Fluorescence spectra and lifetime of 2-naphthol in H2O- and D2O-ice(Ih) single crystal

_

Pergamon

Solid State Communications,

00381098(95)00247-2

FLUORESCENCE

SPECTRA

AND LIFETIME OF 2-NAPHTHOL SINGLE CRYSTAL

Qi Ping, Katsuhiko

The University

Okazaki, Tomoya Akiyama, and Takeshi Shigenari

Vol. 95, No. 3, pp. 177-180. 1995 Elscvier Science Ltd Printed in Great Britain 0038-1098/95 $9.50+.00

IN HzO- AND D20-ICE(Ih)

Kohji Abe

Department of Applied Physics and Chemistry of Electra-Communications 1-5-1, Chofugaoka, Chofu, Tokyo 182, Japan

(Received 13 October 1994; revised version received 25 January 1995 by H. Kamimura)

Proton dynamics were studied by a new fluorescence method using a cw-mode locked picosecond laser. Fluorescence spectra of P-naphthol(ZNpOH) doped in single crystals of ice I* (Hz0 and DzO) were measured from about 10K to 300K. In Hz0 ice, a broad band emission and six sharp bands were found in the near UV (340-385nm) range, while in DzO ice, besides a very weak broad band, only three sharp bands were observed. The broad band emission can be assigned to a neutral 2-NpOH interacting with a large number of water molecules and the shar bands are considered to be due to an isolated cluster of 2-NpOH with a small num ! er of water molecules. The less structured spectra in DzO ice and the lifetime difference between Hz0 and DzO ice are consistent with the fact that the rate of excited state proton transfer (ESPT) is retarded by deuteration. One of the sharp bands at 370nm was found to be peculiar since its lifetime (w0.2ns at T<40K) is much shorter than that of the broad band N5.2ns). The behaviour suggests that it is determined by the so-called intrinsic ES $ T time.

1. Introduction

tion, would show any isotopic effects. In this work, we present the fluorescence spectra and lifetime of 2-NpOH in Hz0 and DzO ice, and discuss the behavior of the fluorescence from a view point of the difference between protons and deuterons.

The proton dynamics plays an essential role for the physical properties of ice crystal. It is also of current interest from the following two points of view: (1) it is directly connected to the proton ordering transition to XI phase [l], [2] i.e. the 72K transition in KOH-doped ice [3], and (2) i t may be related to the existence of two different kinds of hydrogen bonds in an ice crystal which was recently found by the neutron scattering study [4]. It is well known that the proton dynamics is primarily controlled by the motion of defects in ice [5]. There are four kinds of defects in ice, ionic HaO+ and OH- defects, and the orientational L and D defects [5],[6]. Proton dynamics in ice was investigated by several methods such as the dielectric relaxation [3], photoconductivity [8], and isotopic exchange effect on IR spectrum [9]. Recently, we proposed a new experimental method[7], the laser-fluorescence method, which has the advantage of probing a local property of the bulk crystal with no effect of surfaces [lo]. In the previous paper [7], the temperature dependence of the lifetime of 2-NpOH (ClOHsOH, hereafter referred to as 2-NpOH) in Hz0 ice was explained in terms of a trapping process of proton with L defects. Since both the proton trap ping rate by L defects in ice [8] and the rate of excited state proton transfer (ESPT) of 2-NpOH [13] have been known to be affected by the isotopic exchange, it is of interest to see if the present fluorescence method, which is sensitative to a short-lived large-amplitude fluctua-

2. Experimental

and Results

2-NpOH (Wako Chem. Ltd.) was dissolved in commercially available deionized purified HzO. The concentration of the solutions was 2x10m3M. Single crystals were grown by a modified Bridgman method [3]. A sample with 6x6 x 8mm was set to an optical cryostat with temperature stability of -0.2K during an experiment. DzO water with the purity of 99.8% was purchased from EURISO-TOP, Ltd.. The concentration df 2-NpOH in D20 ice is the same as that in Hz0 ice. Since no remarkable polarization and crystal orientation dependences were observed, the following data were taken in a configuration (H, H+V), with the incident light directed along the a-axis of the sample. A synchronously mode-locked dye laser produces a pulse train at 4MHz with a width of 8wlOps, which is converted to UV pulses (X=295nm) with energy less than lnJ/pulse. The fluorescence spectra between 320N540nm were measured by a spectrometer (Acton 275) with a multichannel ICCD detector. The lifetime of the fluorescence was detected by a band pass filter (Ax = f5nm), a photomultiplier (HTV R1564U), and a time correlated single photon counting system. 177

178

FLUORESCENCE

Figure1 shows the temperature dependence of fluorescence spectra of 2-NpOH in Hz0 and DzO ice. In Hz0 ice (Fig.l(a)), at very low temperature (10~ 50K), six sharp bands are superposed on a broad band around X N350nm[ll]. The wavelengths of the sharp bands are 346, 349, 361, 370, 376 and 385nm. Among them, 370 and probably 346nm bands behave differently from others and are needed a special comment: the intensities of these two bands decrease rapidly between lo-50K, and they become shoulders of 361, and 349nm bands, respectively. The other four bands as well as the broad band change their intensities relatively slowly with temperature. Above 150K up to the melting point, no sharp band exists. In DzO ice (Fig.l(b)), the broad emission is very weak even at the lowest temperature (weaker than that in Hz0 ice above 1.5OK), and almost no remarkable temperature dependence was observed. Among the six sharp bands appeared in Hz0 ice, only three sharp bands exist in DsO ice at 370, 376 and 385nm. The 370 and 376nm sharp bands in DzO ice vary in a similar way with that in Hz0 ice, except that the intensity of the 370nm becomes weaker than 376nm at different temperature (about 46K for Hz0 ice and 64K for DlO ice). The fluorescence decay curves observed at 350nm (broad band) and 370nm (sharp band) in Hz0 ice are shown in Fig.2(a) and (b), respectively. The corresponding data in DzO ice are given in Fig.2(c) and (d). For the broad band (Fig.Z( a) and (c))? the fluorescence decays faster with the increase in temperature, and after melting, it becomes slow. These decay curves were analyzed with two components: a fast one rf, and a slow one r3[12]. The fast component, becomes more prominent at high temperature (above 200K). For the sharp bands (Fig.2(b) and (d)), however, the behavior of the decay is different from the broad

SPECTRA AND LIFETIME

band. Below 50K in Hz0 and 150K in DsO, where the 370nm emission is much stronger than that of the broad band, it was found that the decay curve has another component with a very fast lifetime r; (<0.5ns), in addition to the slow r, (N5.2ns) and rf (N0.8ns) components which are originated from the superposed broad band. The intensity of 7; decreases gradually with temperature. In Hz0 ice (Fig.2(b)), the sharp line is almost completely suppressed above 80K, and finally above 1OOK the decay curve resembles that of the broad band (Fig.a(a)). A similar behavior was also observed in DzO ice (Fig.2(d)), but the decay with r; remains up to 200K. This would be the reason why the clear 7; ‘suppression‘ observed in the Hz0 case (Fig.2(b)), was not seen in DzO ice (Fig.2(d)).

3. Discussion

sharp

Let, us first discuss the origin of the broad and bands of the fluorescence spectra. 2-NpOH is

0

n

183.8K

A

165.2K

-

WAVELENGTH

[nm]

a

2

4

6 Inal

146.5K

/\

A

Vol. 95, No. 3

XII5

92.7K

xl/5

64.OK

x I,5

48.2K

x L/6

24.5K

WAVELENGTH

Fig.1: Temperature dependence of the spectra NpOH doped in (a) HlO- and (b) D20-ice.

[run] of 2-

Fig.2: Temperature dependence of fluorescence decay curves observed at (a) 350nm and (b) 370nm in Hz0 ice, and (c) 350nm and (d) 370nm in DzO ice. The small bumps near ?-=2ns in several decay curves are the experimental artifact.

Vol. 95, No. 3

FLUORESCENCE SPECTRA AND LIFETIME

one of the typical substances which are chosen for the study of the ESPT in a solution [14],[15]. In an aqueous solution, the excited state 2-NpOH’ is dissociated to an excited anion 2-NpO-’ and a proton. As shown in the top of Fig.1, both species 2-NpOH’ and 2-NpO-* give two broad emissions at 350nm and 420nm region, with the widths of about 50nm and 60nm, respectively. Therefore, the fluorescence from ice in Fig.1 must be originated not from the anion but from the neut.ral 2NpOH’. The kinetics of ESPT were also studied in solution. From the studies on solutions, however, the observed values are mainly controlled by the solvents and the intrinsic nature of ESPT can not be obtained [17]. In order to get an insight into the intrinsic nature, matrix isolation [17] and free molecular jet [la] methods have been developed. So, it is interesting to compare the present result with them. According to the studies by Leutwyler [19] on l-NpOH cooled molecular jet with HzO, three stages are involved in a ESPT process. First, when a small fraction (n<=8)of Hz0 molecules is attached on the OH radical of l-NpOH, only a few sharp bands appeared in the fluorescence spectra. Second, as the number of Hz0 molecules in a cluster increases (840) of HZ0 molecules approaches a bulk size (solution), the sharp bands disappear and a broad emission of 1-NpO-* anion appears in addition to the broad band of l-NpOH’. ‘The t,emperature dependence of our fluorescence spectra is very similar to the Leutwyler’s cluster size dependence. Assuming that similar situation holds for 2-NpOH in ice as well, we believe that the sharp bands are originated from either a bare 2-NpOH that happens to be isolated from the “matrix” (ice) , or a cluster (here cluster-like structure in ice) of 2-NpOH with a very small number (say n<8 ) of HZ0 molecules (type(a)). The observation of the sharp band may depend on whether such a clust.er (n=O-8) can be formed or not’. The fact that the sharp bands were observed only in the low temperatures region is in agreement with Leutwyler’s since his study was performed with the cooled molecular cluster beam. As for the broad band, it appears in every sample in contract to the relatively poor reproducibility of the sharp bands. Since a 2-NpOH molecule is geometrically a little larger than the cavity of ice I,, crystal lattice, it would be difficult to be doped as an interstitial. It may be included by locally breaking the lattice and is stabilized by HZ0 molecules attached on the naphthyl ring. As described in the previous paper [7], we refer to such a 2-NpOH and Hz0 structure as a clathrate (type (b)). Then in Fig.l(a), the emission at low temperature is a superposition of type(a) and type(b), and as the tem‘It wm recognized from repeated experiments that, in spite of using the same solution for the growing of ice crystal, the sharp bands were not always observed. The appearance of the sharp bands and therefore the formation of the isolated NpOH may be critically dependent on the crystal-growth conditions.

179

perature is increased, the emission varies from type(a) to type(b) It is clear from the comparison of Fig.l(b) with Fig.l(a), that the broad band intensity of 2-NpOH in DsO is much weaker than that in Hz0 ice. This suggests that D20 molecules can hardly take a configuration in which the naphthyl ring is wetted enough to be stabilized. The weakness of the broad band in DzO ice is consistent with the fact that the ESPT in aqueous DzO solution is retarded compared to Hz0 (the ratio of the dissociation rate in Hz0 water and DzO water is ,&(H)/kd(D) ~5.10 at T=303K [13]). The temperature dependences of 7-Sare shown in Fig.3. In our previous work [7], The characteristic change in the lifetime between 130K and 200K was attributed to the increase of the mobility of L defects which is an acceptor of a proton of 2-NpOH. We notice that the amount of the lifetime shortening, which are denoted as AT, and Are in Fig.3, are significantly different. The small Aro (N2.5ns) in DzO ice indicates that the proton trapping rate by a L defect is less than that in H20 ice (w5.0ns). This is in agreement with the Kunst’s observation on photoconductivity [8]. It is interesting that the intensity of 370nm band rapidly decreases at low temperature (below 46K in Hz0 and 64K in DzO ice), while the 376nm (possibly a vibrational side band of 370nm) remains up to a higher temperature (80K in Hz0 and 200K in DzO ice). The origin of these bands are not known at the present stage, but the higher ‘vanishing’ temperature in D20 indicates that an isolated 2-NpOH is more stable in DzO than in H20. In contrast to the expected difference in the broad band behavior between Hz0 and DzO ice, it was unexpected that the lifetime 7; of the sharp band at 370nm was found to be very short (w0.2ns) in both Hz0 and DzO ice (Fig.2(b) and (d)). It is shorter than the lifetime of the broad band (7-# N 5.2ns in Hz0 ice). This

0

1.1,

I,,

1.I..

100

1,.

1

I,,

200

1..

.I k 300

TEMPERATURE[K]

Fig.3: Temperature dependence of the slow component of lifetime r, of the broad band (350nm). ASH and Are are the amount of shortening of T* between 120K and 200K for H20- and DzO-ice, respectively. Solid lines are guide for eyes.

FLUORESCENCE

180

SPECTRA AND LIFETIME

Vol. 95, No. 3

tem. Further studies to clarify this problem would be needed. In conclusion, the cw-mode-locked picosecond laser fluorescent method was found to be a sensitive probe of proton dynamics in ice crystal. From the comparison of the fluorescence study of 2-NpOH doped in Hz0 and DXO ice, it was revealed that; (1) The differences in the spectra (less structured in DzO than Hz0 ice) and the broad band fluorescence lifetime are consistent with the L-defect model [7] if one takes into account the reduced mobility of deuterons both in L-defect trapping and ESPT. (2) The lifetime of the 370nm sharp bands (<0.2ns at T<40K) suggests that it resulted from the intrinsic ESPT. The authors would like to thank K.Morita for his collaboration in the experiment. This study has been supported by Grant-in-Aid from the Ministry of Education 1Science and Culture, Japan.

excludes the possibility of 370nm sharp band being a zero-phonon line emission, since if the lifetime is simply determined by the interaction with thermal phonons, 7; would be longer than r, as has been observed in a biphenyl crystal[21]. Furthermore, according to Brucker and Kelley [17], a zero-phonon line of 2-NpOH cannot be located in a wavelength region longer than 340nm, even the red shift effect of proton acceptor is taken into account. A possible explanation of 3’ is that it is related to the intrinsic ESPT of 2-NpOH in ice. In other words, once an appropriate localized cluster of 2-NpOH with HxO/DzO molecules is formed, the proton of 2-NpOH would be deprived within a very short period, which occurs only inside the cluster of 2-NpOH with H20/D20. The intrinsic ESPT lifetime has been known to be about 20~s [17] in 2-NpOH.(NHa), with n>3[22]. Our value of 7; <=2OOps seems to be much larger than 2Ops, since our T# is limited by the resolution of the detecting sys-

References 1. A.V.Zaretskii, R.Howe and R.W.Whitworth, Phil. Mug. B 63 757 (1991). 2. H.Suga, T.Matsuo, O.Yamamuro, in: Physics and Chemistry of Ice, eds. N.Maeno and T. Hondoh, (Hokkaido Univ. Press, Sapporo, Japan.) p.1 1992. 3. Y.Sakabe, M.Ida and S.Kawada, J. Phys. Sot. Jpn. 28 265 (1970). S.Kawada, J. Phys. Chem. Solids 50 1177 (1989). 4. J.Li and D.K.Rosss, in: Physics and Chemistry of Ice, eds. N.Maeno and T. Hondoh, (Hokkaido Univ. Press, Sapporo, Japan.) p.27 1992. J.Li and D.K.Ross, NATURE, 365 327 (1993). 5. D.Eisenberg and W.Kauzmann, The Structure and Properties of Water, (Oxford, London, 1969). 6. J.F.Nagle, in: Proton nansfer in Hydrogen Bonded System, ed. T.Bountis, (Plenum Press, New York.) p.17 1992. 7. Qi Ping, K. Okazaki, T. Akiyama, K. Abe and T.Shigenari, Chem. Phys. Lett., 230 322 (1994). 8. M.P.de Haas, M.Kunst, J.M.Warman and J.B.Verberne, J. Phys. Chem. 87 4089 (1983). M.Kunst, J.M.Warman, J. Phys. Chem. 87 4093 (1983). 9. P.J.Wooldridge and J.P.Devlin, J. Chem. Phys. 88 3086 (1988). W.B.Collier, G.Ritzhaupt and J.P.Devlin J.Phys. Chem. 88 363 (1984). 10. T.Shigenari, E.Kojima, Y.Ino and K.Abe, Phys.

Rev.

Lett.

66 2112 (1991).

T.Shigenari, K.Abe, K.Morita, Qi Ping and E.Kojima, J. Phys.:Condens. Matter 6 7469 (1994).

11. In [7], fluorescence spectra of X < 400nm were not obsereved due to the sensitivity of the spectrometer. 12. The analysis was done as described in [7], and the details will be presented elsewhere. 13. T.Kishi and J.Tanaka, Chem. Phys. Lett. 41 497 (1976). 14. C.M.Harris and B.K.Selinger, J. Phys. Chem. 84 891 (1980). 15. J.Lee, R.D.Griffin, and G.W.Robinson, J. Chem. Phys. 82 4920 (1985). 16. M.Ofran and J. Feitelson, Chem. Phys. Lett., 19 427 (1973). 17. G.A.Brucker and D.F.Kelley, J. Chem. Phys. 90 5243 (1989). 18. A.Oikawa, H.Abe, N.Mikami and M.Ito, J. Phys. Chem. 88 5180 (1984). 19. R.Knochenmuss and S. Leutwyler, J. Chem. Phys. 91 1268 (1989). 20. In our experiment, both ‘L, and lLb are excited, while in Brucker’s experiment, only ‘Lb state is excited. 21. K.Shinjyo, K.Morita, K.Abe and T.Shigenari, Ferroelectrics (in press). 22. In 8-hydroxy-1,3,6-pyrene trisulfonate (HPTS), this value is loops for Hz0 and 350~s for DzO, respectively. See, D.H.Huppert, A. Jayaraman, R.G.Mines, Sr.,D.W.Steyert and P.M.Rentzepis, J. Chem. Phys., 81 5596 (1984).