Optical properties of 1,4-dihydroxyanthraquinone covalently bonded to SiO2AlO32 gel

JOURNAL OF

1,I,11 ELSEVIER

Journal of Non-CrystallineSolids 203 (1996)43-48

Optical properties of 1,4-dihydroxyanthraquinone covalently bonded to SiO2-A103/2 gel Tomohiro Ishiwaki *, Hiroyuki Inoue, Akio Makishima Department of Materials Science Facultyof Engineering, Universityof Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan

Abstract Transparent SiO2-AI20 3 gels doped with 1,4-dihydroxyanthraquinone (DAQ) were prepared by the sol-gel process (Q/SiO2-AIO3/2 system). The absorption band of DAQ in the SiO2-AI203 matrix shifted to longer wavelengths compared to that of DAQ doped SiO 2 gel (Q/SiO 2 system), and the absorption spectrum of Q/SiO2-A103/2 system was similar to the reported fluorescence excitation spectrum of the Q/~/-AI20 3 system. A binding model for the Q/~,-A120 3 system, i.e., one proton of DAQ forming an intramolecular hydrogen bond was replaced by an A1 atom has been proposed. It is believed that DAQ in the Q/SiO2-AI203/2 system forms a chemical bond with an AI atom. To investigate excited electronic states of the Q/SiO2-AI203/2 system, fluorescence lifetimes were measured from 80 to 280 K by means of time-correlated single photon counting. It was found that the dominant lifetimes of this system were different from those of the "y-Al203 doped systems. This difference indicates that the relaxation process from the excited state of the Q/SiO2-AIO3/2 system differed from that of the ",/-AI203 doped system.

1. Introduction The sol-gel method enables preparation of organic-inorganic composite materials [1,2]. Many studies have been performed concerning preparation and properties over the last decade that have revealed the states of dye molecules in inorganic matrices [3-6]. A recent trend in the preparation of organic-inorganic composite materials is to make chemical bonds between organic guests and inorganic hosts. Improvements of properties due to fixation have been reported [7,8]. This tendency can be regarded as one direction for the design of hybrid materials.

* Corresponding author. Tel.: + 81-3 3812 2111 ext. 7114; fax: + 81-3-3815 8363; e-mail: [email protected].

DAQ (1,4-dihydroxyanthraquinone) is one well known PHB (photochemical hole burning) active molecules. Br'~iuchle et al. have reported that stable holes were burned at 77 K for Q/~/-AI20 3 system [9,10]. In the present study, transparent SiO 2-A12 03 gels doped with D A Q were prepared by the sol-gel process ( Q / S i O 2 - A 1 0 3 / 2 system). The optical properties of the Q / S i O z - A I O 3 / z system were measured using absorption, fluorescence lifetimes, and FLN (fluorescence line narrowing) spectra.

2. Experimental DAQ, tetraethoxysilane (TEOS) and Al(OBu-s) 3 purchased from the Tokyo Kasei were used without additional purification. Since the rate of hydrolysis for both alkoxides are different, a two-step hydrolysis process was utilized to prepare homogeneous

0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0022-3093(96)00332- 8

44

T. lshiwaki et al. / Journal of Non-Crystalline Solids 203 (1996) 43-48

Table 1 The composition of prehydrolysis solution (molar ratio) TEOS

EtOH

H 20

HC1

1

8

2

10 -4

,I

, r , , : ,

'

r ,

.... ~

i

~ r l l - , , l

t

b

//

-

solutions. After holding the prehydrolysis solution at 60°C for 24 h, Al(OBu-s) 3 dissolved in BuOH, EtOH solution with DAQ, and HCI were added to the solutions. The solutions were kept at 40°C for about a month, after which transparent samples were obtained. The chemical compositions of the prehydrolysis-solution and samples are listed in Tables 1 and 2. As reference samples, DAQ doped SiO= gel ( Q / S i O 2 system), Q/~-A1203 system by the method of Bf~iuchle et al. [10,18] and Q/'y-AI203 embedded in polyvinyl-pyrolidone-vinyl acetate (Q/~/Al=O3/polymer) system were also prepared. Absorption spectra were recorded on a spectrophotometer (Hitachi U-3410). Fluorescence lifetimes were measured by means of a photon-counting system (Horiba NAES-550). The measurements of FLN spectra were performed with an OPO laser (Mirage 500, Hoya Continuum) and a high-resolution monochrometer (1000M, SPEX).

3. Results

The absorption spectrum of the Q/SiO=-AIO3/= system is shown in Fig. l(a). The absorption band of DAQ around 480 nm decreased with an increase in concentration of A1 atoms, and a new absorption band appeared around 560 nm. This red shift was complete when the [AI]/[Q] ratio exceeded 700. The spectrum of Q/SiO=-A103/2 system was similar to the fluorescence excitation spectrum of the Q/~/A1203 system determined by Br~iuchle et al. [10] (Fig. l(b)). Fluorescence lifetimes of Q/SiO=-A103/2, Q / S i O 2, Q/'c-A1203 and Q/"y-Al=O3/polymer Table 2 The composition of Q/SiO 2 -AIO3/2 system (molar ratio) SiO z

AI=O 3

DAQ

1

5 × 10 - 3

5×

10 -6

i

v71

::

J

/

,.

L I 7

300

350

400

450

500

550

Wavelon~h

(rim)

600

650

700

Fig. 1. (a) Absorption spectrum of the Q/SiO 2-A103/2 system and (b) fluorescence excitation spectrum of the Q/3,-AI203/2 system.

systems were measured from 80 to 280 K. The decay curves of each system could be fitted with a sum of two exponentials over all temperatures. The residuals, X:, were less than 1.3 for each system. The fluorescence lifetimes of each system (%, i = 1, or 2) are shown in Fig. 2 and listed with their relative quantum yields (Qi) in Table 3. The experimental errors of the fluorescence lifetimes and relative quantum yields were estimated to be less than 10% and 15%, respectively. To determine microstructural information in the Q/SiO2-AIO3/2 system, FLN spectra were measured. A typical FLN spectrum in the Q / S i O 2A103/2 system excited at 568.2 nm is illustrated in Fig. 3(a). The peak structure around 605.9 nm is closely examined in Fig. 4(a). These spectra were measured at 5 K. Fig. 3(b) and Fig. 4(b) are those of the Q/'y-AI203 system and Fig. 3(c) and Fig. 4(c) are those of the Q/~/-AI203/CH2CI 2 system in which Q/~-A1203 was embedded. These spectra were measured at 1.6 K by Br~iuchle et al. [17,18]. According to them the sideband at several cm -I lower energy from the zero phonon line (ZPL) shown in Fig. 4(b) was due to hindered translational or rotational motions of the adsorbed DAQ, and the

T. Ishiwaki et al. / Journal of Non-Crystalline Solids 203 (1996) 43-48 16

!(a)' -

•

.....

~'~. . . . . . . . . .

A

iibi

45

'

V

'

,I

t ]

~o 3

i 4

i .

.

.

.

.

S0

.

.

100

.

.

.

.

.

.

1 S0

.

.

.

.

.

200

~

. . . .

2S0

300

50

100

16

. . . . (C)

~- . . . . . . . ,

i

150

200

250

300

Temperature (K)

Temperature ( K ) ~ . . . . .

16 ;(d)

i

F

i

.~10-

L_

'

;'

50

100

150

J

J

i

200

250

300

Temperature (K)

.... 50

100

150

i

. . . . . . .

200 Temperature (K)

250

± _

300

Fig. 2. (a) Fluorescence lifetimes of Q/SiO2-AIO3/2 system. (b) Fluorescence lifetimes of Q / S i O 2 system. (c) Fluorescence lifetimes of Q / - / - A I 2 0 3 system. (d) Fluorescence lifetimes of Q/~/-Al203/polymer system.

peak which is separate from the ZPL shown in Fig. 4(c) was built up by bulk phonons of CH 2C12 [17,18].

4. Discussion 4.1.

Absorption

The spectra at each [AI]/[Q] ratio were expressed by the mixture of the spectrum of Q / S i O 2 system

and that of Q/SiO2-A103/2 system, and had an isosbestic point which indicated that the reaction in this system was a one-step process. This result and the fact that a crushed sample kept in CHC13 did not show dye extraction indicate that DAQ in the Q/SiO2-A103/2 system formed a chemical bond with an AI atom on the surface of the SiO2-AIO3/2 matrix (see Fig. 5). This model is supported by the similarity of Fig. l(a) and (b). Br'~iuchle et al. have proposed the same binding model for the Q/y-A1203

46

T. lshiwaki et a l . / Journal of Non-Crystalline Solids 203 (1996) 43-48

!

(e)

(b)

560

580

600

620

640

660

Wavelength (am)

16440

16460

16480

'16500 16520

16540

16560

Fig. 3. FLN spectra o f ( a ) Q / S i O 2 -A103/2 system (T = 5 K), (b) Q/~/-A1203 system, and (c) Q/3,-A1203/CH2C12 system (T = 0.16 K) excited at 568.2 nm. (b) and (c) were measured by Briiuchle et al.

system [10,11], i.e., one proton of DAQ forming an intramolecular hydrogen bond was replaced by an A1 atom. The absorption energy-shifts of substituted anthraquinone were dependent on the electron donation power of the substituent [12]. Fig. 6 shows the magnitude of energy-shift of DAQ adsorbed on vari-

Fig. 4. FLN spectra of each system around 605.9 nm. (a) Q / S i O 2-AIO3/2 system (T = 5 K), (b) Q/~/-A1203 system, and (c) Q/~/-AI203/CH2CI 2 system (T = 1.6 K). (b) and (c) were measured by Br'~iuchle et al.

ous oxides against microscopic optical basicity, A, which represents the electron donation ability of the oxy-groups in the glass [13]. Increasing A tended to increase the red shift. It is believed that the electron donating character of matrices were responsible for the red shift in the absorption spectra.

Table 3 Fluorescence lifetimes (ns) of each system at 80-280 K

Q/SiO2-AIO3/2 80 K 120 K 160 K 200K 240 K 280 K

Q/SiO 2

Q/~/-AI203

7i

Qi

7"i

Qi

14.2 5.0 14.5 5.3 14.3 5.4 14.1 (90.1) 5.1 14.3 5.3 14.1 5.3

(89.8) (11.2) (87.4) (12.6) (88.1) (11.9)

2.8 5.7 2.6 5.0 2.4 4.7 2.5 5.7 2.4 5.0 2.0 5.0

(90.0) (10.0) (84.3) (15.7) (85.0) (15.0) (94.0) (6.0) (93.0) (7.0) (95.1) (4.9)

(9.9) (87.0) (13.0) (85.7) (14.3)

16580

Wavenumbera(cm])

ri 5.5 14.5 5.6 15.2 5.4 15.0 5.3 15.0 4.9 14.4 4.5 (63.8) 14.0

Q/~/-Al203/polymer Qi (67.0) (23.0) (68.5) (22.5) (66.7) (23.3) (66.8) (23.2) (64.7) (25.3) (26.2)

7i 5.2 13.7 5.1 14.0 4.8 (72.0) 13.8 (28.0) 4.5 13.5 4.6 13.5 4.4 12.8

Qi (74.3) (25.7) (73.6) (26.4)

(73.8) (26.2) (73.5) (26.5) (73.0) (27.0)

T. lshiwaki et al. / Journal of Non-Crystalline Solids 203 (1996) 43-48

(a)

I

II ~Olo, I

I

O

o

O

si - O - - S i

47

6)

I

(b)

OH

HO

(+)

O

OH

OH

O

Fig. 5. Model of a D A Q molecule covalently bonded to SiO 2 A103/2 gel. I[

4.2. Fluorescence lifetimes

Fig. 7. (a) Model of CT transition. (b) Model of IHB transition.

The result that the fluorescence lifetimes of Q/SiO2-AIO3/2, Q / 7 - A 1 2 0 3 and Q / 7 A12Oa/polymer systems were longer than that of Q / S i O 2 system can be explained as follows. The S O-~ Sj transition of DAQ is considered to be a charge-transfer-like (CT) excitation between the electron-donating hydroxyl group and the electronaccepting carbonyl group [ 14-16], as is symbolically depicted in Fig. 7a. After that the succeeding nonradiative relaxation to the lower state is caused by an intramolecular hydrogen bonds (IHB) transition [15,16] (see Fig. 7b). In the case of DAQ-OA1 the

0.9

,

,

Q/TiO2 @

-

0.85 r

Q/Y203

:~ 0.8 ,~

•

Q/AI~O 3

i ~ 0.75 i !

~

I

0.7 •

4.3. FLN spectra

Q/B20:,

0.65

I J

0.61200

rate of IHB-like transition, kIHB, was assumed to be much smaller than that of DAQ-OH. It is suggested that the decrease of kin B results in the decrease in the total rate of each transition, k, which is the reciprocal of the fluorescence lifetime. While the Q/SiO2-AI03/2 system was similar in fluorescence lifetimes to Q/',/-AI203, and Q / 7 Al2Oa/polymer systems, the dominant lifetimes in the Q/SiO2-AIO3/2 system were different from 7-A1203 doped systems. For the Q/SiO2-AIO3/2 system the relative quantum yields of the longer components were about 90%, but those of the other two systems were about 30%. With respect to the temperature dependence of the fluorescence lifetimes there were also differences between Q/SiO2-A103/2 system and 7-A1203 doped systems. For the former system, both components of fluorescence lifetimes were independent of temperature, whereas for the latter systems they decreased slightly at high temperatures. These results suggest that there are distinctions between Q/SiO2-A103/2 system and ~t-A1203 doped systems in microscopic structure.

~

. . . . . . . 1300 1400

i ......... 1500 1600

~ ,. 1700

Energy shift of absorption band (cm"l) Fig. 6. Relationship between magnitude of energy-shift of D A Q adsorbed on various oxides and microscopic optical basicity.

The FLN spectra of Q/SiO2-A103/2 system were more similar to those of Q/7-A1203 system rather than to those of Q/'y-A1203/CHzC12 system, but a few differences were observed. In spite of being measured at higher temperature, the linewidth of the peak around 605.9 nm in the Q/SiO2-A103/2 sys-

48

T. lshiwaki et al. / Journal of Non-Crystalline Solids 203 (1996) 43-48

tem was smaller than the corresponding peak in the Q/SiO2-AI03/2 system. In addition, a broad sideband was present in the spectrum shown in Fig. 4(a). It is suggested that two phonons as were seen Fig. 4(b) and 4(c) were present as a mixture in the Q/SiO2-AIO3/2 system. DTA measurement showed that residual BuOH and EtOH existed in the gel. It was believed that these residual solvents suppressed hindered translational or rotational motions of the adsorbed DAQ. These results showed the difference between the Q/SiO2-AI03/2 system and the Q/~/-A1203 system, but the difference of dominating lifetimes between the two systems could not be explained. As was seen for rhodamine B adsorbed on oxides [19], two different electron transfer rates from adsorbate to substrate would be present. The comparison of surface states in SiO2-AIO3/2 and "y-Al203 would result in the difference of dominating lifetimes. To reveal the microstructure of Q/SiO2-A103/2 system more quantitative results are required.

5. Conclusions DAQ doped SiO2-A103/2 (Q/SiO2-A103/2 system) was obtained by the sol-gel method. From the absorption spectrum it is hypothesized that DAQ molecules were chemisorbed to AI atoms on the surface of gel similar to DAQ adsorbed on the surface of the ~/-AI203 (Q/~/-AI203 system). The dominant lifetimes of the Q/SiO2-A103/2 system differed from that of the ~/-A1203 doped systems. In addition FLN spectra showed the microstructural differences of both systems.

Acknowledgements The authors express their appreciation to Asahi Glass Foundation for their support.

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