Quenching of TICT fluorescence by electron donors

J. Photochem.

Photobiol. A: Chem., 81 (1994) 33-37

Quenching

of TICT fluorescence

33

by electron

donors

G. Schopf”,“, W. Rettigasb and J. Bendig” ‘IN. Stranski-Znstimt $5 Physikalirche und Exxwetische Chemie, Technixhe Universit& Berlin, Strasse des X7. Juni, D-10623 Berlin (GeT??XJny) bW. Nemst-Znstitut fiir PhysikuILsche und Theoretirche Chemie, Humboldt-Universitdz Berlin, Bunsensh. I, D-IO11 7 Berlin (Germany] ‘Zwfitit fiir Organische und Bioolganische Chemie, Humboldt-Universitiit, Hessische Str. l-2, D-10115 EMin {Grrrnany~ (Received

September

27, 1993; accepted

March

17, 1994)

Abstract The quenching of twisted intramolecular charge transfer (TICT) Auorescence of selected biaryls by methoxybenzenes, methylbenzenes and N, N-dimethylaniline was studied and shown to be of dynamic origin by comparison of fluorescence intensity and lifetime measurements. The results demonstrate that fluorescence quenching by formation of a two-centre, three-electron cr bond is not important here. The correlation between the ionization potentials of the quenchers and the quenching rate constants can be best described by a direct intermolecular electron transfer between the quencher and the TICT compound. The quenching rate constant is somewhat dependent on the structure of the acceptor group of the TICT compound with N-(9-anthryl)carbazole and N-(4-cyanophenyl)carbazole exhibiting a larger reactivity than N-(l-naphthyl)carbazole. The use of TICI compounds as photocatalysts is discussed.

1. Introduction

slower

quenching rates. The deactivation of the state was attributed to the formation of a two-centre, three-electron bond interaction between two amino nitrogens. Recently, the fluorescence quenching of a larger TICT compound without an amino group was studied by Habib .Iiwan and Soumillion [9]. The occurrence of intermolecular electron transfer between the TICT state of N-(l-naphthyl)carbazole (ClN) and electron acceptor and donor compounds was demonstrated. The quenching rate constants could be correlated with the oxidation potentials of the quenchers. Rehm and Weller [lo] described the intermolecular electron transfer reaction in solution in two steps: (1) diffusion; (2) intcrmolecular electron transfer. The free enthalpy AG of this reaction can be calculated according to eqn. (l), where E&D/D+) is the oxidation potential of the donor, E&A/A-) is the reduction potential of the acceptor, AE,, is the excitation energy and C is the Coulomb term TICT

Some 30 years ago the dual fluorescence of dimethylaminobenzonitrile (DMABN) in polar solvents was discovered by Lippert et al. [l] and, subsequently, the anomalous long-wavelength fluorescence was attributed to a twisted intramolecular charge transfer (TICI?) state [Z]. The TICT state is characterized by a twisted conformation between the donor and acceptor groups and charge separation between them (Fig. 2, see Section 4). Several reviews [3-6] have shown the large variety of TICT systems, including heteroaromatic donor systems, such as pyrrole and carbazole. For some of the combinations (e.g. anthracene or benzonitrile and pyrrolc), detailed model compound studies have been performed, attributing the anomalous fluorescence behaviour to the twisted geometry; in all cases, typical TICT emission (forbidden radiative transmission, large excited state dipole moment) is taken as sufficient evidence for TICT assignment. The quenching of normal and TlCT fluorescence of DMABN by tertiary saturated amines has been studied by Wang [7, 81. The quenching rate constants were not correlated with the oxidation potentials of the amines, but controlled by steric effects. Amines with bulkier alkyl groups showed

1010.6030/94/$07.00 0 1994 Elsevier SSDZ 1010-6030(94)03839-M

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S.A. All rights reserved

AG =EoX(D/D+)

-Ered(A/AP)

- AE,,(A*)

-C (1)

The quenching rate constants k, increase as AG becomes more negative, reaching a constant value for highly exergonic electron transfer reactions.

G. Schopf et al. / Quenching of TICT&mscence by electron donors

34

In this case, the diffusion is the rate-determining step. According to Marcus theory [ll], the quenching rate constants kq due to electron transfer increase (normal region), reach a maximum and decrease again (Marcus inverted region) with increasing exergonicity. According to eqn. (Z), the ionization potential (IF) and oxidation potential (E,) of the electron donor are linearly correlated [12] and thus are the main factors which determine its quenching activity with respect to a given electron acceptor IP = 1.473&,, + 5.821

2. Experimental

(2)

details

N-(1-NaphthyQcarbazole (ClN) [13], N-(4cyanophenyl)carbazole (CBN) [14] and 4-(N-pyrrolo)benzonitrile (PBN) [15] were purified by vacuum sublimation and subsequent recrystallization and N-(9-anthryl)carbazole (C9A) was synthesized as described in ref. I6 and extensively chromatographed. 4-(N-1ndolo)benzonitrile (IBN) was synthesized by reacting indole with 4-iodobenzonitrile according to ref. 17 and was purified by chromatography and sublimation. The quenchers 1,2,4-tri-, 1,2,3-tri-, 1,2-di-, 1,3di- and 1,4_dimethoxybenzene (Merck), 1,3,5trimethoxybenzene (Aldrich), anisole (Merck), 1,2,4,5_tetramethylbenzene (Fluka) and hexame(TEWE) were used as received. thylbenzene 1,2,4,5_Tetramethoxybenzene as quencher was obtained from H. Staerk, Gottingen. N,N-Dimethylaniline was freshly distilled in vacuum before use. The UV-visible absorption spectra in acetonitrile (Uvasd, Merck) were recorded on a UVIKON 930 spectrophotometer. Corrected fluorescence spectra were measured in acetonitriie on a Hitachi-Perkin-Elmer MPF 2A spectrofluorometer. The fluorescence quenching experiments were performed at 26 “C. The concentration of TICI compounds was about S x IO-5 mol 1-l and the quencher concentrations were between 0.2 X lo-' and 18X lop2 mol I-‘. The fluorescence lifetimes in acetonitrile were determined using the timecorrelated, single-photon-counting technique [.l8]. For excitation, synchrotron radiation from the Berlin electron storage ring BESSY was used together with the equipment described in ref. 19.

3. Results The structure of the TICT compounds investigated, their abbreviations and UV-visible absorption properties are listed in Table 1. The TICT compounds were excited into the longest wavelength absorption band in order to avoid absorption by the quenchers at the excitation wavelength. The compounds studied here emit only one fluorescence band (TICT) in contrast with DMABN. The quenchers used, their ionization potentials and oxidation potentials are listed in Table 2. No quenching test was carried out with PBN, since PBN absorbs at shorter wavelengths at which the quenchers also absorb and disturb the quenching results. The quenching rate constants (Table 2) were obtained according to the Stern-Volmer equation. The lifetimes TOof the TICT compounds in aerated acetonitrile ar also given in Table 2. The Stern-Volmer constants (KY,,) were determined from the fluorescence intensities. for CYA-1,2,4-

TABLE 1. W-visible pounds in acetonitrile

absorption

COA

properties

of the TICI

com-

G. Schopf et al. I Quenching of TICT @o~~ence TABLE

by electron donm

35

2.

Excitation wavelengths A,, emission wavelengths h,,, fluorescence lifetimes TO,Stem-Volmtr coefficients KN (I mol-‘) and quenching of TICT compounds with several quenchers in acetonitrile; oxidation rate constants kq (ld 1 mol-’ s) included in parentheses potentials E, and ionization potentials IP in eV. *Calculated according to eqn. (2).

4 10

8 6

4

4

Fig. 1. Stern-Volmer plots for quenching of the TACT fluorescence intensities (X) and fluorescence lifetimes (m).

trimethoxybenzene, the fluorescence intensity and lifetime measurements were compared (Fig. 1) and the results were equal within experimental error. No interactions between the quenchers and the TICT compounds in the ground state were observed in the UV-visible absorption spectra. The absorption of the quenchers at the excitation wavelength can be ruled out, since the pertinent absorbance in the I-IV-visible spectrum did not change with increasing quencher concentration.

2

of C9A by 1,2,4-trimethoxybenzene,

determined

by the fluorescence

4. Discussion Quenching of TICT fluorescence can be observed for quenchers with sufficiently low ionization po tential (IP ~7.8 eV) (or low oxidation potential, E,61.34 eV). The quenching rate constants increase with decreasing IP of the quenchers {Table 0 The fluorescence quenching by specific TICT-quencher interactions, e.g. the formation of

a three-electron CTbond as observed by Wang [7] for DMABN, can be ruled out because fluorescence quenching occurred with 1,2,4,5tetramethoxybenzene, 1,2,4_trimethoxybenzene and 1,4-dimethoxybenzene, but not with 1,2,3_trimethoxybenzene, 1,2_dimethoxybenzene, 1,3,5trimethoxybenzene, 1,3-dimethoqbenzene and anisole, which possess similar methoxy groups. This behaviour is reminiscent of the different quenching behaviour of DMABN and PBN with respect to inorganic anions [23] and can be explained by the different distribution of positive charge on the TICT donor: localized on nitrogen for amines such as DMABN, and hence prone to specific solute-solvent and/ or solute-quencher interactions; delocalized for heteroaromatic donors, such as pyrrole, indole and carbazole, i.e. in the compounds studied here. The correlation between the ionization potential, E,, and the quenching rate constants k,, listed in Table 2, can be best described by a direct intermolecular electron transfer between the donor group of the TICT compound (D+) and the electron donor as quencher QD {Fig. 2(a)). In the TICT state, the primary donor group D (carbazole and indole group) of the TICT compound gains electron acceptor properties and will be the electron acceptor D+ during the intermolecular electron transfer. The ionization potential IP of the quencher must bc lower than the IP of the donor radical cation D + in order to cause fluorescence quenching (Fig. 2(b)). The intermolecular electron transfer is the ratedetermining step of quenching, since the k, values are lower than the diffusion rate constant kD in acetonitrile (k,,= 1.87~ 10” 1 mol-’ s-l [24]) in all cases. If the quenching was diffusion controlled, the k, values would be independent of a variation in IP of the quenchers [lo]. On the basis of the k, values (Table 2), it is not possible to distinguish between the two electron transfer mechanisms according to Marcus theory [ll] or Rehm and Weller [lo]. We observe an increase in kg with decreasing oxidation potential E,, {more negative reaction enthalpy AG (eqn. (I)). This correlation corresponds to the normal Marcus region. Quenchers with considerably lower E,, values would have to be chosen in order to reach the Marcus inverted region, and to distinguish between the Marcus and Weller theories of electron transfer with the aid of the k, values. Although not directly involved in the intermolecular electron transfer process, the primary acceptor group of the TICT compound (Fig. Z(a)) influences the quenching rate. This can be seen

if D+ (here carbazole) is held constant and Aof the TICT compound is varied. CBN with an electron-attracting benzonitrilc substituent group and C9A with the large aromatic anthryl group show similar kg values which are significantly larger than that of ClN {Table 2). The fact that the quenching rate of the intermolecular electron transfer (ET, Fig. 2(a)) depends on the properties of the donor as well as the acceptor group of the TICT compound, but only D+ is a part of the intermolecular electron transfer, indicates that donor and acceptor are not completely decoupled in the TICT state of these larger aromatics (especially ClN), and that the charge separation within the TICT state is not complete. A possible interpretation could involve the effect of different TICT state equilibrium conformational distributions, which may be broader for ClN, which has a sizeable fraction of molecules that deviate significantly fkom the “classical” perpendicular TIC’T state conformation [8]. The Stem-Volmer constants (J&v) for C9A-1,2,4-trimethoxybenzene were determined using both fluorescence intensities and fluorescence lifetimes (Fig. 1). The values are equal within experimental error and demonstrate a dynamic quenching mechanism. This is consistent with the observed absence of interactions between the TICI’ compound and the quencher in the ground state. The other quenchers can also be interpreted to undergo the same dynamic quenching mechanism.

bi Fig. 2. The principle of TICI fluorescence quenching by electron donors. (a) Sequence of photophysical steps. (b) Orbital occupation pattern showing the different possibilities for electron transfer from the locally excited (LE) state towards the TICT state and the competition between TICT fluorescence and electron transfer quenching.

G. Schopf et al. I Quenching of TICTfluorescence

by eleciron donors

37

References

o+- A-

Fig. 3. A TICT compound

(D-A)

as possible

photocatalyst.

The electron transfer quenching mechanism discussed above could be applied in practice using TICT compounds as oxidative photocatalysts (Fig. 3). A TICT compound (D-A) is excited and converted into the TICT state (D+-A-) (step 1). The oxidation of an added agent (Q * Q’) effects the reduction of D+ of the TICT compound (D-A-, step 2) described as the quenching process in this paper. The next step (step 3) is the regeneration of the starting state D-A with an added oxidizing reagent R. In this way, D-A can oxidize further molecules of Q in the cycle acting as a photocatalyst (Fig. 3).

Acknowledgments Support by the Bundesministerium fiir Forschung und Technologie (project 05 314 FA I 5) is gratefully acknowledged. We wish to thank Dr. H. Staerk (Max-Planck-Institut fiir Biophysikalische Chemie, Gettingen) for a gift of 1,2,4,5tetramethoxybenzene.

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