Picosecond diffuse reflectance laser photolysis study on 9,10-dichloroanthracene and 9,10-dibromoanthracene microcrystals

6 May 1994

Chemical Physics

ELSEVIER

CHEMICAL PHYSICS LETTERS

Letters 222 (1994) 123-128

Picosecond diffuse reflectance laser photolysis study on 9,10-dichloroanthracene and 9,10-dibromoanthracene microcrystals Norimasa Fukazawa, Hiroshi Fukumura, Hiroshi Masuhara DepartmentofApplied Physics,Osaka University,Suita, Osaka565, Japan Received 20 January 1994; in final form 19 February1994

Abstract Absorption spectra of excimer states of 9, IOdichloroanthracene (9,10-DCA) and 9, IO-dibromoanthracene(9,1O-DBA) mierocrystalswere measuredand their formation wasconfirmedto take placewithin a fewps after excitationby picoseconddiffuse reflectancelaser photolysismethod. Excimer states in 9,10-DCA and 9, IO-DBA microcrystals reflect the difference in the geometrical structure of the excimers.

1. Introduction The relaxation processes of electronic excited states in an organic molecular crystal (exciton) strongly depend on its crystal structure. In some cases, localization of excitation energy (formation of self-trapped exciton) takes place, accompanying a large deformation of the crystal lattice. A relation between such a deformation and the crystal structure is of course an interesting topic and has received much attention. A representative study is on excimer dynamics of 9, 10-DCA, because it has two crystal forms and both of them show excimer emission. Their crystal structures were reported in the literature [ l-41, which is summarized here briefly. The molecules are stacked in the direction of the crystallographic a axis in the u-form crystal. The molecular planes are approximately perpendicular to the a axis and the interplanar distance is 3.52 A. The long axes of adjacent molecules form an angle of 60” to each other, giving partial overlap between two anthracene rings. On the other hand, the molecular axes of translationally equivalent molecules are parallel in the &form crys-

tal, although the molecules are also stacked in the direction of the a axis. Furthermore, there are two inequivalent symmetry-independent molecular stacks. The distance of adjacent two molecules are 3.48 and 3.52 A. Therefore, the relative geometrical structures of excimer state in the a- and p-form crystals are different. This leads to the difference of stabilization of the excited state, and consequently, the difference of the excimer dynamics between two types of crystal. Although investigation for excimer dynamics has been carried out by emission spectral analysis, it is important to measure the transient absorption spectrum since a non-emissive transient species sometimes contributes to excimer dynamics. It is well known that electron transfer between identical molecules upon excitation is induced for geometrically restricted systems such as zeolite [ 51 and intramolecular excimer compounds [ 6 1, and is closely related to excimer formation. It is expected also in crystals that such a phenomenon is involved in excimer dynamics, because the relative geometry of two molecules is spatially fixed. In the present work, we have measured for the first

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time the excimer dynamics of 9,10-DCA and 9,1 ODBA crystals by picosecond diffuse reflectance laser photolysis method, and discuss the relation between spectrum and geometrical structure of the excimer. We have succeeded in finding a new behavior which could not be revealed by fluorescence measurement.

2. Experimental 9,10-DCA and 9,10-DBA (Nacalai Tesque) were purified by repeated recrystallization from benzene. The u-form crystal of 9,10-DCA was grown from its benzene solution, while the p-form crystal was obtained by heating the a-form crystal in an evacuated, sealed tube at 180°C for 24 h. Phase transition from the a- to the B-form was confirmed by measuring the fluorescence spectrum before and after annealing. 9,10-DBA crystal was grown from benzene solution. Anthracene crystal (Merck scintillation grade) was zone-refined ( 100 passes) before use. All the samples described above were grounded on a mortar and contained in a rectangular suprasil cell with 2 mm thickness. Poly (methyl methacrylate ) (Kuraray, abbreviated as PMMA) powder samples where 9,10-DCA and 9,10-DBA were molecularly dispersed were prepared in the following way. Benzene solutions of anthracene derivatives and PMMA were prepared, where the concentration of anthracene derivatives was adjusted to 2 wt% of PMMA polymer. After stirring over 2 h, the solution was frozen on the flask wall, cooled down to - 10 ’ C, and the flask was evacuated to remove benzene. The picosecond diffuse reflectance laser photolysis system was similar to that described previously [ 7 1. The third harmonic pulse (355 mn) of a mode-locked Nd3+ : YAG laser (Quantel International, YG501 C; 1064 nm, 30 ps) was used for excitation of the sample. A picosecond continuum, which was generated by focusing the fundamental pulse ( 1064 nm) into a quartz cell containing a DzO and HZ0 mixture, was used as analyzing light. Diffuse reflected light from the sample was collected into a polychromator and detected by a multichannel photodiode array (Otsuka Electronics Co. Ltd., MCPD 11O-A). The intensity of transient absorption was displayed as percentage absorption [ 8,9], defined as

%Abs.(l)={l-[R(I)/&(A)]}XlOO,

(1)

where R and R. represent the intensity of diffuse reflected picosecond continuum with and without excitation, respectively. The time axis of the system was determined by measuring the rise curve of benzophenone triplet absorption in crystal state. The origin of the time axis was set to the delay when the O/oAbs.is half of the plateau value of the rise curve [ 71. The zero time corresponds to the time when exciting and monitoring pulses take the maximum overlap. The rise time ( 10°X+900hrise for the plateau value) of benzophenone triplet absorption was about 50 ps, and this corresponds to the response of our system. The detection of temperature-dependent fluorescence of the u-form crystal was performed by a multichannel photodiode array equipped with image intensifier (Otsuka Electronics Co. Ltd., IMUC 7000) under the same excitation arrangement to laser photolysis. All data were accumulated over ten times excitation.

3. Results and discussion As the reference of the present anthracene systems, we first describe the transient absorption spectra of anthracene microcrystal and PMMA powders in which 9,10-DCA and 9,10-DBA were molecularly dispersed. The latter powders are expected to give the absorption spectrum of the monomer excited singlet state. As shown in Fig. 1, the transient absorption spectra of 9,10-DCA and 9,10-DBA dispersed in PMMA had a rather sharp peak at 600 and 6 10 nm, respectively. These are assigned to the excited singlet state of monomer anthracene because of spectral similarity to S,-S, absorption spectra of anthracene derivatives in solution [ 1O-121. On the other hand, the transient absorption spectrum of anthracene microcrystal showed a broad band whose peak was located around 580 nm (Fig. lc). This spectrum is broader and slightly blue-shifted compared to the S,S, absorption of anthracene in solution [ 131; however, the spectrum of anthracene microcrystal was similar to that of the single crystal reported by Kotani et al. [ 141. It is well known that the excimer state is not formed in the anthracene crystal and the ab-

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2.3ns

t I

400

I

I

I

1

1

-i

I 500

700 600 500 Wavelength (nm)

600

(

700

Wavelength (nm)

Fig. 1. Transient absorption spectra of anthracene systems; (a) 9,10-DBA dispersed in PMMA, (b) 9,10-DCA dispersed in PMMA, (c) anthracene microcrystal.

sorption band of anthracene microcrystal is ascribed to the monomer excited singlet state [ 141. The result in Fig. 1 confirms that the spectral data obtained by the diffuse reflectance laser photolysis is accurate and reliable, and it can be used as the reference of monomer. The spectral difference between anthracene derivatives in PMMA and anthracene microcrystal is ascribed to the formation of the excitation state in the crystal. Transient absorption spectra and time profile of P_ form 9,10-DCA microcrystal are shown in Fig. 2. A broad band with a peak around 500 nm was observed. These transient spectra were dominated only by one transient species, since the spectral shape did not significantly change over the delay time of our measurement. The spectrum is not similar to those of Fig. 1 but to that of intramolecular excimer state of 1,2-di( 1-anthryl)ethane and 1,2-di( 9-anthryl)ethane in solution [ 11,12 1, although the peak position of this crystal is shifted toward short wavelengths. Therefore, this transient species is assigned to the excimer (self-trapped exciton) state of p-form 9,10-DCA microcrystal. The spectral shift can be due to a difference of the relative configuration (distance and overlap) of two anthracene rings of the excimer state. The formation of the excimer state in p-form 9,1 O-

Time (ns)

40

0

40 Time (~JJ)

80

Fig. 2. (a) Transient absorption spectra of bform 9, IO-DCA microcrystal. The delay times are given in the figure. (b) Time profile of the transient absorption intensity observed at (0 ) 500 and (0) 600 nm.

DCA microcrystal was finished within a few ps after excitation at room temperature, because its absorption rise curve corresponds to the response function of our photolysis system. This is in agreement with the result reported by Mayer et al. [ 15 1. They measured the excimer fluorescence of 9,10-DCA crystal and concluded that the observed rise time at 300 K was determined by the time resolution of the detection system ( 5 ps ) . The present decay behavior could not be analyzed with a single exponential function. This might be due to an effect of high excitation in-

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tensity, since the exciton-exciton annihilation process in p-form 9,10-DCA crystal was already discussed in detail [ 15,16 1. The flat absorption spectrum at 2.3 ns might be due to the overlap with other spectrum such as that of triplet state. The transient absorption spectra of 9,10-DBA microcrystal are shown in Fig. 3a. The time profile up to 2 ns of the absorption was independent of the observation wavelength as shown in Fig. 3b. Thus, we conclude that the only one transient state was formed in 9,10-DBA microcrystal as similarly to the p-form

9,10-DCA. Indeed, this crystal has the similar relative configuration of two anthracene rings with b-form 9,1 O-DCA [ 17 1, and the spectra were similar to those shown in Fig. 2 except for a small shift of the peak position toward long wavelength (Fig. 3a). Consequently, this transient species is also assigned to the excimer state. The small shift for 9,10-DBA compared with p-form 9,10-DCA is attributed to a difference in x-electron overlap. Such a difference results

t (a)

660

600

700

OPS I

600 600 Wavelength (nm)

700

Wavelength (nm)

lime (ns)

Time (ns)

Time (ps)

Time (ps)

Fig. 3. (a) Transient absorption spectra of 9, IO-DBAmicrocrystal. The delay times are given in the ti6ore. (b) Time profile of the transient absorption intensity observed at (0 ) 500 and (0) 600 nm.

Fig. 4. (a) Transient absorption spectra of a-form 9,10-DCA microcrystal. The delay times are given in the figure. (b) Time profile of the transient absorption intensity observed at (0) 550 and (0) 480 nm.

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from the fact that the van der Waals radius is smaller for a chlorine atom than for a bromine atom [ 17 1. Fig. 4 shows transient absorption spectra and their time profile of u-form 9,10-DCA microcrystal. At early delay times, a broad absorption band and a slight shoulder were observed around 550 and 470 nm, respectively. The rise of both bands was instantaneous after excitation, while the decay time of 550 nm was faster than that of 480 nm. Therefore, we consider that at least two absorbing transient species exist in the a-form crystal and that the transient absorption around 550 nm is due to the excimer state. The absorption maximum (5 50 nm) is between those of anthracene ( 580 nm) and @form 9,1O-DCA ( 500 nm ) , which is due to an excimer state with less overlap of neighboring anthracene rings of the u-form compared to the gform. Actually, the fluorescence of this crystal at room temperature was assigned to the extimer emission [ 18,19 1. The formation of this excimer was also completed within a few ps, because the rise curve of the transient absorption at 550 nm corresponds to the response function of the system as similarly in the p-form crystal. The absorption around 480 nm was tentatively assigned to the triplet state of 9,10-DCA, because the decay time of this band was longer than that of fluorescence measured under the same excitation condition. The temperature effect on fluorescence spectra of u-form 9,10-DCA crystal was studied by some researchers [ 17- 19 1. The spectra changed from a broad spectrum (excimer emission) to a structured one (monomer emission) by decreasing the temperature. The reason for this fluorescence spectral change was that the formation of excimer state is a thermally ao tivated process and therefore the excimer giving a broad structureless emission is not formed at low temperature [ 18,191. From the above discussion concerning the peak position of the S,-Sr absorption spectrum, the peak position of the absorption spectrum should show a red-shift, if the concentration of monomer excited state increased by decreasing temperature as pointed out [ 18,191. However, the transient absorption spectra did not significantly change between room temperature and 77 K except for the slight sharpening of absorption band as shown in Fig. 5, which contradicts with the fluorescence study. As is generally well known, a fluorescence measurement is very sensitive even to a minor species, while transient absorption measurement probes the major

500 600 wavelength (nm)

400

127

700

500 600 Wavelength (nm)

Fig. 5. The temperature dependence on transient absorption spectra at 0 ps (a) and emission spectra (b ) . The temperature is given in the figure.

component because it needs high concentration of species for detection. Therefore, it is considered that the excimer state is a main species even at 77 K. The dynamics of monomer fluorescence and its relation to the excimer is beyond our knowledge at the moment and should be clarified, which systematic study is being performed.

4. Conclusion We have demonstrated that the picosecond diffuse reflectance laser photolysis is a useful technique for investigating excimer dynamics in crystal system. The S,-S, absorption of the excimer state was observed for 9,10-DCA and 9,10-DBA. The spectra of &form 9,10-DCA and 9,10-DBA resemble each other. This is due to the similarity of the geometrical structure of the excimer, because the relative contiguration of two

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anthracene rings in these crystals is quite similar. On the other hand, the absorption spectrum of the u-form crystal which has a different crystal structure is redshifted. Furthermore, it is found that the formation of the excimer state takes place within a few ps after excitation in both crystal structures. From the examination on the temperature dependence of the transient absorption spectrum, it was found out that the excimer dynamics in the a-form crystal proposed on ground of fluorescence measurement does not hold.

Acknowledgement

We acknowledge Otsuka Electronics for their generous affording multichannel photodiode arrays. The present work is supported by a Grant-in-aid from the Japanese Ministry of Education, Science and Culture (63430003).

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