J. Phys. Chem. 1994, 98, 47-52
47
High-Temperature Non-Photochemical Hole-Burning of Phthalocyanine-Zinc Derivatives Embedded in a Hydrated AIP04-5 Molecular Sieve M. Ehrl,? F. W. Deeg,*J C. Briiuchle,t 0. Franke,t A. Sobbi,# G. Schulz-Ekloff,* and D. Wiibrlei Institut f i r Physikalische Chemie, Uniuersitat Miinchen. Sophienstrasse 1 1 , 80333 Mhchen, Germany, Institut f i r Angewandte und Physikalische Chemie, Universitiit Bremen, 28334 Bremen, Germany, and Institut flir Organische und Makromolekulare Chemie, Uniuersitat Bremen, 28334 Bremen, Germany Received: July 16, 1993; In Final Form: October 1 1 , 1993’
We report on persistent spectral hole-burning studies of phthalocyanine-zinc derivatives encapsulated in the pores of an A1P04-5 alumophosphate molecular sieve. Persistent holes are only formed if the porous crystal is saturated with additional solvent molecules, e.g. water or chloroform. The characteristics of the hole formation indicate that the burning mechanism is non-photochemical due to interaction with the two-level systems of the amorphous solvent shell within the pores. The hydrated molecular sieve allows efficient hole formation up to 80 K, the highest temperature for which stable non-photochemical holes have been reported so far. This is attributed to the combined effect of a stiff matrix and a hydrogen-bonded amorphous environment leading to a high Debye-Waller factor. The inorganic crystal provides a stiff framework in which water constitutes an amorphous hydrogen-bonded environment containing an appreciable number of high-barrier two-level systems. The data are discussed in the context of the prerequisites of high-temperature hole-burning, especially the distribution of barriers and the temperature dependence of the Debye-Waller factor.
Introduction Persistent spectral hole-burning has been used as a powerful tool in low-temperature spectroscopy since its first observation’ in 1974. It was suggested as the basis for high-density frequencydomain optical storage (FDOS) in 1978.2 One of the problems still unsolved and impeding practical application has been the limitation of hole-burning phenomena to liquid helium temperatures. First indications of the possibility of persistent spectral holes above 30 K appeared in the literature of the early 1980~.~-$ These observations were given under circumstances equivalent to a temperature-cycling experiment? but were not checked seriously. In 1988 persistent spectral holes burnt at 80 K were reported for the first time.7 The hole-burning system was an anionic porphine derivate in the amorphous polymer poly(viny1alcohol) (PVA). Since then, a lot of work has been done to improve the high-temperature properties of dye-polymer Nonpolymer materials have been checked as well. Crystals usually suffer from some disadvantages, particularly a small inhomogeneous broadening. Yet, it was shown that therearealso inherent advantages over polymer hosts in high-temperature hole-burning (HTHB). Systems based on nitrogen vacancies in diamondI4 and on earth-alkaline halides of the PbFC1-type doped with Sm+2 15316 undergo spectral hole-burning up to 120 K in the case of diamond14 and up to room temperature in the case of Sm+*doped SrFClI/ZBr1/2single crystals.16 Furthermore, adsorbatesubstrate systems were found to show strong inhomogeneous broadening as well as HTHB. Two systems consisting of the dye molecules quinizarine17J8and o~taethylporphine~~ adsorbed on the heterogeneous surface of y-alumina have been reported to undergo spectral hole-burning at 77 and 90 K, respectively. In all HTHBcases discussed so far the holeburning mechanism has been attributed to a photochemical change of the excited chromophore. In this paper HTHB in a new class of materials will be demonstrated. The new host material is a low molecular weight solvent (here water and chloroform) embedded in an UnivenitHt MIlnchen (FAX 49-89/5902 602). t Institut fur Angewandte und Physikalische Chemic, Universitat Bremen
(FAX: 49-4211218 4042). I Institut far Organischcund Makromolekulare Chemie,UniversitltBremen (FAX: 49-421/218 4042). Abstract published in Adounce ACS Absrrocrs, December 1, 1993.
inorganic molecular sieve. To understand the relationship between the properties of the chromophore-host system and the characteristics of HTHB the essential prerequisities, i.e. high producteduct barriers and a large Debye-Waller factor, will be reviewed in the first section and our experimental results will be discussed later in the context of these factors.
Prerequisites of High-Temperature Hole-Burning in Amorphous Hosts To understand the properties of spectral hole-burning at elevated temperatures, e.g. 77 K, a number of factors which are in general of minor importance at liquid helium temperatures have to be considered. A first major factor is the microscopic process responsible for the appearance of a persistent spectral hole. The published literature tries to distinguish between photochemical (PHB) and non-photochemical (NPHB) hole-burning processes. PHB, first discussed by Voelker et a1.,2O was assigned to the intramolecular tautomerization reaction of free base porphine. This reaction causes a change in the a-bonding system of the molecule. If the chromophorefully retains its structure and electronic system upon excitation, hole-burning is still possible in amorphous hosts. The interaction of the chromophore with degrees of freedom of the host (so-called two-level systems (TLSs)) may change, and in this case the burning process is called non-photochemical. This process was first described by Hayes and SmalP giving several phenomenological indications for NPHB. For instance, the widely investigated molecule quinizarine is reported to undergo holeburningZ2in alcohol matrices. The burning process is known to be a concerted action of matrix and dye molecules. Therefore, the mechanism cannot be strictly assigned to one of the terms. Central for the feasibility of HTHB is the height of the producteduct barrier, which determines the reconversion rate and therefore the stability of the spectral hole. Up to now there are basically three different classes of HTHB materials with organic chromophores. Best characterized are derivatives of free base porphine.7-Q.l’.’Q,2~,24 In this case intramolecular proton tautomerization provides a photochemical burning mechanism. The internal barrier for the tautomerization is rather high, inhibiting thermal conversion up to about 100 K.20 Metal complexes of porphine derivatives are reported to undergo one- or two-photon
0022-3654/94/2098-0047S04.50/0 0 1994 American Chemical Society
48
The Journal of Physical Chemistry, Vol. 98, No. 1 , 1994
electron transfer25 with well-stabilized products.1° For quinizarine on y-alumina17 the most plausible burning process is a change from intramolecular to intermolecular hydrogen bonding. In general it is assumed that the barrier in thesesystems isdetermined by the chromophore and has a relatively well-defined value. For NPHB the stability of the holes depends on the barriers associated with the TLSs, which in amorphous matrices are characterized by a broad distribution of barrier heights. Therefore, the number of TLSs which allow hole formation decreases continuously with increasing temperature. There is no welldefined maximum HB temperature, but in the systems investigated so far no clearly non-photochemical hole has been observed at elevated temperature. Extending hole stability measurementsz6to high temperatures,Z7 there have been some indications that even in the case of intramolecular product-educt barriers, the main limiting factor is given by the barrier distribution of the host. Temperature cycling of tetraphenylporphine (TPP) in a series of alkyl esters of poly(methacry1ic acid) between 4.7 K and 50 K revealed irreversible hole-broadening effects ranging from a few GHz up to some five wavenumbers, depending on the nature of the alkyl substituent. Because of the well-known intramolecular producteduct barrier of free base porphines, no hole area decay was expected. However, effective hole filling even at excursion temperatures well below 50 K was observed. The authors concluded configurational changes of host alkyl groups to exhibit low-energy barriers but to cause large changes in transition energy of the guest chromophores. This indicates a strong coupling of the chromophore transition energy to host degrees of freedom. From a phenomenological point of view, hole filling is caused by any process which shifts the transition frequency of a molecule from outside to inside the spectral range under investigation. Hole broadening is observed for a process which shifts transition frequencies within the observed spectral range. In general, holefilling processes are associated with the dynamics of strongly coupled so-called extrinsic TLSs, wherein hole broadening is attribued to the relaxation of weakly coupled so-called intrinsic TLSsaZ8Temperature-cycling experiments to high excursion temperatures2' suggest that the difference between extrinsic and intrinsic TLSs may vanish parallel to the difference of PHB and NPHB a t high temperatures. There may be TLSs of different physical nature resulting in a dispersion of the chromophoreTLS coupling strength additional to the distribution of TLS parameters. If the chromophore interacts with low-barrier TLSs which cause hole broadening in the range of some tens of wavenumbers, no persistent spectral holes can be burnt. The second major factor in this context of HTHB is the change of the line shape or respectively the hole shape with increasing temperature. The burnt spectral hole has two contributions: the zero phonon hole (ZPH) and the phonon sideband (PSB) due to electron-phonon coupling. These two parts are related by the Debye-Waller factor CYD. It is given by the quotient of the hole area (integrated intensity) of the ZPH, AzPH(T),and the sum of AzPH(T) and the area of the PSB, Apse(T): CYD= A Z P H ( T ) / ( A Z P H ( T )
+ABB(T))
(1) A decrease of CYDwith temperature corresponds to a transfer of spectral intensity from the zero phonon hole (ZPH) to the phonon sideband (PSB). AZPHis proportional to the product of the halfwidth (I'ZPH) and peak intensity (Zmx)of the hole, assuming a Lorentzian hole profile: At liquid helium temperatures, r Z P H and the PSB half-width (I'psB) have typical values of 1 GHz and 20 cm-I, Le. 600 GHz, respectively. At 80 K, r Z P H is increased to 15-20 cm-I, whereas I'mB is not changed significantly. This line-broadening effect inherently causes a dramatic loss in resolution on the intensity
Ehrl et al.
8
2 c1
1.0
!.
Figure Debye-Waller factor abw as a function of temperature and energy Y of the coupled phonon for (a) linear electron-phocon coupling strength S = 0.15 and (b) S = 0.3. A small S and a large Y is required to obtain a large value of a h at elevated temperature.
scale of the spectrum, independent from changes of CYD. Therefore, a t liquid helium temperatures, a satisfying separation of the ZPH from the PSB is provided by CYDvalues ranging from 0.01 to 1.0.29 As a contrast, a t 80 K even an CYDof 0.1 is too small a value to resolve a ZPH a t all. If the widths of ZPH and PSB come into the same order of magnitude because of homogeneous line broadening, zero phonon holes will only be resolved if the area of the PSB is still small in comparison to AZPH. This will only occur for the case of very weak electron-phonon coupling. Therefore, CYDvalues close to unity are especially important a t high temperatures. In the simplest model of linear electron-phonon coupling one assumes that the electronic transition couples with the strength S to one dominating phonon. Under these circumstances the temperature dependence of the Debye-Waller factor CYDof a single site absorption line profile is given by30
Due to the two-step nature of burning and reading out in a holeburning experiment, the hole profile is different from the corresponding absorption line profile. For example the width of the ZPH is twice the width of the zero phonon absorption line,29 and the temperature dependence of the Debye-Waller factor obtained from the hole profile function is given by31 %iOLE
(n= cxp( -2s coth( &))
(4)
In the analysis of many hole-burning experiments eq 3 is erroneously used instead of eq 4. Though the one-phonon approximation seems to be an oversimplification, particularly a t elevated temperature with increasing population of several phonon modes, recent experiments have shown that the approximation is in good agreement with the experimental results up to 80 K.11J3J7-19,23.32 CYDdecreases with increasing temperature. A large value of the Debye-Waller factor at elevated temperature requires a high phonon mode and a weak coupling strength S. For an instructive illustration of this dependence see Figure 1. The strength of the linear electron-phonon coupling can be correlated with the polarizability of the chromophore or, more specific, the change of the molecular dipole moment A p between ground and excited state.33 This coupling strength S can be
The Journal of Physical Chemistry, Vol. 98, No. I, 1994 49
Hole-Burning of PhthalocyanineZinc Derivatives
a cd Y
b4-
13.7 A
A Z P O ~5
Figure 2. Schematic picture of the AIPO4-5framework. A1 and P atoms are represented by dots, and oxygen is centered in the middle of each connecting line. The expansion of the hexagonal unit cell is represented by the two trigonal prisms. For further explanation, see text.
evaluated from the temperature dependence of
(
Y
D
~
~
~
aDHOLE( T - 4 ) = exp(-2S)
The value of ;of the coupling phonon mode can be determined by the spectral separation of ZPH and PSB. It was reported to be a characteristic value of the host matrix, as investigated by hole-burning of tetraphenylporphine (TPF) in polymer system~.ll.*4J*.3~.35Only minor variations of u were found due to different dye molecules in poly(methy1 methacrylate) (TPPversus quinizarine) and PVA (tetrasulfonatophenylphorphine versus quinizarine). Even though it may be considered as a rule of thumb that the phonon frequency is correlated-to the host, there are some notable exceptions. For iptance, Y = 45 cm-I was reported for oxazine 1 in PVA36 and Y = 50 cm-1 for quinizarine on y-alumina.'* These values are twice as large as the values usually reported from those hosts. A common rule is that the values of; for polymer matrices fall into two clearly divided subgroups. Polymers containing hydroxy groups usually exhibit phonon modes above 20 cm-1, and allothers, polar or nonpolar in nature, do not.12 All known HTHB systems which show hole widths in the range 15-20 cm-l at 80 K contain a considerable quantity of hydroxy groups. In this work a hole-burning study of dye molecules embedded in the porous crystalline structureof a molecular sieve is presented. The pores are filled with a low molecular weight solvent, which forms an amorphous matrix and allows NPHB. These systems offer host crystallinity combined with large inhomogeneous br0adening.3~938 The stiff framework of these crystals promises to get rid of bothering low-frequency modes and large spectral diffusion, which frequently limit the suitability of polymer hosts to hole-burning at elevated temperature^.^^
Experimental Section A1P04-539is a pore-building neutral oxide framework of edgelinked oxygen tetrahedrons, containing one atom of aluminum or phosphorus each. There is a strict alternation of A1 and P. Each A1 is neighbored by four P and vice versa. The AlP04-5 framework contains well-separated parallel channels of 7.8-A diameter (see Figure 2). It may exhibit some local polarizability due to the differences in electronegativity between A1 (1 -5) and P (2.11.39 Details of dye-doped sample synthesis will be published e1sewhere.a In brief, dye molecules were prepared as iodine salts of tetrakis[N-methyl-3-(pyridyloxy)]phthalocyaninezinc (ZnTpyP) and tetrakis(N-ethyl-2,3-pyridino)tetraazaporphyrin-zinc (ZnTaP), respectively (see Figure 3). Dye-loaded AlP04-5 samples were synthesized by adding the dye, solved in water, to the aqueous starting mixture of the template synthesis. Microcrystalline needles were obtained, containing crystals up to 5 pm in length. Atomic absorption spectroscopy was used to determine
500 530 560 590 620 650 680 710 740 770 800
excitation wavelength [ n m ] Figure 3, Fluorescence excitation spectra of ZnTaP (dashed line) and ZnTpyP (solid line) in hydrated AlP04-5. Both dyes consist of up to four structural isomers, as is usual for tetra-substituted phthalocyanine derivatives. Hole-burning experiments were carried out near the maxima of 650 and 680 nm.
the dye content of 6.2 X mol/g in the case of ZnTpyP and of5.3 X 10-7mol/gin thecaseofZnTaP. Thedensityofhydrated AlP04-5 is about 2 g/cm3; therefore, the unity (mol/g) has to be converted by multiplication by a factor of 2 X 103 into moles/ liter. This reveals concentrations of about 1 X mol/L, which correspond to a mean molecular distance of about 120 A. Maximum fluorescence intensities were obtained at this concentration. After synthesis the crystal pores contain dye molecules, water, and a large quantity of the regular template molecule, usually triethylamine. In order to guarantee reproducible conditions, the pore-filling molecules were removed by heating the sample by means of a temperature ramp of 0.5 OC/min up to 120 OC under the reduced pressure of 1e5Torr. Then different treatments were used. Samples called dehydrated were sealed off in glass cuvettes. Samples called hydrated were stored in open glass cuvettes for at least 2 days over saturated aqueous KCI solution (83% relative humidity a t 20 "C). Samples treated with organic solvents were immersed in the solvent and sealed off in cuvettes. The sample cuvettes were held in a Cryovac helium bath/flow cryostat (1.3-300 K). Spectral holes were burnt using a CR 899-29 autoscan laser supplied with DCM dye and dumped by an Innova 200 argon ion laser. The dye laser line width was 5 1 MHz. The spectra were recorded in fluorescenceexcitation mode using a CRC C31034 A photomultiplier and a SI single-photon counting system. Appropriate cutoff filters were used to select fluorescence light only. High-resolution spectra were recorded scanning the autoscan laser. Broad band and high temperature hole spectra were recorded, using a 150-W Xe arc lamp disperged by a Spex 1402 monochromator with a 3-cm-l bandpass. In order to investigate the temperature dependence of the Debye-Waller factor, a hole of about 5% in depth and about 15 cm-1 in width was burnt at 50 K. After scanning the hole once, it was cooled down to 2 Kand successively read out a t temperature between 2 and 50 K. In this way, spontaneous hole filling was found to be negligible during the experiment. The burning temperature of 50 K was used because a proper separation of the ZPH from the PSB is not possible for higher temperatures in the systems considered.
Results and Discussion Burning Mechanism. The fluorescence excitation spectra of the samples investigated are essentially independent of the presence of additional solvent in the pores, indicating that the large inhomogeneous broadening is provided by the molecular sieve itself. Hole-burning experiments were carried out near the
50 The Journal of Physical Chemistry, Vol. 98, No. 1, 1994
Ehrl et al. S = 0.16
B
se
0.88
1
1
I
I
2 0.8 ZnTPYP $.r3-;
0.0
.-x
z
1 0
Y
14
28
42
56
70
temperature [K]
1.00
*
Figure 5. Temperature dependence of the Debye-Waller factor a
.-
h
for ZnTpyP and ZnTaP in hydrated Alw4-5: open circles, normalized experimental data, derived from a fit due to eq 6; solid curve, values calculated from eq 4. For a discussion, see text.
8 0.98' G
8 0.96 0 "
a photoionization mechanism. Thus, a single-proton electrontransfer mechanism can be ruled out. .In order to investigate the possibility of electron transfer from higher triplet states, two-color hole-burning experiments with A2 E 0.92 ZiiTaP = 468 nm and A2 = 5 14nm were carried out in samples containing E o - -240 -120 0.0 120 240 the solvent chloroform as a suitable electron acceptor. A2 was fi - Lj/>,i,r, [ C I I l C ' ! chosen close to the triplet-triplet absorption maximum of phthalocyaninezinc a t 480 nm.45 Varying the burning power of Figure 4. Deep saturated holes of phthalocyanine-zinc derivatives in XI (685 nm in the case of ZnTpyP, 655 nm in the case of ZnTaP) hydrated AIPO4-5. (a) Chromophore = ZnTpyP, hole burnt at 20 K and between 5 X lo-' and 1 X 10-* W/cm2, and of A2 between 1 X 685.0 nm with 1.5 mW/cm2 for 1800 s. At both sides of the ZPH the 1W and 1 W/cm2, no gating effect on the hole-burning efficiency PSB can clearly be seen (28 cm-1). On the high-energy side of the hole, the maximum of the product band is marked by an arrow at 75 cm-I. The could be detected at 2 and 20 K. Thus, a two-photon ionization dip at -220 cm-1 results from a vibronic pseudo side-hole. Its product mechanism can also be ruled out. band lies 75 cm-I to higher energy at -145 cm-I. (b) Chromophore = According to temperature-cycling experiments, the stability ZnTaP, hole burnt at 20 K and 655.0 nm with 1.0 mW/cm2 for 1800 of holes is low. Even in hydrated samples, a hole burnt at 2 K s. At both sides of the ZPH the PSB can clearly be seen (30 cm-I). On is completely erased if the excursion temperature is raised to 30 the high-energy side of the hole, the maximum of the product band is K. Spontaneous hole filling is logarithmic in time and comparable marked by an arrow at 75 cm-I. Vibronic pseudo side-holesare at -250 to the behavior found in bulk glasses. However, hole broadening and -130 cm-I. as a function of time, i.e. spectral diffusion, was found to be significantly reduced in AlFQ-5 samples in comparison to ethanol maximum of the S&l transition band on the long wavelength glass. This effect will be discussed in detail elsewhere.46 side (around 685 nm in the case of ZnTpyP, 655 nm in the case Summarizing the results above, we conclude the burning process of ZnTaP; see Figure 3). The dehydrated samples do not show to be a non-photochemical mechanism associated with rearany persistent hole-burning effects using burning powers in the rangements of the amorphous solvent shell within the pores. range 5 X 10-7 to 5 X W/cml and an exposure time of 200 Temperature Dependence of the Debye-Waller Factor. As can s at 2 K. Yet, they do exhibit fluorescence line narrowing. These be seen from eq 1, the area of the zero phonon line ( A ~ His ) observations demonstrate that there is no intramolecular phoproportional to (YD if the oscillator strength of the transition, i.e. tochemical hole-burning mechanism. the sum of the integrated intensities of ZPH and PSB, is Phthalocyanine and porphine derivatives are generally known independent of the temperat~re.~2 The DebyeWaller factor was to undergo photon-induced electron-transfer reactions. This by monitoring the hole area of the zero phonon hole possibility is usually enhanced in metal c o m p l e x e ~ . ~ ~ ~ ~ ~ - ~ determined * for temperatures T ITburn.To determine the linear electronIn samples with solvent-filled pores, Lorentzian-shaped holes phonon coupling strength S, the experimental data were fitted could easily be burnt in both dye-A1P04-5 systems at temperatures to the function (see eq 4) between 2 and 20 K. The solvents used were water and chloroform. The highest achievable temperature which allowed the generation of detectable holes varied with the kind of solvent used for pore h - d T ) = N exp( -2s coth( (6) filling: 50 K was found for chloroform and 85 K for water. with N being a constant relating A z p ~and ah,,. vis extJacted Assuming a NPHB mechanism, which is usually related to a rearrangement of hydrogen bonds, that is not a surprising result. from the hole-burning-spectrum. As shown in Figure 4, Y = 28 The existence of hydrogen bonds involving chlorine and the acid cm-I for ZnTpyP and Y = 30 cm-I for ZnTaP. From these data proton of chloroform is evident,43 but those bondings are known a coupling strength S = 0.16 was found in the case of ZnTpyP, to be weaker than hydrogen bonds formed by hydroxy groups. corresponding to a ahoLe( T-0) = 0.73. In the case of ZnTaP In order to detect the photoproduct, deep and saturated holes S = 0.28 and aDnoLB(T+O) = 0.56 were found (see Figure 5). To discuss the difference in coupling strength, two main aspects were burnt at 20 K. Figure 4 presents the results for ZnTpyP should be considered. Firstly, the electronic transition in ZnTpyP and ZnTaP in hydrated samples. The samples solvated with should be insensitive to any polarizing effect on the cationic chloroform looked very similar. Both spectra show photoproduct N-methyl-3-(pyridyloxy) groups as the aromatic system of the bands on the short wavelength side of the ZPH. This effect is well-known and considered to be an indication of NPHB.44 The pyridinium ring is not conjugated to but isolated from the real phonon sideband is partially overlapped by the product band. phthalocyanine ring system due to the oxy bridge. In contrast to this, in ZnTaP the cationic nitrogen is part of the aromatic The maximum of the product band is about 75 cm-l away from phthalocyanine ring system, and this molecule should be more the ZPH. This value is considered to be too small an offset for e: e 0.94
-
I
I
g)) -
Hole-Burning of Phthalocyanine-Zinc Derivatives
.-3
1.02
I
c)
.-cOi
1.01
H8
1.00
Pb
O*gg
3
0.98
$
0.97
' z
I
v T T,
ZnTaP
30
a
A -40 -20 0.0 20 40 fi - fihhrrrn [cm-ll
-160 -80 0.0
80
160
fi - f i / m r n [cm-'I
Figure6. (a) Shallow spectral hole of ZnTaP in hydratedAlPO,-S burnt at 80 K and 655.0 nm with 13 mW/cmz for 100 s. Saturation effects
and sideband enhancement (marked by arrows) are still absent and allow reasonable detection of the hole width, which is 19 cm-1 in this case. (b) Deep spectral hole of ZnTpyP in hydrated AIPO4-5 burnt at 77 K and 685.0 nm with 10 mW/cm2 for 1000 s. The holes width is 67 cm-1. Saturation effects and PSB enhancement (marked by arrows) dominate the line shape of the hole. polarizable than ZnTpyP. Therefore, an enhanced electronphonon coupling is expected for ZnTaP compared to ZnTpyP, in agreement with experiment. Secondly, AIP04-5 is a hydrophilic substance39 and has to be considered as a partially polar matrix. Water fills the free void volume (33% of unit cell volume). In the undistributed pore system (7.8-Aporediameter), each water molecule (3-Adiameter) is adjacent to the framework and bulk behavior is not expected. At local defects, water is able to aggregate to clusters and furthermore to form a bulk system. This bulk water may shield the dye molecule from dipolar interaction with the framework. Due to its four aromatic substituents, ZnTpyP is a more extended molecule than ZnTaP. Both molecules are larger than the pore
The Journal of Physical Chemistry, Vo1. 98, No. I , 1994 51 diameter and must be immobilized at local structure defects. The molecular diameters are approximately 25 A for ZnTpyP and 15 A for ZnTaP, respectively. Assuming the local defects to be spheres with the diameter of the dye molecule, the sphere around ZnTypP (280 HzO) has about 5 times the volume of that around ZnTaP (60 HzO). If the coupling to the water is weaker than to the molecular sieve framework, this should result in a reduced coupling constant for ZnTpyP. We expect that the structure of water in the defects and pores of AlP04-5 is different from bulk ice because of the high surface/bulk ratio experiencedby a solvent in a molecular sieve. Further hole-burning investigations which should contribute to this subject are in progress. The phonon frequencies found here are comparable to values found for a number of physisorbed adsorbate-substrate systems,17 higher than in organic bulk glasses and smaller than in chemisorbed adsorbate-substrate systems. This confirms the dominating influence of matrix stiffness and interaction of the chromophore with this matrix on the frequency of the coupled phonon. High-TemperatureBehavior. Spectral holes were burnt up to 85 K in hydrated AlP04-5 samples. Hole widths of 19 cm-1 for ZnTaP at 80 K and of 21 cm-I for ZnTpyP were found for holes with a depth of less than 5%. These values fit well into the range of hole widths of currently known high-temperature dye systems. As can be seen from a comparison of Figure 6, parts a and b, a proper resolution of the zero phonon hole is achieved only in the case of hole depths well below 5%. Figure 6b shows a hole of ZnTpyP with 7% depth and a full width at half-maximum of 67 cm-1. The hole shape represents a convolution of ZPH and PSB and is dominated by saturation effects due to sideband absorption. As a consequence of the changed profile mentioned above, sideband absorption and saturation effects are severe factors influencing hole-burning at elevated temperatures. Relying on the majority of data known, one must assume that significant deviations from the 15-20-cm-l rangeare due to saturation effects. In Table 1 we have compiled material from the literature concerning properties of HTHB materials, especially parameters related to the coupling phonon mode and the burning efficiency. A similar compilation with data on electron-phonon coupling and the DebyeWaller factor will be published elsewhere.47As a measure for the burning efficiency, one can use the ratio of hole area (depth (9%) X width (cm-1)) and fluence. Table I reveals that the high-temperature burning efficiency in the AIP04-5 samples discussed here is small but comparable to other HTHB systems currently known. Besides the samples investigated in this paper, all other systems listed are associated with a photochemical burning mechanism. This demonstrates that
TABLE 1: Properties of Molecular HTHB Systems. cph r(80 K) depth irradiance time fluence efficiency system (cm-1) (cm-1) (mW/cm2) (s) (mJ/cm2) (% cm/J) source TCPP(Na)/PVA 23 15 110 0.6 8 Quilr-AlzO3 48 15 5 12 500 6000 13 17 TSPP(Na)/PVA 23 16 3 0.6 120 72 667 7 TPP/Phr 15 16 2.5 0.75 300 225 178 9 TPP/Er(HMDA) 17 16 3 75 60 4500 11 11 TMAP(PTS)/PVA 23 17 110 0.6 8 ZnTapP/hydrated AlPO4-5 22 this work 30 19 1.5 13 100 1300 TpyP-l(I)/PVA 23 19 110 0.6 8 16 this work 28 21 4.5 15 ZnTpyPfhydrated Alm4-5 400 6000 TSTP(H)/PVA 23 22 110 0.6 8 OEP/y-Al203 20 30 5 0.86 200 172 872 49 TZT/PMMA:CHCIa 80 $10 photon gating 10 * vpb: phonon mode, taken from the spectral distance of the ZPH and PSB maxima. r(80 K): full hole width at half-maximum burnt at 80 K. TCPP(Na): tetrasodium 5,10,15,2O-tetrakis(4-carbonatophenyl)porphine. TSPP(Na): tetrasodium 5,10,15,20-tetrakis(4-sulfonatophenyl)porphine. TPP tetraphenylporphine. TMAP(PTS): 5,10,15,20-tetrakis[4-(N~,N-trimethylamino)phenylporphinetetra(ptoluenwu1fonate). TPyP-l(1): 5,10,15,2O-tetrakis[4-(N-methylpyridinium)porphine] tetraiodide. TCPP(H): 5,10,15,20-tetrakis(4-carboxyphenyl)porphine.TSTP(H): 5,10,15,2Otetrakis(5-sulfothieny1)porphine. T Z T meso-tetra-p-tolyltctrabcnzoporphyrin-zinc.PVA: poly(viny1 alcohol). Phr: phenoxy resin. Er(HMDA): epoxy rwin, cross-linked with hexamethylenediaminc. PMMA poly(methy1 methacrylate).
52 The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 NPHB allows high-temperature burning efficiencies comparable to PHB materials. The main question remaining is which specific property of the hydrated samples of ZnTaP and ZnTpyP in AlP04-5 is the basis for the HTHB properties in this system. In all cases the process of hole-burning is due to the permanent selection of a site. This site is defined not only by its transition energy but also by a subensemble of barriers not likely to be crossed at the burning temperature. In NPHB systems the educt-product barriers are determined by the properties of the extrinsic TLSs and are characterized by a broad distribution. This results in a wide distribution of burning efficiencies at any temperature as well as in a burning efficiency which decreases with increasing temperature due to elimination of low barrier height TLSs. In contrast to this, PHB systems exhibit rather uniform burning efficiencies which are independent of temperature, e.g. for octaethylporphine on y-alumina a constant burning efficiency is found in the temperature range 2-90 K.19 Therefore, NPHB at elevated temperature is due to a small subensemble of chromophores coupling to extrinsic TLSs with high barriers, allowing the formation of stable product states. The data here demonstrate that water in the pores provides TLSs of this kind. There are various approaches to obtain materials of this kind in the polymer area. Using as host stiff polymers created by cross-linking the polymer chains or covalently binding the chromophore to the chain48 did succeed in good cycling stability but not in HTHB, probably because the DebyeWaller factor CYD in these materials is too low a t elevated temperature due to lowenergy phonon modes. So, a prerequisite for high temperature NPHB is the existence of a stiff environment which gives rise to high energy barrier TLSs and a high value of CYDsimultaneously.
Conclusions High-temperature NPHB was demonstrated for chromophores in the water-filled pores of an inorganic molecular sieve. Hole width and burning efficiency are comparable to values found for HTHB materials based on a photochemical mechanism. HTHB is attributed to the simultaneous existence of a high DebyeWaller factor and stable high energy barrier two-level systems. Both conditions are found to be fulfilled by the stiff framework of AlP04-5 and the strong hydrogen bonding of pore-filling water.
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