the ground state), whtle the A* state is attnbuted to the form twrsted in such a way that the lone pair is located in the plane of the ring. The latter conformatron promotes a full electron transfer from thendonor lone pair to the lowest vacant orbital of the cyanobenzene rrelectronic system, thus producmg a twisted mtramolecular charge-transfer excited state (TJC-0 111.
444
The syntheses of CBQ wrll be descrrbed elsewhere [2]. The compound was punfied by means of TLC (SIO,, eluent: ethyl acetate), followed by sublimation at 75” C m high vacuum. Solvents. rsooctane and CH3CN “Uvasol” Merck, CH,OH and CH,Cl, for fluorescence spectroscopy were used wrthout further pullfication. 2-methyltetrahydrofuran (MTHF, Fluka-pure) was dried over KOH, passed twice through a column ftied with basrc Al,O,, then through a silica-gel column, dried over Na and finally distilled from T_.iAlH, under N,. Ethanol and propanol @a POCh-Gliwice) were stirred for 24 h with charcoal, filtered and distilled. All chemicals were checked for purity before use. Commercial 0, was used for fluorescence quenching. Quantum yields of fluorescence in various solvents were measured against quinine sulphate as standard, those of CBQ phosphorescence with a benzonitrile standard, @pt., = 0.43 +- 0.02 [3]. Fluorescence, phosphorescence, emission anisotropy and low-temperature absorption spectra were measured with the multifunctional Jasny spectrofluorimeter [4].
Volume
CHEMICAL
70, number 3
PHYSICS
3. Results The absorption
spectrum
of the lack of symmetry). Assuming p, = 4.05 D from quantum-chemical calculations [8] we”obtained 13 < peG15D. The fluorescence quantum yield & has been measured in several solvents in the temperature range 120-295 K; in all solvents ef < 10b2. in CH2C12 9 is temperature independent, but in alcohols (E009, n-propanol, n-butanol)we observed a slight decrease in @f with cooling. As CBQ is not excited by the N2 laser beam, we could not determine the fluorescence lifetime in a direct measurement; it was determined indirectly in CH,OH solution by means of the 0, quenching technique [9], under pressures up to 80 atm To evaluate the lifetime we have used the Stem-Volmer equation
of CBQ in the region of
3200047000 cm-1 isshown in fig. 1. The first weak transition
(e =
100)
is overlapped
by the stronger
sec-
ond
band. Better resolution is achieved in aprotlc solvents due to the red-shift of the weak band with increasing solvent polarity. in protic solvents this band slufts to higher energ?es, resembling a ‘(n, n*) transition. The second band (II) etibits nearly the same transition energy and vibrational structure as the first absorption band of the protonated molecule (CBQH+, cf. fig. 1) or of benzonitnle (BN) [S]. The fluorescence spectrum of CBQ (fig. 1) IS broad and structureless with a large Stokes shift (900012000 cm-l) which increases with solvent polarity. The change of dipole moment on excitation esti- mated according to Kawski [6] on the basis of Stokes shifts is 9 < Ap* < 11 D. We have assumed the same polarisabtity (Yfor the ground and excited state, an Onsager radius 4 G a G 5 A (cf. encounter radius RO for 3-methyM-cyano-N,Ndlmethylamline (MCDMA) [7]) and a/ax=;. The dipole moment in the excited state, per has been estimated as a sum of the ground-state dipole moment, pg, and A/J (ii fact cc, f pg because
n 7-l
F
/
/I
20
!’
MO/9
= 1 + k@*I
,
421
where Go/4 is the fluorescence quantum yield ratio in the absence and presence‘ of 0,; W is the fraction of molecules quenched in a diffusion-controlled stationary process according to Weller [lo] withencounter radlusRO = 4 A(cf. ref. [7]). A relative diffusion coefficient for reagents has been evaluated from the reMion Dq = constant(7 - solvent viscosity), taking for CH,CN: D = 9.5 X 1O-5 cm2 s-t [7], g = 0,442 cl?,
/c--l. .
/’
\
\ \
I
I
15 March 1980
LETTERS
\
1’
\ I
25
\
,n, I
‘<
.
,’
I’
30
-
:
.
35
5 1
40 = lO%m-’
1
acetonitrile (eeucitation specFig. 1. Absorption and fluorescence (F) spectra of CBQ III- --isooctane; methanol; -o-o4trum momtored at 23000 cm-’ : open points - experimental values overlap exactly with the absorption spectrum) at room temperature; (Ph) phosphorescence at 77 K III n-propanoi as solvent. __ Absorption and fluorescence spectra of CBQHC in acidified propanolic solution at room temperature.
445
Volume
70. number
3
CHEMICAL
PHYSICS
and for CHsOH Q= 0.611 CP [I I]. The rate of quenching with 02, kg, has been assumed equal to the diffusion-limited rate constant for a senes of naphthalene derivatives [12],kg=3.1 X 10IOM-I s-*,asthe Size of those molecules was comparable to that of CBQ. [O,] has been determmed from Henry’s law, with [02] = 2 X 10e3 M in air-saturated CH,OH [I 1 J. From the good linear Stern-Volmer plot, we obtained rr = 8 X 10-g s, hence, using the value tif = 8 X IOW3, the radiative hfetime 7, = T& =2x 10-7s. The phosphorescence of CBQ (fig. 1) exlubits a strong similarity to BN phosphorescence. the same transitron energy, band shape, vibrational structure and the hfetirne, rph = 3.15 s (cf. ref. 131). The zerofield splitting parameter, D*, is very sirndar for both molecules: 0.1379 cm-l (CBQ) and 0.1386 cm-1 (BN). There is a difference, however, in phosphorescence quantum yields: 0.8 and 0.43 for CBQ and BN, respectively. Anisotropy of CBQ luminescence is shown m fig. 2.
15 March
LETTERS
That of fluorescence, measured in n-propanol and MTHF solutions at 150 K, is small but positive when excited at the absorption band II. Excitation to the *(n, n*) state leads to higher positive values. Phosphorescence excited at the second singlet i(rr, a*) (band 11) is negatively polarised, and almost depokrised when excited to the I(n, n*) state.
4. Acid-base propertiesof CBQ ln its ground state, the CBQ molecule exhibits weak basic properties;pK, = 5.6 + 0.1 was determined spectrophotometricalIy in buffered aqueous solutions. The error was mainly due to the simdarity between the absorption spectra of the neutral molecule and its catIon. A decrease in basic properties of CBQ with respect to the parent compounds (table 1) points to a weak but not negligble (un?) interaction between the lone electron pair of the donor and the x-electromc system of the acceptor in the ground state. In aqueous solutions, the fluorescence of both the cation and unprotonated molecule are observed. The ratio of quantum yields of both emissions is dependent on the concentration of two forms m the ground state (fluorescence of CBQH+ increases with ddution). Taking mto account the eneraes of O-O transitions of CBQ and CBQH+ estimated from absorption and fluorescence spectra, one might expect a drastic increase of acidity in the excited state (A@,* =Z -10). ln spite of that, us weakly acid solutions an almost unquenched cation fluorescence is observed. ln a concentrated acetate buffer (0.5 M, pH = 4.8), in which Table 1 Comparison
of acid-base
ABC0
properties Benzquinuchdme
CBQ
7.88
5.6
N 10’
1
I
32
35
3
38=10%m.’
Fig. 2. CBQ fluorescence and phosphorescence ewmtron amsotropy spectra, rf and rph, momtored at 23000 cm-’ and 24000 cm-‘, respectrvely, and correspondmg absorption spectra- m propanol as a solvent at 150 K (rph at 77 K); --m MTHF as solvent ff at 150 K, fPh at 77 K; absorption at room temperature. 446
1980
pKa = 10.79
[13]
[ 131
CHEMICALPHYSICSLETTERS
Volume 70, number3
CH3COO- acts as proton acceptor, the quantum yield of cation fluorescence decreases in favour of the fluorescence of the CBQ molecule, giving evidence of decreasing basic properties of CBQ m its excited state. A low rate of proton transfer to the solvent probably reflects a negative A8 * for the reactron CB*QH+ + ROH + -CB*Q+
i- ROH; ,
(3)
leadmg to a molecule with an internal charge separation between both subunits. Fluorescence of the cation was also observed m dilute alcoholic solutions cooled below 180 K, strongest in methanol and considerably weaker in n-butanol.
5. Discussion The lowest singlet state observed in the absorptron spectrum of CBQ (fig. 1) IS most probably the ‘(n, n*) state responsible for electron transfer from the dlmethylamino group to the aromatic acceptor subunit. Thus it would be identical with a postulated TICT state. The next one is a l(7r, R*) state - an almost unperturbed local state of the acceptor (see for companson the fist absorption band of BN). The existence of this local transition is a consequence of the weakness of coupling of the perpendicular substituent to the ring in the locally exerted 1 L,, state. The identity of phosphorescence spectra of CBQ and BN mdicates that this emrssion originates from the practically pure locai excited state of the benzonitnle subunit. Population of the tnplet state is more efficient in CBQ than m BN. This seems to result from the different character of singlet precursors in these molecules and from the existence of the 3(n, 7~*) state close to the singlet in CBQ (for TICT states the S-T separation is expected to be very small, as in a radicalion pair). The fluorescence of CBQ resembles the long-wave fluorescence of CDMA observed in polar solvents and assigned to originate from a TICT state. Both emissrons exlubrt simrlar band shape and transition energy. The excited state dipole moment, pe, is between 13 D and 15 D for CBQ, and 16.2 D for CDMA (the last value from electro-optical measurements [l]). The estrmated value of the radiative rate constant, kf = 5 X lo6 s-l, is approximately equal to the temperature-independent term of a radiative rate constant of
15 March1980
the TICT states [14]. In contrast to the TKT state and its derivatives, in the case of CBQ we do not observe any thermal activation of fluorescence in aprotic solvents (a weak dependence of & in alcoholic sohetions may be due to thermal activation of the breakage of hydrogen bonds which oppose an electron transfer from the lone electron pair of the donor). The ammo group in the CBQ molecule is rigidly fifed with respect to the acceptor moiety; thus, the vibration which might enhance the radiative process must be of sufficiently high frequency that no contriiution from the corresponding vrbrationally excited emitting state is observed. The unexpectedly large Stokes shift, observed even in hydrocarbon solvents, could make somewhat doubtful the identity of the ‘(n, rr*) state from absorption and of the fluorescence emitting state in CBQ. However, we have not found any other weak absorption down to E > 10 cm* mrnol-1 below the ‘(n, n*) transrtion. Moreover, in spite of the rigidity of the CRQ molecule we can expect some structural changes in the excited state, e.g. by analogy to the structurally related ABC0 molecuIe. In this latter case the Stokes shift, even in the gas phase, is about 4000 cm-l [ES] _ Most probably in both CBQ and ABC0 in the emitting state the configuration around the ammo nitrogen tends to be planar, as in the R3N+ ions. On the other hand, fluorescence of several tertiary ahphatic amines, including the cage amine ABCO, also exhibits in liquid solutions a considerable Stokes shift which increases with solvent polarity, and a long radiative lifetime of the order IO-7 s. The origin of this fluorescence is still controversial. The positive value of the anisotropy of fluorescence excited within the r(n, rr*) + So absorption band strongly supports the identity of this state and the fluorescing one. Because of the striking similarity to BN, the phosphorescence of which is mainly out-ofplane polarised 1161, it seems reasonable to assume the same for CBQ phosphorescence. Thus the fhtorescence and phosphorescence anisotropies would have overlapped if the fluorescence were out-of-plane polarised, as is the emission from the ‘(n, a*) state. They are, however, mutually shifted (fig. 2). it implies that either the fluorescence or phosphorescence! has an in-plane component (aside of the main out-ofplane one). Because of fluorescence depolarisation by excitation to the fust 1(rr, #) state, we conclude that
Volume 70, number 3
CHEMICALPHYSICS LETTERS
both internal charge-transfer fluorescence and the r(n, a*) + So transitron in absorption have mrxed polam&ions, with both m- and out-of-plane components. The present results with CBQ are good evidence in favour of the TICT hypothesis, although an effort to find a clear-cut proof led us unexpectedly to a series of new probIems. However, all of them find their explanations on the basis of that hypothesis.
Acknowledgement We express our thanks to professor Z.R. Grabowski for valuable drscussions and for his most helpful comments on the present work. The authors are grateful to Dr. A. Grabowska and Dr. A. Siemiarczuk for their cntical readmg of the manuscript, and thank Mr. A. Mordziriski for the CBQ phosphorescence quantum yield measurements.
15 March 1980
References [ 11 Z.R Grabowski, [2] [3] [4] [S] [6] [ 71
[ 81 [9] [ 101 [ II] [ 121 [13] [ 14j (151 [ 161
K. Rotkiewicz, A. Sremiarnuk, D.J. Cowley and W. Baumann, Nouv. J. Chum. 3 (1979) 443, and references therein. A. K&wczy&ki, to be pubbshed. N. Kanamaru, H.R. Bhattacharjee and EC. Lun. Chcm. Phys. Letters 26 (1974) 174. J. Jasny, J. Luminescence 17 (1978) 149. W Atlas of Orgamc Compounds, VoL 2, D8/2 fVerlag Chemie, Wemheim/Butterworths, London, 2966-1971). L. Blot and A. Kawski, Z. Naturforsch. 17a (1962) 621. K. Rotkrewicz, 2.R Grabowskr and J. Jasny, Chem. Phys. Letters 34 (1975) 55. J. LIP&~, unpubhshed results. G. Weber and J.R. Lakowicz, Chem. Phys. Letters 22 (1973) 419. A. Weller, Z. Physrk. Chcm. NF 13 (1957) 335; 15 (1958) 438. Landolt-Bornstein (Spnnger, Berbn) B.II-Sa, p. 246, p. 208, B-II-2b, pp. l-75. P. Lentz, H. Blumc and D. Schulte-Frohhnde, Ber. Bunsenges. Physlk. Chem. 74 (1970) 484. B.M. Wepster, Rec. Tray. Chim. 71 (1952) 1171. Z.R. Crabowski, K. Rotkiewrn, W. Rubaszewska and E. Kukor-Kammska, Acta Phys. PoIon. A54 (1978) 767. A.M. Halpem, J. Am. Chem. Sot. 96 (1974) 7655. G.L. LeBel and J.D. Laposa, J. MoL Spectry. 41 (1972) 249.
70, number
3
CHEMICAL
INTRAMOLECULAR Krystyna Inshhttt!
ELECTRON-TRANSFER
ROTKIEWICZ
1979;
15 March 1980
LETTERS
EXCITED
STATE
IN 6-CYANOBENZQUINUCLlDlNE
and Wresrawa RUBASZEWSKA
of PhyacalChermsny,
Recervcd 23 November
PHYSICS
Polrsh Academy of Science, 01-224 Warsaw,Poland m final form
10 December
1979
6-cyanobenzquinuchdmc has a ngd structure wrth the lone-pau orbrtal of the ammo-group mtrogen atom and the norbrtals of the aromntrc rmg mutually perpcndrcular. It 1s a model for the previously postulated twrsted rnternal chargetrnnsfcr cxcrted states. The fluorcsccnt smglet state was rdentrfied as a strongly polar state with a full charge separation, observed m absorption as the t(n, rr*) ewttcd state. The results strongly support the twtsted Internal charge-transfer state hypothesrs.
1. Introduction Ln prevrous papers [l] we interpreted the double lurrunescence of 4-cyano-N,N-dimethyianilme (CDMA) in terms of two excited forms of this molecule, B* and A*.
6-cyanobenzquirmclidine (CBQ) has in its ground state a geometry close to that postulated for the TICT states (orthogonal donor and acceptor systems) and can be used as a model compound for testing the TICT hypothesrs.
2. Experimental
B’
A’
CBQ
The structural difference between B* and A* 1s due to the conformation of the dunethylamino group (electron donor) with respect to the benzene ring (acceptor): m the case of B* the lone electron pau of the donor is perpendrcular to the ring (resembhng the geometry of the ground state), whtle the A* state is attnbuted to the form twrsted in such a way that the lone pair is located in the plane of the ring. The latter conformatron promotes a full electron transfer from thendonor lone pair to the lowest vacant orbital of the cyanobenzene rrelectronic system, thus producmg a twisted mtramolecular charge-transfer excited state (TJC-0 111.
444
The syntheses of CBQ wrll be descrrbed elsewhere [2]. The compound was punfied by means of TLC (SIO,, eluent: ethyl acetate), followed by sublimation at 75” C m high vacuum. Solvents. rsooctane and CH3CN “Uvasol” Merck, CH,OH and CH,Cl, for fluorescence spectroscopy were used wrthout further pullfication. 2-methyltetrahydrofuran (MTHF, Fluka-pure) was dried over KOH, passed twice through a column ftied with basrc Al,O,, then through a silica-gel column, dried over Na and finally distilled from T_.iAlH, under N,. Ethanol and propanol @a POCh-Gliwice) were stirred for 24 h with charcoal, filtered and distilled. All chemicals were checked for purity before use. Commercial 0, was used for fluorescence quenching. Quantum yields of fluorescence in various solvents were measured against quinine sulphate as standard, those of CBQ phosphorescence with a benzonitrile standard, @pt., = 0.43 +- 0.02 [3]. Fluorescence, phosphorescence, emission anisotropy and low-temperature absorption spectra were measured with the multifunctional Jasny spectrofluorimeter [4].
Volume
CHEMICAL
70, number 3
PHYSICS
3. Results The absorption
spectrum
of the lack of symmetry). Assuming p, = 4.05 D from quantum-chemical calculations [8] we”obtained 13 < peG15D. The fluorescence quantum yield & has been measured in several solvents in the temperature range 120-295 K; in all solvents ef < 10b2. in CH2C12 9 is temperature independent, but in alcohols (E009, n-propanol, n-butanol)we observed a slight decrease in @f with cooling. As CBQ is not excited by the N2 laser beam, we could not determine the fluorescence lifetime in a direct measurement; it was determined indirectly in CH,OH solution by means of the 0, quenching technique [9], under pressures up to 80 atm To evaluate the lifetime we have used the Stem-Volmer equation
of CBQ in the region of
3200047000 cm-1 isshown in fig. 1. The first weak transition
(e =
100)
is overlapped
by the stronger
sec-
ond
band. Better resolution is achieved in aprotlc solvents due to the red-shift of the weak band with increasing solvent polarity. in protic solvents this band slufts to higher energ?es, resembling a ‘(n, n*) transition. The second band (II) etibits nearly the same transition energy and vibrational structure as the first absorption band of the protonated molecule (CBQH+, cf. fig. 1) or of benzonitnle (BN) [S]. The fluorescence spectrum of CBQ (fig. 1) IS broad and structureless with a large Stokes shift (900012000 cm-l) which increases with solvent polarity. The change of dipole moment on excitation esti- mated according to Kawski [6] on the basis of Stokes shifts is 9 < Ap* < 11 D. We have assumed the same polarisabtity (Yfor the ground and excited state, an Onsager radius 4 G a G 5 A (cf. encounter radius RO for 3-methyM-cyano-N,Ndlmethylamline (MCDMA) [7]) and a/ax=;. The dipole moment in the excited state, per has been estimated as a sum of the ground-state dipole moment, pg, and A/J (ii fact cc, f pg because
n 7-l
F
/
/I
20
!’
MO/9
= 1 + k@*I
,
421
where Go/4 is the fluorescence quantum yield ratio in the absence and presence‘ of 0,; W is the fraction of molecules quenched in a diffusion-controlled stationary process according to Weller [lo] withencounter radlusRO = 4 A(cf. ref. [7]). A relative diffusion coefficient for reagents has been evaluated from the reMion Dq = constant(7 - solvent viscosity), taking for CH,CN: D = 9.5 X 1O-5 cm2 s-t [7], g = 0,442 cl?,
/c--l. .
/’
\
\ \
I
I
15 March 1980
LETTERS
\
1’
\ I
25
\
,n, I
‘<
.
,’
I’
30
-
:
.
35
5 1
40 = lO%m-’
1
acetonitrile (eeucitation specFig. 1. Absorption and fluorescence (F) spectra of CBQ III- --isooctane; methanol; -o-o4trum momtored at 23000 cm-’ : open points - experimental values overlap exactly with the absorption spectrum) at room temperature; (Ph) phosphorescence at 77 K III n-propanoi as solvent. __ Absorption and fluorescence spectra of CBQHC in acidified propanolic solution at room temperature.
445
Volume
70. number
3
CHEMICAL
PHYSICS
and for CHsOH Q= 0.611 CP [I I]. The rate of quenching with 02, kg, has been assumed equal to the diffusion-limited rate constant for a senes of naphthalene derivatives [12],kg=3.1 X 10IOM-I s-*,asthe Size of those molecules was comparable to that of CBQ. [O,] has been determmed from Henry’s law, with [02] = 2 X 10e3 M in air-saturated CH,OH [I 1 J. From the good linear Stern-Volmer plot, we obtained rr = 8 X 10-g s, hence, using the value tif = 8 X IOW3, the radiative hfetime 7, = T& =2x 10-7s. The phosphorescence of CBQ (fig. 1) exlubits a strong similarity to BN phosphorescence. the same transitron energy, band shape, vibrational structure and the hfetirne, rph = 3.15 s (cf. ref. 131). The zerofield splitting parameter, D*, is very sirndar for both molecules: 0.1379 cm-l (CBQ) and 0.1386 cm-1 (BN). There is a difference, however, in phosphorescence quantum yields: 0.8 and 0.43 for CBQ and BN, respectively. Anisotropy of CBQ luminescence is shown m fig. 2.
15 March
LETTERS
That of fluorescence, measured in n-propanol and MTHF solutions at 150 K, is small but positive when excited at the absorption band II. Excitation to the *(n, n*) state leads to higher positive values. Phosphorescence excited at the second singlet i(rr, a*) (band 11) is negatively polarised, and almost depokrised when excited to the I(n, n*) state.
4. Acid-base propertiesof CBQ ln its ground state, the CBQ molecule exhibits weak basic properties;pK, = 5.6 + 0.1 was determined spectrophotometricalIy in buffered aqueous solutions. The error was mainly due to the simdarity between the absorption spectra of the neutral molecule and its catIon. A decrease in basic properties of CBQ with respect to the parent compounds (table 1) points to a weak but not negligble (un?) interaction between the lone electron pair of the donor and the x-electromc system of the acceptor in the ground state. In aqueous solutions, the fluorescence of both the cation and unprotonated molecule are observed. The ratio of quantum yields of both emissions is dependent on the concentration of two forms m the ground state (fluorescence of CBQH+ increases with ddution). Taking mto account the eneraes of O-O transitions of CBQ and CBQH+ estimated from absorption and fluorescence spectra, one might expect a drastic increase of acidity in the excited state (A@,* =Z -10). ln spite of that, us weakly acid solutions an almost unquenched cation fluorescence is observed. ln a concentrated acetate buffer (0.5 M, pH = 4.8), in which Table 1 Comparison
of acid-base
ABC0
properties Benzquinuchdme
CBQ
7.88
5.6
N 10’
1
I
32
35
3
38=10%m.’
Fig. 2. CBQ fluorescence and phosphorescence ewmtron amsotropy spectra, rf and rph, momtored at 23000 cm-’ and 24000 cm-‘, respectrvely, and correspondmg absorption spectra- m propanol as a solvent at 150 K (rph at 77 K); --m MTHF as solvent ff at 150 K, fPh at 77 K; absorption at room temperature. 446
1980
pKa = 10.79
[13]
[ 131
CHEMICALPHYSICSLETTERS
Volume 70, number3
CH3COO- acts as proton acceptor, the quantum yield of cation fluorescence decreases in favour of the fluorescence of the CBQ molecule, giving evidence of decreasing basic properties of CBQ m its excited state. A low rate of proton transfer to the solvent probably reflects a negative A8 * for the reactron CB*QH+ + ROH + -CB*Q+
i- ROH; ,
(3)
leadmg to a molecule with an internal charge separation between both subunits. Fluorescence of the cation was also observed m dilute alcoholic solutions cooled below 180 K, strongest in methanol and considerably weaker in n-butanol.
5. Discussion The lowest singlet state observed in the absorptron spectrum of CBQ (fig. 1) IS most probably the ‘(n, n*) state responsible for electron transfer from the dlmethylamino group to the aromatic acceptor subunit. Thus it would be identical with a postulated TICT state. The next one is a l(7r, R*) state - an almost unperturbed local state of the acceptor (see for companson the fist absorption band of BN). The existence of this local transition is a consequence of the weakness of coupling of the perpendicular substituent to the ring in the locally exerted 1 L,, state. The identity of phosphorescence spectra of CBQ and BN mdicates that this emrssion originates from the practically pure locai excited state of the benzonitnle subunit. Population of the tnplet state is more efficient in CBQ than m BN. This seems to result from the different character of singlet precursors in these molecules and from the existence of the 3(n, 7~*) state close to the singlet in CBQ (for TICT states the S-T separation is expected to be very small, as in a radicalion pair). The fluorescence of CBQ resembles the long-wave fluorescence of CDMA observed in polar solvents and assigned to originate from a TICT state. Both emissrons exlubrt simrlar band shape and transition energy. The excited state dipole moment, pe, is between 13 D and 15 D for CBQ, and 16.2 D for CDMA (the last value from electro-optical measurements [l]). The estrmated value of the radiative rate constant, kf = 5 X lo6 s-l, is approximately equal to the temperature-independent term of a radiative rate constant of
15 March1980
the TICT states [14]. In contrast to the TKT state and its derivatives, in the case of CBQ we do not observe any thermal activation of fluorescence in aprotic solvents (a weak dependence of & in alcoholic sohetions may be due to thermal activation of the breakage of hydrogen bonds which oppose an electron transfer from the lone electron pair of the donor). The ammo group in the CBQ molecule is rigidly fifed with respect to the acceptor moiety; thus, the vibration which might enhance the radiative process must be of sufficiently high frequency that no contriiution from the corresponding vrbrationally excited emitting state is observed. The unexpectedly large Stokes shift, observed even in hydrocarbon solvents, could make somewhat doubtful the identity of the ‘(n, rr*) state from absorption and of the fluorescence emitting state in CBQ. However, we have not found any other weak absorption down to E > 10 cm* mrnol-1 below the ‘(n, n*) transrtion. Moreover, in spite of the rigidity of the CRQ molecule we can expect some structural changes in the excited state, e.g. by analogy to the structurally related ABC0 molecuIe. In this latter case the Stokes shift, even in the gas phase, is about 4000 cm-l [ES] _ Most probably in both CBQ and ABC0 in the emitting state the configuration around the ammo nitrogen tends to be planar, as in the R3N+ ions. On the other hand, fluorescence of several tertiary ahphatic amines, including the cage amine ABCO, also exhibits in liquid solutions a considerable Stokes shift which increases with solvent polarity, and a long radiative lifetime of the order IO-7 s. The origin of this fluorescence is still controversial. The positive value of the anisotropy of fluorescence excited within the r(n, rr*) + So absorption band strongly supports the identity of this state and the fluorescing one. Because of the striking similarity to BN, the phosphorescence of which is mainly out-ofplane polarised 1161, it seems reasonable to assume the same for CBQ phosphorescence. Thus the fhtorescence and phosphorescence anisotropies would have overlapped if the fluorescence were out-of-plane polarised, as is the emission from the ‘(n, a*) state. They are, however, mutually shifted (fig. 2). it implies that either the fluorescence or phosphorescence! has an in-plane component (aside of the main out-ofplane one). Because of fluorescence depolarisation by excitation to the fust 1(rr, #) state, we conclude that
Volume 70, number 3
CHEMICALPHYSICS LETTERS
both internal charge-transfer fluorescence and the r(n, a*) + So transitron in absorption have mrxed polam&ions, with both m- and out-of-plane components. The present results with CBQ are good evidence in favour of the TICT hypothesis, although an effort to find a clear-cut proof led us unexpectedly to a series of new probIems. However, all of them find their explanations on the basis of that hypothesis.
Acknowledgement We express our thanks to professor Z.R. Grabowski for valuable drscussions and for his most helpful comments on the present work. The authors are grateful to Dr. A. Grabowska and Dr. A. Siemiarczuk for their cntical readmg of the manuscript, and thank Mr. A. Mordziriski for the CBQ phosphorescence quantum yield measurements.
15 March 1980
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