Excited state dynamics in UHV-deposited films of pyrene

260

Journal of Luminescence 45 (1990) 260—262 North-Holland

EXCITED STATE DYNAMICS IN UHV-DEPOSITED FILMS OF PYRENE H. PORT, P. FISCHER and R. SEYFANG 3. Physika/isches Inst itut, Universität Stuttgart, D-7000 Stuttgart 80, FRG

In thin pyrene films deposited on a silicon substrate the high temperature crystalline phase is found to persist down to He-temperature. In optical CW and ps time-resolved measurements spectral characteristics and excited state dynamics of this phase are investigated in a largely extended temperature range (4.2—300 K). The quantitative analysis ultimately confirms our concept of a two-step excimer formation process in pyrene via the metastable B-state being observed in the whole temperature range.

1. Introduction This work is aimed at solving the remaining ques-

The corresponding temperature dependent decrease/ increase of blue/excimer fluorescence quantum yields are shown in fig. 2.

tions about excimer dynamics in pyrene and the nature of the precursor state B discovered by our group [1,2].

The existence of the B-state still had been questioned due to the fact that by passing through the crystal phase transition temperature T= 113 K the excimer dynamics drastically changes and the B-fluorescence abruptly disappears [1].

Thin pyrene films offer the unique possibility to extend the accessible temperature range of the crystalline high temperature phase down to He-temperature. The films are deposited on a silicon substrate with thicknesses of 100 to 1000 A. According to X-ray investigations they consist of microcrystals of the pyrene high temperature phase. The existence of this phase is further substantiated and controlled via triplet excitation spectra. CW-fluorescence emission and excitation spectra

have been measured with laser or lamp excitation using standard photon counting detection (T= 4.2—300 K).

Triplet excitation spectra have been detected after narrow-band dye-laser excitation. The ps time-resolved spectra and transients have been recorded using time correlated Single-Photon-Counting after sync-pumped

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2. Fluorescence emission spectra and quantum yields

Temperature-dependent fluorescence spectra are shown in fig. 1. A continuous transition occurs from the

typical crystalline excimer fluorescence band at high temperature to a structured blue fluorescence component predominant at low temperature. A residual ternperature independent excimer fluorescence contribution prevails below T = 60 K. 0022-2313/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

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UHV-depositedfilms ofpyrene

261

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Fig. 2. Temperature dependent fluorescence quantum flux of blue (•) and excimer fluorescence (~)component.

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film with strong 00-line of high temperature phase pyrene at

16890 cm and weak satellite of low temperature phase. Left: single crystal low temperature phase as reference.

3. Excitation spectra: verification of supercooled high temperature phase Singlet excitation spectra (fig. 3) detected at the maximum of the blue fluorescence (AD = 406 nm) exhibit the characteristics of the high temperature crystal-

1. Upon line phase, with 00-transition at =26700 cm above 30 excimer fluorescence detection (AD 470 nm) K the same characteristics are observed, whereas at T < 30 K a red-shifted and less structured spectrum appears, originating from the residual low temperature

excimer component (see above). According to our previous studies (3) of triplet excitation spectra a clear distinction can be made between the high and low temperature phases of pyrene crystals.

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Fig. 3. Fluorescence excitation spectra detected at maximum of excimer (left) and blue fluorescence (right),

corresponding transients at various detection lengths.

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262

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UHV-depositedfilms of pyrene

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Fig. 6. Arrhenius plot rise and blue fluorescence decay time constants. TB-values given for detection at fluorescence maximum and origin,

In the films, T 1 ~— S0 excitation spectra of the high temperature phase are found (fig. 4). A residual contribution of the low temperature crystalline phase appears which is consistent with the observation of residual excimer fluorescence in the singlet emission spectra. Both types of spectra verify the occurrence of the supercooled high temperature phase in the thin pyrene films. The similar singlet spectra previously observed [4] probably are due to this phase and not to surface defects.

5. Conclusions These experiments ultimately prove the concept of the two-step excimer formation process via the metastapyrene ble B-state [1,2]. in the Theyhigh provide temperature a consistent crystalline quantitative phase of description of dynamics and quantum yields between He- and room temperature adopting a two-minima potential energy surface.

The intermediate B-state is formed without an additional activation barrier which is assumed in [5]. It exhibits much weaker electron phonon coupling than the excimer state and a vibronic substructure is resolved in the B-fluorescence spectra (partly smeared out at higher temperatures). Based on the present data the comparative analysis of self-trapping parameters for Band excimer fluorescence is now possible and in progress.

Acknowledgement

4. Fluorescence dynamics In ps time-resolved spectra (fig. 5) the blue fluorescence component, which disappears for T> 70 K in the CW-spectra of fig. 1, is still observable up to T> 200 K. The spectral characteristics and relative quantum yields with respect to excimer fluorescence at T> 115 K are identical to the B-fluorescence in single crystals of high temperature phase. The corresponding fluorescence transients demonstrate the drastic wavelength dependence between the blue and excimer fluorescence spectral regions. The interrelationship between excimer rise and blue fluorescence decay in the ps time regime is obvious. The quantitative analysis (fig. 6) proves the one-toone correspondence between excimer rise and blue fluorescence decay in the temperature range between 60 and

We acknowledge helpful discussions with M. Wiechmann and H.C. Wolf and thank M. Möbus for preparation of the thin films. This work has been supported by the Deutsche Forschungsgemeinschaft.

References [11 H. Port, R. Seyfang and H.C. Wolf, J. de Phys. C7-391. [21 R. Seyfang, H. Port and H.C. Wolf, J. Lumin. 127. [3] K. Mistelberger and H. Port, Mol. Cryst. Liq. (1980) 203. [~l N.Y.C. Chu and DR. Kearns, Mol. Cryst. Liq.

46 (1985)

42 (1988) Cryst. 57

Cryst. 16 (1972) 61. 151 K. Mizuno and A. Matsui, J. Lumin. 38 (1987) 323.