Radiative and radiationless processes in charge-transfer complexes

Volume

3. number

8

CHEMICAL

RADIATIVE IN

AND

PHYSICS

LETTERS

RADIATIONLESS

CHARGE-TRANSFER

August

1969

PROCESSES COMPLEXES

J. PROCHOROW and R. StEGOCZYI&Kt of Physics ~ Polish Academy of Sciences.

Institute

Hoza 69. Warsaw.

Received

Poland

23 June 1969

The measurements of absorption and fluorescence spectra, fluorescence quantum yields and r-l?cay times for charge-transfer complexes of tetrachlorophthalic anhydride-he.xamethylbenzene and pyromeLlitic dianhydride-hexamethylbenzene. in different solvents at room temperature, were made. The results have been discussed in terms of changes of the radiative and radiationless transition probabiLities.

1. TNTRODUCTION Fluorescence

of charge-transfer

(CT)

com-

plexes in liquid solutions at room temperature is a rather unusual phenomenon. The first case of such a fluorescence was observed by Czekalla et al. [I! 21 for the tetrachlorophtalic anhydride (TCPA) - hexamethylbenzene (HMB) complex. in slightly polar solvents. In 1966 Rosenberg and Eimutis [3] reported the observations of fluorescence of CT complexes formed between pyromellitic dianhydride (PMDA) and substituted methylbenzenes in CC14 at room temperature. Short and Parker [4] reported in their paper the results of measurements of fluorescence quantum yields of TCPA and PMDA complexes (with HBM and naphthalene) in several solvents at room temperature. They found the quantum yield rather low - about 0.01 and less. It must be emphasized, however, that most of the nonfluorescent complexes in solutions at room temperature,

exhibit

fluorescence

in rigid

phase,

i.e. in crystalline phase (even at room temperature) and in low-temperature glassy solutions and under these circumstances their [1,5,6], fluorescence quantum yield may become as large as = 0.1 - 0.4 [7]. So far no systematic investigations of a complete set of spectral data (including absorption and fluorescence spectra, fluorescence quantum yield and decay times as well as their dependence on solvent polarity and viscosity or on temperature) of CT complexes have been made. Such investigations are undoubtly necessary if one wanted to explain the deactivation mechanism operating in the excited state of the complex.

Measurements of the fluorescence spectra, the fluorescence quantum yield and decay time in different solvents were made by Mataga et al. [8] and by Weller et al. [9] for so-called heteroextimers (charge-transfer complexes formed only in the excited state, unstable in the ground state). Although the heteroexcimers are in some respects similar to the common weak CT comproxes they cannot be identified with the latter, and the conclusions arising from the observations of heteroexcimers’ fluorescence may be of littfe value when applied in a simple way to the CT complexes. In this letter we report our preliminary rasults on absorption and fluorescence spectra, the fluorescence quantum yield and decay times in different solvents at room temperature for TCPAHMB and PMDA-HMB CT complexes. 2. EXPERIMENTAL All compounds and solvents were very thoroughly purified and checked spectroscopically. Recorded fluorescence spectra, obtained in a common way (monochromator + photomultiplier + electronic recording set) were corrected for the spectral sensitivity of the photomultiplier. Fluorescencoe of TCPA-HMB complex was excited by the 3660A mercury line. In the case of the PMDA-HMB complex which has a double-maximum absorption band, excitation was carried out by means of 3660 and 435OA lines which coincide with respective sub-bands of the absorption band (the fluorescence is the mirror image of the low635

Volume

CHEMICAL PHYSICS LETTERS

number 8

3.

frequency sub-band). Attempts to observe fluorescence in highly polar solvents such as acetonitrile were unsuccessful. The fluorescence yields were measured in a way described by Parker and Reec [lo]. As standards of known fluorescence yield quinine bisulphate in 0.1 N sulphuric acid [lo], and trypaflavine in ethyl alcohol [ll] were used. For the PMDA-HMB complex the fluorescence yield was measured separately for excitation in low-frequency and high-frequency sub-bands. No oxygen quenching of fluorescence was found (there are no differences in the fluorescence yield of solutions prepared in the ordinary way and the deaerated ones). The fluorescence decay times were measured by means of the phase fluorimeter described in ref. [12]. These measurements were limited to the rather intense emissions, and to the spectra lying not too far in the red; e.g. for the PMDAHMB complex sure results could be obtained only in the cases of n-hexane and CC14 solutions.

WAVE

of the TCPA-HMB

complex

previously reported by Czekalla and Meyer [2]. The Stokes’ shift increases with increasing solvent polarity but this increase, similar to the red

Absorption complexes

(q?) and fluorescence in different solvents

Solvent

Table 1 (zf) maxima and the Stokes’ shift (Acs = za - zf) of TCPA-HMB and PMDA-HMB at room temperature. Refractive indices. fl. and dielectric constant. E, for the solvents used are also indicated

n

E

i;a

TCPA-HMB

(cm-l)

Ff (cm-l)

ya* (cm-l)

PMDA-HMB Cf (cm-l)

ACs (cm-l)

n-hexane

1.373

1.890

26 670

19 650

7020

22 880 (28 100)

16 600

6 280

cc14

1.463

2.238

25 970

18 700

7 270

22 730 (28 600)

16 160

6 570

1.380

3.390

26 670

17900

8 770

23 530 (28 570)

16420

7 110

m-chlorotoluene

1.521

5.550

25 130

18 000

7 130

22 730 (27 300)

16 100

6 630

1. 1-dichloroethane

1.416

10.46G

27 780

17 000

10 750

23 260 (30400)

n-propyl

ether

* The maximum

636

1O’Cm“

shift of fluorescence. does not closely follow the changes in solvent polarity. Nevertheless, the observed spectral data clearly show that the structures (electronic as well as internal geometrical structure) of excited and groud states of the complex are different, and that this difference becomes larger in more polar solvents. Such a tendency may be easily understood in terms of interaction between nearly ionic excited complex and surrounding polar solvent molecules. The direction of the solvent shift of fluorescence points out that the excited-state dipole moment is larger than the ground-state one. Using the well-known formula given by Bilot and Kawski [13] we calculated the value of dipole moment in the excited state of the complex. As a groundstate dipole moment for TCPA-HMB we accepted the value 3.6 D given in ref. [Z]. In the case of

As an example the typical observed absorption and fluorescence spectra are indicated in fig. 1. The maxima of fluorescence exhibit a red shift with increasing dielectric constant of the solvents (table 1). This shift is not strictly regular, simas in the case

NUMBER,

Fig. 1. Absorption (a) and fluorescence (f) spectra of TCPA-HMB -, and PMDA-HMB --complexes in nhexane at room temperature.

3. RESULTS AND DISCUSSION

ilarly

August 1969

of the high-frequency

absorption

sub-band

is given in brackets.

--

--

Volume 3, number 8

CHEMICAL PHYSICS LETTERS

the case of PMDA-HMB the quantum yield was measured with selective excitation of fluorescence in bcth sub-bads of the absorption band of the complex. Quantum yield of fluorescence excited in the low-frequency sub-band was always larger than that excited in the high-frequency sub-band. These results are in agreement with those we obtained earlier in the case of complexes of tetracyanoethylene with methyl-substituted benzenes which are fluorescent in organic glasses at 77’K [7]. It seems that the ratio of quantum yields of fluorescence excited in lowfrequency and in high-frequency sub-bands varies with solvents. However, as the quantum yields are very small, this dependence needs more detailed investigations. Fluorescence decay times for the complexes under consideration are very short, but they have reasonable values as compared with those reported by Czekalla et al. [l, 5] for weak CT complexes in the crystalline phase (e.g. 7.7 nsec for TCPA-HMB, 2.6 nsec for trinitrobenzene-HMB, 1.3 nsec for trinitrobenzene-durene, etc.). The decay time decreases with increasing dielectric constant of the solvent, particularly for the TCPA-HMB compex (table 2). The decrease of fluorescence quantum yields and of decay times with increasing solvent polarity were also observed by Mataga et al. [8] and by Weller et al. [9] in the case of heteroexcimers. Mataga explained the decrease of quantum yield

PMDA-HMB the ground-state dipole moment is unfortunately unknown. It seems reasonable to assume, however, that its value is close to zero as both components of the complex are nonpolar. The values of excited-state dipole moments, obtained in this way are as follows:

for TCPA-HMB

pe = 9.6 D,

for PMDA-HMB

r-t, = 6.OD.

August 1969

The obtained value of pe for TCPA-HMB is in gocd agreement with those obtained by Czekalla and Meyer [2], ~_c~= 10 D, and by Bilot and Kawski [13], /Je = 9.6 D. For PMDA-HMB no other experimental data concerning the exciled-state dipole moment exist. Table 2 lists quantum yields (Q), decay times (rd) and calculated transition probabilities (kf and ki). It is easily seen that the changes of the fluorescence quantum yield in different solvents are pronounced in the case of TCPA-HMB and rather slight in the case of PMDA-HMB. Such a difference between both investigated complexes is probably due to the different values of their excited-state dipole moments. If we assume that the interaction between excited complex and solvent molecules is in a first approximation similar to the analogous interaction of an excited molecule, then the complex with larger value of excited-state dipole moment should interact more strongly with solvent than the complex with smaller dipole moment of excited state. Hence the changes of fluorescence yield of the former would be more pronounced. The measured quantum yields are of the same order of magnitude as those previously reported by Short and Parker [4] (in CC14: Q = 0.004 for TCPA-HMB, and Q = 0.003 for PMDA-HMB). In

as due to the decrease bability in more polar

of radiative transition prosolvents, which in turn is

die to the changes of particular terms in the dipole transition moment. In terms of Mulliken’s theory [l4] the ground (g) and excited (e) states of a CT complex may be described by the wave functions (including lo-

Table 2 Quantum yields (Q), decay times (rd) and transition probabilities (kf and ki) in different solvents Solvent

TCPA-HMB Q

‘;-a

(nsec)

pphexane

0.017

2.0

cc14

0.007

1.5

n-propyl ether

0.005

0.7

m-chlorotoluene

0.0006

1,1-dichloroethane

-0.0001

(106L&-l) 8.5

Q* (108

z-q

PMBA-HMB ‘;-d (106::c-9 (nsec)

1.96

0.007 (0.004)

l-6

6.62

0.005 (0.003)

Il.5

0.003 (0.0007)

-

-

4.3

(206&) 6.20 6.63

-

* The quantum yields of the fluorescence excited in the high-frequency

sub-band

of absorption

are given in brackets.

637

Volume 3; number 8 tally

CHEMICAL PHYSICS LETTE EIS

excited configurations)

qg = ati

+ F b&j(A-

+e = a’&(AD)

- F b;$(A-

of the following

type:

D+) + 7 cj+AD)* D+) + 7 c&(AD)*

where &(AD) is the wave function of so-called no-bond structure, @i(A-D+) - dative structure, $(AD)* represents the locally excited configurations (within donor and/or acceptor molecule). As a rule weak (r, r) type complexing takes place a >> bi, ij- and 62 >> CL’[15]. Hence, in a first approximation the most important contributions to the transition moment are given by the following matrix

The the

elements:

first term is rather small as compared with second,

transition

but

its

relative

moment is large.

weightin

the

full

With increasing

sol-

vent polarity the ionic character of the excitedstate wave function (i.e. the relative weight of &(A- D+)) increases,

August 1969

of the radiative transition constant, and that the increase of radiationless transition probability is mainly responsible for the observed decrease of fluorescence yield. The radiationless processes which take place in the CT complex include the direct internal conversion to the ground state of the complex, and the intersystem crossing to the triplet state of the complex or of one of its components. For experimental reasons our results are complete only for solvents of low polarity. Therefore, we cannot exclude the possibility that in more polar solvents the radiative transition probability would diminish. In polar solvents there are two additional factors acting in opposite directions which lower the fluorescence yield. The first is the strong solvation of the donor and acceptor molecules which makes the intermolecular distance between them larger. This in turn is reflected in the decrease of the (@(AD)]Eer

1Oi(A- D+))

matrix element, being very sensitive to the intermolecular distance.

Hence the solvation of components of the.complex leads to the decrease ofthe radiative transition

whilts_ the role of locally excited configurations decreases. According to

probability [8]. The secondfactoris

Mataga’s interpretation [8] of observed behaviour of heteroexcimers’ fluorescence the diminution of weight of locally excited configurations d(AD)* is responsible for the decrease of the probability of radiative transition, and consequently ior the observed decrease of fluorescence quantum yield. Mataga suggests that the same mechanism could be taken into account in the case of the weak CT complexes. Our present results do not seem to confirm this suggestion. The decrease of the observed decay times approximately follows the changes of the fluorescence yields. Hence the probability of radiative transition kf may be treated as being constant in the investigated series of solvents, which is roughly confirmed by the data of table 2. On the other hand, radiationless transition probability ki increases with increasing dielectric constant of the solvent, particularly in the case of TCPA-HMB complex. Although, as it was shown by Iwata et al. [16], the locally excited configurations should be taken into account in building up the charge-transfer states and in calculations of the transition moment, there is no conclusive evidence that their role is as important in the case of weak CT complexes as in the heteroexcimers. We may conclude that the increasing ionic character of the excited-state wave function with increasing solvent polarity seems to maintain the probability

tion of the complex in the exc.ted state into the solvated radical ions A,. . . D,. This effect, which is discussed in more detail.by WellerSet al. [9], increases the total probability of the*racliationless transition. The formation of radicaI.ions accompanied by heteroexcimer. fluorescence was observed both by Weller et al. [9],:and by Mataga et al. [8]. It is impossible now to say which of these two effects is more important in the case of CT complexes in more polar solvents. It seems, however, that the effect of complex ciissociation into radical ions may be predominant. ?Xroll [l’i] estimated the infiueuze of dielectric medium on ‘he potential curves of the ground and excited state of the complex, and showed that the solvent not only stabilizes the excited state (i.e., lowers the energy minimum of the potential curve) but also shallows the potential curve, which makes the dissociation easier. Finally let us pay attention i? the fact that observed quantum yields of fluorescence are in general very small even in nonpolar. solvents such as n-hexane. Furthermore, most of the complexes fluoresce in rigid media exhibiting quite large quantum yields. It seems that a strong change in relative configuration of the components of the complex in excited state (in relation to the ground-state configuration) may be responsible for such behaviour. This problem needs, however, a further study.

638

the dissocia-

Volume

3,

number 8

CHEMICAL

PHYSICS

ACKNOWLEDGEMENTS We wish to thank Dr. A. Tramer for the interest shown in this work and many helpful suggestions. Our thanks are also due to Professor Z. R. Grabowski for his valuable discussions and corrections to the manuscript. We are also grateful to Dr. R. K. Bauer for the use of his phase fluorimeter.

REFERENCES [l] J. Czekalla. A. Schmillen and K. J. Mager. Ber. Bunsenges. Physik. Chem. 61 (1957) 1053, [2] J. Czekalla and K.-O. Meyer, 2. Physik. Chem. 27 (1961) 185. [3] H. M. Rosenberg and E. C. Eimutis. J. Phys. Chem. 70 (1966) 3494. [4] G. D. Short and C. A. Parker, Spectrochim. Acta 23A (1967) 2487.

LETTER9

August

1969

I51 _ . J. Czekalla,

A. Schmitten and K. J. Maser. Ber. Bunscngcs. Physik. Chem. 63 (1959) 622. 161 _ _ J. CzekaIla. G. Briegleb and W. Herre. Ber. Bunsenges. Physik. Chcm. 63 (1959) 712. [7] J. Prochorow and A. Tramer. J. Chem. Phys. 47 (1967) 775. [S] N. BIataga. T. Okada and N. Yamamoto. Chem. Phys. Letters I tiYi7) 119. [9] H. Knibbe. K. j:tillig. F. P. Schiifer and A. Welter. J. Chem. Phys. 47 (1957) 1184. [lo] C. A. Parker and W. T. Rees. Analyst 85 (lS60) S87. [ll] Th. Fi3rster. Fiuorescenz organischer Verbindungen (Vandehoeck and Ruprecht. Gottingen, 1951). [12] R.K. Bauer and K. I. Rudik. Acta Phys. Polon. 35 (1969) 259. 1131 L. Bilot and A. Kawski. Z. Naturforsch. L7a (L962) 621. [14] R. S. Mulliken. J. Chim. Phys. 61 (1963) 29. [15] G. Briegleb. ELektronen-donator-acceptor-Komplexe (Springer-Verlag, Berlin, 1961). [16] S.Iwata. J. Tanaka and S. Nagakura. J. Am. Chem. Sot. 88 (1966) 894. [17]M. Kroll. J-Am. Chem. Sot. 90 (1968) 1097.

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