excited state relaxation of n-(1-anthryl)-2,4,6-trimethyl...

Vol. 06 INTERNATIONAL JOURNAL OF PHOTOENERGY 2004

Excited state relaxation ofN-(1-anthryl)-2,4,6-trimethyl-pyridinium cation

M. N. Khimich,1 N. I. Makarova,2 M. I. Knyazhansky,2 and B. M. Uzhinov1,†

1 Department of Chemistry, M.V. Lomonosov Moscow State University,

Vorob’evy Gory, 119899 Moscow, Russian Federation2 Institute of Physical and Organic Chemistry, Rostov State University,

Stachki str. 194/3, 344104 Rostov on Don, Russian Federation

Abstract. The fluorescence spectrum of N-(1-anthryl)-2,4,6-trimethylpyridinium cation (1) has an anoma-lously high Stokes’ shift. The fluorescence spectra of 1 in ethanol and butyronitrile are shifted to short-wavelength region and fluorescence quantum yield increases as the temperature decreases. The fluorescencerate constant of this compound changes considerably (6 times in ethanol and 15 times in butyronitrile) asthe temperature decreases from 293 K (relaxed state) to 77 K (mainly nonrelaxed state). It points out that atthese temperatures the fluorescence takes place from two species with different structures. It is concludedthat anomalously high fluorescence Stokes’ shift of 1 is caused by both solvent orientation relaxation andexcited state structural relaxation consisting in the mutual rotation of anthracene and pyridinium fragmentsof the cation and resulting in the formation of a specie with different structure. The rates of these processesare determined by the temperature-dependent viscosity of the medium.

1. INTRODUCTION

The unusual fluorescence and absorption properties ofN-(1-anthryl)-2,4,6-trimethyl-pyridinium cation (1) havebeen described previously [1]. The fluorescence spec-trum has an anomalously high Stockes’ shift. The flu-orescence spectra of 1 at room temperature and at77 K differ considerably. The fluorescence spectrum at77 K is shifted to shorter wavelengths in comparisonwith the spectrum at room temperature. The fluores-cence spectrum at intermediate temperatures (141 K)is a mixture of two different spectra belonging to dif-ferent emitters [2]. The fluorescence quantum yield in-creases considerably by temperature decreasing. Suchspectral behaviour of 1 has been explained by the ex-istence of excited state relaxation process (probablystructural relaxation), inhibited by the increasing sol-vent viscosity [2]. According to mechanism suggested[1] in the ground state anthracene fragment is orthogo-nal to pyridinium one and the conjugation ofπ -systemsis absent. In liquid solutions the formation of planarstructure due to torsion fragments vibrations is possi-ble and charge transfer takes place. At the excitation of1 in rigid matrix the charge transfer common for thesystems having donor and acceptor groups does nottake place and the fluorescence spectrum correspondsto anthracene one.

In the present paper the absorption and fluo-rescence spectra, fluorescence kinetics of 1 in 298–77 K temperature range are investigated to clarifythe mechanism of excited state relaxation process.

†E-mail: [email protected]

2. EXPERIMENTAL DETAILS

Compound 1 was prepared as perchlorate salt byknown methods [3], purified by recrystallization andthe product was checked for purity by fluorescence af-ter each recrystallization step as described previously[1].

CH3

CH3N

+

H3C

I

Solvents—ethanol, butyronitrile and dichlorometh-ane—were purified according to methods known [4].The absorption spectra were registered by a Shi-madzu UV-3100 spectrophotometer, and the fluores-cence spectra—by an Elyumin 2M spectrofluorimeter.The fluorescence quantum yields were measured bycomparison of the corrected fluorescence spectra areasof the compound investigated and quinine bisulphatein 1N sulphuric acid (ϕf = 0.546) [5]. The optical den-sities of the solutions investigated were in the range0.2–0.4 at the excitation wavelength. The fluorescencequantum yields at various temperatures were corrected

70 M. N. Khimich et al. Vol. 06

with respect to the absorption spectra of solutions atthe same temperatures. The optical cryostat with liq-uid nitrogen vapor cooling was used for absorptionand fluorescence spectral and kinetic measurementsin the range 77–298 K. The fluorescence kinetics wasregistered by a SP-70 nanosecond spectrometer by themethod of time-correlated counting of single photonswith excitation by air-filled flash lamp radiation (exci-tation pulse duration 0.8 ns, registration channel width0.054 ns). The decay times were fitted using the iterativedeconvolution procedure which allowed a time resolu-tion down to 0.1 ns and a precision of better than 0.1 ns.

3. RESULTS AND DISCUSSION

The fluorescence spectra of 1 at different temperaturesin ethanol are shown in Figure 1 and in butyronitrile inFigure 2.

100

80

60

I,a.

u.

40

20

01

23

4

5

6

7

8

400 500λ, nm

600 700

1–290 K2–212 K ×103–191 K4–171 K5–151 K6–130 K7–115 K8–77 K

Figure 1. The fluorescence spectra of 1 in ethanol at dif-

ferent temperatures (1–293, 2–210, 3–191, 4–171, 5–150,

6–130, 7–115, 8–77 K).

100

80

60

I,a.

u.

40

20

0400 500

λ, nm600 700

1–293 K2–210 K ×103–150 K4–115 K5–77 K

12

3

4

5

Figure 2. The fluorescence spectra of 1 in butyronitrile at

different temperatures (1–293, 2–210, 3–150, 4–115, 5–

77 K).

1,0a

0,8

0,6 b

ϕf

0,4

0,2

0,0

50 100 150 200T , K

250 300

Figure 3. The dependence of the fluorescence quantum

yield of 1 in butyronitrile (a) and in ethanol (b) on the tem-

perature.

The fluorescence spectra in ethanol are shifted toshorter wavelengths by 127 nm as the temperature de-creases from 290 to 77 K, and the fluorescence quan-tum yield (ϕf ) increases from 0.014 to 0.55 (Figure 3a).Similar results were obtained for 1 in butyronitrile. Thefluorescence spectra are shifted by 125 nm, and ϕf in-creases from 0.015 to 0.84 (Figure 3b). These data pointout the existence of a relaxation process in the ex-cited state. The rate of this process is determined bythe temperature-dependent properties of the medium.Ethanol viscosity [6] and polarity [7] increase as the tem-perature decreases.

The short-wavelength shift of fluorescence spectraof 1 with decreasing temperature (at increasing solventviscosity) indicates that the relaxation process rate de-pends considerably on the solvent viscosity. The cru-cial role of medium viscosity, but not the temperaturein the short-wavelength shift of fluorescence spectra issupported by the similarity of the fluorescence spectraof 1 in PMMA at 293 K and in ethanol at 77 K. It is alsosupported by the fact that the greatest shifts of fluores-cence spectra are observed in the temperature intervalswhere the changes of ethanol and butyronitrile viscos-ity are greatest. In PMMA at 293 K and in ethanol at77 K the relaxation process is completely inhibited. Therelaxation process is caused either by intramolecularrotation of fluorophor molecular fragments, accompa-nied by the solvent relaxation, or by solvent orientationrelaxation only. In both cases the solvent orientation re-laxation is inhibited upon solvent viscosity increasing,the fluorescence takes place from the state with less ex-tent of relaxation and fluorescence spectra should beshifted to shorter wavelengths.

The dependence of the fluorescence rate constant(kf ) on the temperature can give an important informa-tion about the relaxation process mechanism. At thesolvent relaxation the molecular structure is changedinsignificantly therefore in this case kf of relaxed and

Vol. 06 Excited state relaxation of N-(1-anthryl)-2,4,6-trimethyl-pyridinium cation 71

Table 1. The fluorescence lifetime of 1 in ethanol and buty-

ronitrile at different temperatures.

T , Kτ , ns

ethanol butyronitrile

77 15 11.3

114 16.4 13.7

130 17.2 −150 12.7 6.2

191 5.5 −210 4.7 4.3

293 2.1 2.8

0,08

b

0,06

0,04

ak f,n

s−1

0,02

0,00

50 100 150 200T , K

250 300

Figure 4. The dependence of the effective radiative rate con-

stant of 1 in ethanol (a) and in butyronitrile (b) on the tem-

perature.

nonrelaxed states should not differ considerably. If therelaxation process is associated with a mutual rota-tion of fluorophor molecular fragments, resulting in an-other molecular structure, then kf of relaxed and non-relaxed states should differ significantly [8]. We havedetermined the excited state lifetimes (Table 1) and flu-orescence quantum yields (Figure 3) at correspondingtemperatures for kf calculation, kf =ϕf/τf .

The fluorescence kinetics was registered at thewavelength which corresponds to the fluorescencespectral maximum at a given temperature. Figure 4shows the dependence of kf of 1 on the temperature inethanol (a) and butyronitrile (b). The values of kf for re-laxed (T = 298 K) and nonrelaxed (T = 77 K) states dif-fer considerably (0,006 and 0,036 ns−1 in ethanol and0,005 and 0,075 ns−1 in butyronitrile, respectively).

The following mechanism of excited state relax-ation processes based on the experimental results re-ceived can be suggested. In liquid ethanol the mutualrotation of pyridinium and anthracene fragments takesplace and orthogonal arrangement of these fragmentsin the ground state is changed to planar one at the ex-citation (structural relaxation). The conjugation of π -

systems of these fragments is possible for planar struc-ture. Such conjugation results in the charge transferfrom anthracene fragment to pyridinium one. The newcharge distribution in flurophore molecule results inanother polar solvent orientation. This is a solvent ori-entation relaxation resulting in the fluorescence spec-trum displacement to longwavelengh region. The valueof this displacement depends on the solvent polarityand is greater in polar solvents. The fluorescence spec-trum of 1 in ethanol (ε = 25.2) is shifted to longerwavelengths by 47 nm in comparison with spectrumat dichloromethane (ε = 9.08). At the temperature de-creasing the ethanol viscosity increases and both thefragments rotation of fluorophore molecule (structuralrelaxation) and solvent orientation relaxation are ham-pered.

Thus in 290–190 K temperature range gradualshortwavelengh shift of fluorescence spectra caused byviscosity hampering of the solvent orientation is ob-served. At further temperature decreasing the fluores-cence band corresponding to anthracene fluorescenceappears. It means that structural relaxation is ham-pered by increasing ethanol viscosity. In this case inthe excited state the rotation resulting in the planarstructure does not take place and charge transfer doesnot occur. It is supported by the dependence of kf onthe temperature. A marked change of kf begins at thetemperature decreasing till 171 K and anthracene bandappears in the fluorescence spectrum.

The values of kf in the 135–230 K temperature re-gion are effective. They characterize the averaged-outemission rate from relaxed (with different relaxationextent) and nonrelaxed states. It should be noted thatthe change of kf for 1 in butyronitrile two times muchthan that in ethanol. It can be caused by specific effectof ethanol on the emission rate constant, which can beconnected with the formation of hydrogen bond com-plex of 1 with ethanol in the ground state. On plots ofkf versus T , curves for the solution in butyronitrile issteeper than that for the solution in ethanol. It is ex-plained by the fact that the viscosity of butyronitrile ischanged in narrower temperature interval than that forethanol viscosity.

4. CONCLUSION

The data resulting from the investigation of fluo-rescence spectra and fluorescence kinetics of N-(1-anthryl)-2,4,6-trimethylpyridinium cation in ethanoland butyronitrile at different temperatures show the ex-istence of excited state structural relaxation consistingin the mutual rotation of anthracene and pyridiniumfragments of the cation followed by the solvent orienta-tion relaxation. The rates of these processes are deter-mined by the temperature-dependent viscosity of themedium.

72 M. N. Khimich et al. Vol. 06

ACKNOWLEDGMENTS

The support of the Russian Foundation for Basic Re-search (grant 03-03-32687) and International ScientificTechnical Center (grant 2117) is gratefully acknowl-edged.

REFERENCES

[1] V. A. Kharlanov, M. I. Knyazhansky, N. I. Makarova,and V. A. Lokshin, J. Photochem. Photobiol. A: Chem.70 (1993), 223.

[2] V. A. Kharlanov, W. Rettig, M. I. Knyazhansky, andN. I. Makarova, J. Photochem. Photobiol. A: Chem.103 (1997), 45.

[3] A. T. Balaban, A. Dinculescu, F. Iordache, F. Chiralen,and D. Patsioia, Chem. Scr. 18 (1981), 230.

[4] A. Weissberger, E. S. Proskauer, J. A. Riddick, andE. E. Toops, Organic Solvents. Physical Propertiesand Methods of Purification, Technique of OrganicChemistry, 7, Interscience, New York, 1955.

[5] W. Melhuish, J. Phys. Chem. 76A (1972), 547.[6] T. K. Sherwood, R. C. Reid, and J. M. Prausnitz, The

Properties of Gases and Liquids, McGraw-Hill, NewYork, 1977.

[7] C. P. Smyth and W. N. Stoops, J. Am. Chem. Soc. 51(1929), 3312.

[8] M. N. Khimich, V. V. Volchkov, and B. M. Uzhinov, J.Fluorescence 13 (2003), 301.

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