Photoionization of N-alkylphenothiazines in mesoporous metal silicoaluminophosphate molecular sieves

Article abstract of DOI:10.1021/jp001500h

The photoionization of N-alkylphenothiazines (PC_n where n = 1, 3, 6, 10, and 16) in mesoporous silicoaluminophosphate (UHM-3) and transition metal-substituted silicoaluminophosphate (MUHM-3 where M = Cu, Ni, Cr, and Mn) molecular sieves has been studied by electron spin resonance with ultraviolet irradiation at room temperature. Mesoporous UHM-3 and MUHM-3 molecular sieves were synthesized at room temperature. The effect of the ratio of Si/Al, the nature of the transition metal, and the concentration of the metal on the photoyield was examined. N-Alkylphenothiazine was incorporated into these mesoporous materials and the photoionization of PC_n was studied.

Full text of DOI:10.1021/jp001500h

J. Phys. Chem. B 2000, 104, 9661-9669
9661
Photoionization of N-Alkylphenothiazines in Mesoporous Metal Silicoaluminophosphate
Molecular Sieves
Jae Young Bae, Koodali T. Ranjit, Zhaohua Luan, R. M. Krishna, and Larry Kevan*
Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5641
ReceiVed: April 19, 2000; In Final Form: August 8, 2000
The photoionization of N-alkylphenothiazines (PCn where n ) 1, 3, 6, 10, and 16) in mesoporous
silicoaluminophosphate (UHM-3) and transition metal-substituted silicoaluminophosphate (MUHM-3 where
M ) Cu, Ni, Cr, and Mn) molecular sieves has been studied by electron spin resonance with ultraviolet
irradiation at room temperature. Mesoporous UHM-3 and MUHM-3 molecular sieves were synthesized at
room temperature. The effect of the ratio of Si/Al, the nature of the transition metal, and the concentration
of the metal on the photoyield was examined. N-Alkylphenothiazine was incorporated into these mesoporous
materials and the photoionization of PCn was studied.
Introduction
Photoionization of molecules in heterogeneous systems1-4
such as vesicles and micelles,5-17 colloidal solutions,18-20
zeolites (molecular sieves),21-27 and silica gels28-39 has been
used as model systems for light energy storage. The design of
efficient artifical photoredox systems1-4 for photochemical
energy conversion and storage must prevent rapid back electron
transfer40-42 to achieve long-lived photoinduced charge separa-
tion. Heterogeneous systems22-27 have been found to be better
hosts than homogeneous systems for such photoredox systems.
N-Alkylphenothiazines with longer alkyl chains are excellent
photoactive electron donors. They can be easily oxidized by
ultraviolet (UV) irradiation to form N-alkylphenothiazine cation
radicals, which have been characterized by electron spin
resonance (ESR) and optical spectroscopic techniques.43-50
Gra¨tzel et al. used flash photolysis to study the photooxidation
of phenothiazine and methylphenothiazine in micelles.15,51-53
Hovey used steady-state photolysis and ESR at room temper-
ature to study the air photooxidation of methylphenothiazine
in micelles.54 Photoactive electron donors such as N-alkyl-
phenothiazines can be incorporated inside heterogeneous hosts,
and such heterogeneous hosts can stabilize photoinduced radical
cations with relative ease.55,56
Figure 1. Structure of N-Alkylphenothiazines.
1). The photoionization of PCn in UHM-3 and MUHM-3
molecular sieves has been studied by ESR after UV irradiation
at room temperature. The photoyield was found to depend on
the Si/Al ratio, the nature of the transition metal ion, the M/Al
ratio, and the alkyl chain length of PCn.
Experimental Section
Researchers at Mobil Corporation synthesized silica-based
mesoporous M41S molecular sieve materials57-59 in 1992. Zhao
et al.60-63 reported the successful formation of mesoporous
hexagonal aluminophosphate (UHM-1) and silicoaluminophos-
phate (UHM-3) molecular sieves as heterogeneous hosts by a
modified ion-pair process. These molecular sieves show a
regular hexagonal array of uniform tubes with a diameter of
about 3.5 nm, a high surface area of about 850 m2/g, and a
pore size distribution nearly as sharp as that of zeolites.
In the present research, mesoporous hexagonal UHM-3 and
metal-substituted UHM-3 (MUHM-3 where M ) Cu, Ni, Cr,
and Mn) molecular sieves have been synthesized by using
optimum mixture and stirring conditions at room temperature
to improve the thermal stability and are used as heterogeneous
hosts for long-lived photoinduced electron transfer from N-
alkylphenothiazines PCn where n ) 1, 3, 6, 10, and 16 (Figure
Synthesis of UHM-3 and CuUHM-3. We have improved
the syntheses of hexagonal mesostructured silicoaluminophos-
phate (UHM-3) and hexagonal mesostructured copper-substi-
tuted silicoaluminophosphate (CuUHM-3) molecular sieves to
increase the thermal stability (calcined at 700 °C) by varying
the mixture composition and the stirring speed. Copper-
substituted silicoaluminophosphate (CuUHM-3) molecular sieves
possess reasonable thermal stability and high specific surface
area by modifying the synthetic conditions reported by Zhao et
al.61 for manganese-substituted aluminophosphate-based meso-
porous molecular sieve. It has been shown that incorporation
of transition metal ions into microporous aluminophosphate
successfully improves its Bronsted acidity and increases the
potential of these materials as catalysts.64-67
Mesoporous hexagonal silicoaluminophosphate (UHM-3)
molecular sieves were synthesized according to Zhao et al.60-63
by using cetyltrimethylammonium chloride (CTACl, Aldrich,
* Author to whom correspondence should be addressed.
10.1021/jp001500h CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/26/2000

9662 J. Phys. Chem. B, Vol. 104, No. 41, 2000
Bae et al.
25% in water) as a structure-directing agent and aluminum
hydroxide (53.5 wt % Al2O3, USP Pfaltz & Bauer Inc.),
phosphoric acid (85 wt %, EM Industries), and tetraethyl
orthosilicate (TEOS, Aldrich) as aluminum, phosphorus, and
silicon sources, respectively, at room temperature. Optimization
for the best synthetic conditions gives P/Al of 1.00, CTACl/Al
of 0.8, slow stirring (300 rpm) and pH 9.0.
work and framework positions. To incorporate metal ions into
extraframework positions, ion exchange of protons in the
calcined H-UHM-3 materials was done by liquid-state ion
exchange. Typical liquid-state ion exchange was performed by
adding 2 mL of 1 × 10-3 M CuCl2‚2H2O, NiCl2‚6H2O, MnCl2‚
4H2O, or CrCl3‚6H2O, and 18 mL of water to 0.2 g of H-UHM-3
and the mixture was stirred overnight at room temperature. The
samples were then filtered, washed with warm distilled water
to remove any excess metal ions on the surface of the sample,
and then dried in air to form MH-UHM-3.
Incorporation of the N-alkylphenothiazines into UHM-3 was
performed by impregnation. For impregnation, 0.1 g of UHM-3
was put into 1 mL of 1 × 10-2 M N-alkylphenothiazine in
benzene for overnight in the dark. The benzene was removed
by flowing nitrogen gas over the sample for 30 min. For ESR
measurements the same amount of the sample was transferred
into Suprasil quartz tubes (2 mm i.d. × 3 mm o.d.) which were
sealed at one end.
Powder X-ray diffraction (XRD) patterns of all UHM-3 and
MUHM-3 samples were obtained on a Philips 1840 powder
diffractometer with Cu KR radiation (40 kV, 25 mA) at 0.01°
step width and 1 s step time over the range 1.5° < 2θ < 10°.
The samples were mounted on aluminum slides in order to avoid
preferential ordering. Chemical analysis was performed by
electron microprobe analysis on a JEOL JXA-8600 spectrom-
eter. The composition of all UHM-3 and MUHM-3 materials
was determined by calibration with known standards and by
averaging over several defocused areas to give the bulk
composition. Thermogravimetric analysis (TGA) of all UHM-3
and MUHM-3 materials was performed on a TGA 2050 analyzer
from TA Instruments with a heating rate of 10 °C min-1 in
oxygen. N2 adsorption isotherms were measured at 77 K using
a Micromeritics Gemini 2375 analyzer. The volume of adsorbed
N2 of all UHM-3 and MUHM-3 materials was normalized to
standard temperature and pressure. Prior to experiments, samples
were dehydrated at 250 °C for 3 h. The BET specific surface
area was determined from the linear part of the BET equation
(P/Po ) 0.05-0.31). The calculation of the pore size distribution
was performed using the desorption branches of the N2
adsorption isotherms and the Barrett-Joyner-Halenda (BJH)
formula.69 Although the BJH analysis underestimates the pore
size,70-73 relative changes in the pore size should be accurately
portrayed. The cumulative mesopore (1.7-10.0 nm) surface
area, ABJH, was obtained from the pore size distribution curves.
The ESR spectra were recorded at room temperature at 9.5 GHz
using a Bruker ESP 300 spectrometer with 100 kHz field
Copper-containing mesoporous hexagonal silicoalumino-
phosphate (CuUHM-3) molecular sieves were synthesized by
using a similar procedure with 0.1 M CuCl2 (Aldrich) added
into the silicoaluminophosphate gel with stirring before the
addition of the CTACl surfactant solution. The composition of
the resultant gel was Al2O3:P2O5:(0.4-2.4)SiO2:(1 × 10-3
-
1)CuCl2:0.8CTACl:(8-9)TMAOH:(455-550)H2O. The organic
structure-directing agent in the pores of these gels was removed
by calcination at 700 °C for 2 h in flowing nitrogen, followed
by 12 h in flowing oxygen.
Synthesis of the N-Alkylphenothiazines (PCn where n )
1, 3, 6, 10, and 16). Commercial methylphenothiazine (PC1,
Aldrich) was used as received. Four N-alkylphenothiazines (PCn
where n ) 3, 6, 10, and 16) were synthesized.68 During the
purification of PCn we used ethyl acetate instead of methyl-
isobutyl ketone as used earlier. The synthetic details for
propylphenothiazine (PC3), N-hexylphenothiazine (PC6), decyl-
phenothiazine (PC10), and hexadecylphenothiazine (PC16) are
as follows.
Ten grams (for PC3), 12 g (for PC6), or 15 g (for PC10 or
PC16) of phenothiazine (Aldrich, 98%) was dissolved in 250
mL (for PC3), 300 mL (for PC6), or 350 mL (for PC10 or PC16)
of anhydrous tetrahydrofuran, and 2.5 g (for PC3), 3.0 g (for
PC6), or 3.5 g (for PC10 or PC16) of sodium hydride (Aldrich)
was added to the solution slowly while the solution was stirred,
respectively. The reaction system was kept under nitrogen. After
the solution was stirred for 4 h (for PC3), 8 h (for PC6), or 10
h (for PC10 or PC16) at room temperature, 10 g (for PC3) of
1-bromopropane (Aldrich, 99+%), 14 g (for PC6) of 1-bromo-
hexane (Aldrich, 98%), 18 g (for PC10 or PC16) of 1-bromo-
decane (Aldrich, 98%) or 1-bromohexadecane (Aldrich, 97%)
was added, and the solution was refluxed at room temperature
for 48 h, respectively. The progress of the reaction was
monitored with thin-layer chromatography. When all of the
phenothiazine was consumed, the reaction was quenched by
pouring the solution into a separatory funnel, followed by the
addition of 100 mL of water and 100 mL of ethyl acetate. The
organic layer was separated and the aqueous layer was washed
with ethyl acetate. The combined organic extracts were washed
with water, and the remaining solvent was concentrated by
evaporation under reduced pressure. The resulting crude prod-
ucts were purified by liquid chromatography (Aldrich silica gel,
60-200 mesh, n-hexane eluant). The compounds were identified
by NMR spectroscopy. The 1H NMR of PC3 in CDCl3 showed
absorption peaks at 1.05 (CH3), 1.92 (CH2CN), 3.94 (CH2N),
and 6.94-7.45 ppm (phenothiazine ring protons). The 1H NMR
of PC6 in CDCl3 showed absorption peaks at 1.03 (CH3), 1.10-
1.60 (C(CH2)3C), 1.90-1.93 (CH2CN), 3.91 (CH2N), and 6.94-
modulation and low microwave power to avoid power saturation.
+
Photoproduced N-alkylphenothiazine cation radicals (PCn
)
yields were determined by double integration of the ESR spectra
using the ESP 300 software. Each photoyield is an average of
three determinations.
The PCn/UHM-3 materials were irradiated using a 300 W
Cermax Xenon lamp (ILC-LX 300 UV) at room temperature.
The incoming light was passed through a 10 cm water filter to
prevent infrared radiation and through a Corning No. 7-54 filter
to filter light of wavelengths shorter than 320 nm. The samples
were placed in a quartz Dewar and rotated at a speed of 4 rpm
to ensure even irradiation.
1
7.56 ppm (phenothiazine ring protons). The H NMR of PC10
in CDCl3 showed absorption peaks at 1.03 (CH3), 1.38-1.42
(C(CH2)7C), 1.95 (CH2CN), 3.55 (CH2N), and 6.97-7.29 ppm
1
(phenothiazine ring protons). The H NMR of PC16 in CDCl3
Results
showed absorption peaks at 0.91 (CH3), 1.12-1.64 (C(CH2)13C),
1.86 (CH2CN), 3.41 (CH2N), and 6.85-7.25 ppm (phenothiazine
ring protons).
Powder XRD patterns of as-synthesized and calcined UHM-3
and CuUHM-3 are shown in Figure 2. Well-defined XRD
patterns show a prominent peak at 2.3° 2θ and some broad but
clearly present peaks in the 2θ range of 3.0°-6.0°. As-
Preparation of Samples and Measurements. Metal ions
were incorporated into the UHM-3 materials in both extraframe-

Photoionization of N-Alkylphenothiazines
J. Phys. Chem. B, Vol. 104, No. 41, 2000 9663
Figure 2. Powder XRD patterns of (a) as-synthesized UHM-3 (Si/Al
) 0.4), (b) calcined UHM-3 (Si/Al ) 0.4), (c) as-synthesized CuUHM-3
(Si/Al ) 0.4 and Cu/Al ) 0.1), and (d) calcined CuUHM-3 (Si/Al )
0.4 and Cu/Al ) 0.1).
synthesized UHM-3 and CuUHM-3 show a d(100) spacing of
3.9 nm and a unit cell parameter (ao ) 2d(100)/31/2 74-82
of 4.5
)
nm. The integrity of the mesoporous structures of UHM-3 and
CuUHM-3 is retained even after complete decomposition of the
organic structure-directing agent by calcination at 700 °C in
flowing nitrogen for 2 h, followed by 12 h in flowing oxygen.
All XRD patterns of calcined UHM-3 and CuUHM-3 are similar
and exhibit a single broad diffraction peak corresponding to a
d(100) spacing of 2.6 nm and a unit cell parameter of 3.0 nm.
The broadening of the d(100) reflection on calcination arises
from less long-range crystallographic order. The d(100) peaks
shift from 3.9 to 2.6 nm for calcined UHM-3 and CuUHM-3.
The pore structure of these materials is believed to be a three-
dimensional, disordered network of short, wormlike, tubular
channels, while the channel widths are uniform.
TGA of UHM-3 shows a large weight loss of 70% on heating
to 700 °C. Three endothermic losses near 70 °C (15% weight
loss), 230 °C (25% weight loss), and 300 °C (30% weight loss)
are observed. The 70 °C weight loss is assigned to water
desorption. The 230 °C and 300 °C weight losses are assigned
to desorption and decomposition of TMAOH and CTACl in
either order. CuUHM-3 shows a similar TGA curve as UHM-
3.
Figure 3. Adsorption-desorption isotherms of nitrogen at 77 K for
(a) calcined UHM-3 (Si/Al ) 0.4) and (b) calcined CuUHM-3 (Si/Al
) 0.4 and Cu/Al ) 0.1). Corrresponding pore size distribution curves
for (c) calcined UHM-3 (Si/Al ) 0.4) and (d) calcined CuUHM-3 (Si/
Al ) 0.4 and Cu/Al ) 0.1).
approximate pore size calculation by nitrogen physisorption is
smaller than the repeat distance ao ) 3.0 nm determined by
XRD (ao ) 2d(100)/31/2, where d(100) ) 2.6 nm), because the
latter includes the thickness of the pore wall. Thus, we can
estimate that the pore walls of these UHM-3 and CuUHM-3
materials are about 1.3 nm thick.
Figure 3 shows typical N2 adsorption isotherms from calcined
UHM-3 and CuUHM-3 samples. Both give typical irreversible
type I behavior as defined by IUPAC,83 indicating the presence
of micropores. Calcined UHM-3 had a BET surface area of 829
m2 g-1 and an average pore diameter of 1.7 nm. Calcined
CuUHM-3 had a BET surface area of 1248 m2 g-1 and an
average pore diameter of 1.7 nm, which suggests that incorpora-
tion of copper can improve the thermal stability. These results
indicate the presence of tubular microporous and mesoporous
mixed-channels in UHM-3 and CuUHM-3 materials. The
For calcined M-UHM-3 samples (M ) Cu, Ni, Cr) ESR
results show that Cu and Ni are in the +2 oxidation state and
that Cr is in the +3 oxidation state.
The photoionization of PCn in the synthesized mesoporous
molecular sieves was examined. The effects of the Si/Al ratio,
the nature of the transition metal, the metal concentration, and
the alkyl chain length on the photoyield were examined. The
photoyields of samples evacuated to remove traces of water or
solvent were similar to those of unevacuated samples so the
possible influence of such adsorbed impurities seems negligible.

9664 J. Phys. Chem. B, Vol. 104, No. 41, 2000
Bae et al.
Figure 5. Increase of ESR intensities at room temperature of PC1/
UHM-3 (Si/Al ) n) (where n ) 0.4, 0.8, 1.2, and 2.4) versus
photoirradiation time.
Figure 4. ESR spectra of PC1/UHM-3 (Si/Al ) 0.4) versus photoir-
radiation time.
The samples with methylphenothiazine introduced into UHM-3
(Si/Al ) 0.4) show only weak ESR signals before irradiation
(Figure 4). This indicates that some methylphenothiazine cation
radicals are generated during the sample preparation process.
After being irradiated by 320 nm light at room temperature for
3 min, the samples show strong ESR signals. With further
increase in irradiation time to 20 min, the intensity of the ESR
signal increases further and then almost reaches a plateau after
60 min. This shows that a substantial number of stable radicals
are produced by the irradiation. These ESR spectra are broad,
partially resolved, and asymmetric. The spectral widths of Figure
4, which show the ESR spectra of samples containing PC1, are
the same as those of the methylphenothiazine cation radicals in
homogeneous solution and in micelles at room temperature.45,54
The ESR spectra observed before irradiation have the same line
shape as those observed after irradiation although they are much
weaker. PC1-impregnated commercial silica gel samples, PC1/
IP2.5, PC1/IP4.0, and PC1/IP1.7,35 show the same ESR signal
as in this sample after 20 min room-temperature irradiation. The
impregnated samples are colorless before irradiation and turn
pink after irradiation.
Figure 6. Increase of ESR intensities at room temperature of PC1/
MH-UHM-3 (Si/Al ) 0.4) (where M ) Cu, Ni, Cr, and Mn) at room
temperature versus photoirradiation time.
Al ) n) with n ) 0.4, 0.8, 1.2, and 2.4, the ESR intensity was
essentially the same for several hours. Hence, for all these
samples, the intensity of the ESR signal was monitored every
24 h. The half-life times (t1/2) for PC1/UHM-3 (Si/Al ) n) with
n ) 0.4, 0.8, 1.2, and 2.4 were estimated to be t1/2 ) 3 days for
Si/Al ) 0.4, t1/2 ) 5 days for Si/Al ) 0.8, t1/2 ) 52 days for
Si/Al ) 1.2, and t1/2 ) 93 days for Si/Al ) 2.4. The stability of
+
the photoproduced PC1 cation radicals was evaluated, and it
Figure 5 shows the increase in the ESR signal intensities of
PC1-containing samples PC1/UHM-3 (Si/Al ) n) with n ) 0.4,
0.8, 1.2, and 2.4 with irradiation time. From Figure 5 it is seen
that the highest photoyield is obtained for PC1/UHM-3 (Si/Al
) 0.4). The ESR signals increase rapidly during the first 20
min and then almost reach a plateau. An irradiation time of 60
min was selected for comparative photoyield and decay studies.
was found that PC1/UHM-3 (Si/Al ) 2.4) exhibited the highest
+
stability toward PC1 cation radicals.
Figure 6 shows the increase in the ESR signal intensities of
PC1-containing samples PC1/MH-UHM-3 (Si/Al ) 0.4) with
M ) Cu, Ni, Cr, and Mn with irradiation time. From Figure 6
it is seen that the highest photoyield is obtained for PC1/CuH-
UHM-3 (Si/Al ) 0.4). The calculated rate constants for the
+
+
The rate of formation of the PC1 cation radicals can be
evaluated from the initial slope in Figure 5 assuming first-order
formation of PC1 cation radicals are k ) 290 × 10-4 s-1 for
PC1/CuH-UHM-3 (Si/Al ) 0.4), k ) 218 × 10-4 s-1 for PC1/
NiH-UHM-3 (Si/Al ) 0.4), k ) 148 × 10-4 s-1 for PC1/CrH-
UHM-3 (Si/Al ) 0.4), and k ) 57 × 10-4 s-1 for PC1/MnH-
UHM-3 (Si/Al ) 0.4). The photoyield of the photoproduced
PC1+ cation radicals was evaluated and it was found that PC1/
CuH-UHM-3 (Si/Al ) 0.4) exhibited the highest photoyield
+
kinetics. The calculated rate constants for the formation of PC1
cation radicals are k ) 380 × 10-4 s-1 for PC1/UHM-3 (Si/Al
) 0.4), k ) 87 × 10-4 s-1 for PC1/UHM-3 (Si/Al ) 0.8), and
k ) 27 × 10-4 s-1 for PC1/UHM-3 (Si/Al ) 1.2 and 2.4). The
+
photoyield of the photoproduced PC1 cation radicals was
+
evaluated and it was found that PC1/UHM-3 (Si/Al ) 0.4)
toward PC1 cation radicals. In the case of decay of PC1/MH-
+
exhibited the highest photoyield toward PC1 cation radicals.
UHM-3 (Si/Al ) 0.4) with M ) Cu, Ni, Cr, and Mn the ESR
intensity was essentially the same for several hours. The half-
life times (t1/2) for PC1/MH-UHM-3 (Si/Al ) 0.4) with M )
Cu, Ni, Cr, and Mn were estimated to be t1/2 ) 22 days for M
+
The decay of the photoproduced PC1 cation radicals as
monitored by ESR is also an important factor in the design of
efficient photoredox systems. In the case of PC1/UHM-3 (Si/

Photoionization of N-Alkylphenothiazines
J. Phys. Chem. B, Vol. 104, No. 41, 2000 9665
Figure 8. Increase of ESR intensities at room temperature of PC1/
CuUHM-3 (Si/Al ) 0.4 and Cu/Al ) n) (where n ) 1 × 10-3, 1 ×
10-2, 1 × 10-1, and 1) at room temperature versus photoirradiation
time.
Figure 7. ESR spectra at room temperature of (a) as-synthesized
CuUHM-3 (Si/Al ) 0.4 and Cu/Al ) 0.1), (b) calcined CuUHM-3
(Si/Al ) 0.4 and Cu/Al ) 0.1) in O2 at 700 °C for 12 h, and (c) PC1/
CuUHM-3 (Si/Al ) 0.4 and Cu/Al ) 0.1).
) Cu, t1/2 ) 4 days for M ) Ni, t1/2 ) 3 days for M ) Cr, and
t1/2 ) 2 days for M ) Mn. The stability of the photoproduced
PC1+ cation radicals was evaluated and it was found that PC1/
CuH-UHM-3 (Si/Al ) 0.4) exhibited the highest stability toward
+
PC1 cation radicals.
Figure 7 shows ESR spectra of (a) as-synthesized CuUHM-3
(Si/Al ) 0.4 and Cu/Al ) 0.1), (b) calcined CuUHM-3 in O2
at 700 °C for 12 h, and (c) PC1-containing samples PC1/
CuUHM-3. The powder ESR spectrum of as-synthesized
Figure 9. ESR spectra at room temperature of PC10/UHM-3 (Si/Al )
2.4) versus photoirradiation time.
CuUHM-3 can be described by an axial g tensor with g )
constants for the formation of PC1+ cation radicals are k ) 123
× 10-4 s-1 for PC1/CuUHM-3 (Si/Al ) 0.4 and Cu/Al ) 1 ×
10-3), k ) 27 × 10-4 s-1 for PC1/CuUHM-3 (Si/Al ) 0.4 and
Cu/Al ) 1 × 10-2), and k ) 3 × 10-4 s-1 for PC1/CuUHM-3
(Si/Al ) 0.4 and Cu/Al ) 1 × 10-1 or 1). The photoyield of
||
2.3554, g 1 ) 2.0716, g 2 ) 2.0504, g 3 ) 2.0362, and an axial
metal ion hyperfine coupling with A ) 147 × 10-4 cm-1. The
||
Cu(II) hyperfine coupling in the perpendicular part of the powder
spectrum was not resolved. In copper-substituted CuUHM-3 the
g values are assigned to the three different environments, one
inside, another outside and another one on the hexagonal tubular
walls, respectively. The powder ESR spectrum of calcined
+
the photoproduced PC1 cation radicals was evaluated and it
was found that PC1/CuUHM-3 (Si/Al ) 0.4 and Cu/Al ) 1 ×
+
10-3) exhibited the highest photoyield toward PC1 cation
CuUHM-3 shows a single Cu(II) species described by g )
radicals. In the case of decay of PC1/CuUHM-3 (Si/Al ) 0.4
and Cu/Al ) n) with n ) 1 × 10-3, 1 × 10-2, 1 × 10-1, and
1, the ESR intensity was essentially the same for several days.
The half-life times (t1/2) for PC1/CuUHM-3 (Si/Al ) 0.4 and
Cu/Al ) n) with n ) 1 × 10-3, 1 × 10-2, 1 × 10-1, and 1
were estimated to be t1/2 ) 23 days for Cu/Al ) 1 × 10-3, t1/2
) 20 days for Cu/Al ) 1 × 10-2, t1/2 ) 14 days for Cu/Al )
1 × 10-1, and 1. The stability of the photoproduced PC1+ cation
radicals was evaluated and it was found that PC1/CuUHM-3
(Si/Al ) 0.4 and Cu/Al ) 1 × 10-3) exhibited the highest
||
2.3554, g ) 2.0716, and A ) 147 × 10-4 cm-1. The
||
observation of only g ) 2.0716 (g value of as-synthesized
1
CuUHM-3) indicates that after calcination metal ions are only
present on the hexagonal tubular walls. The spin Hamiltonian
parameters for PC1/CuUHM-3 are the same as for calcined
+
CuUHM-3 with g ) 2.0055 for PC1
.
Figure 8 shows the increase in the ESR signal intensities of
PC1-containing samples PC1/CuUHM-3 (Si/Al ) 0.4 and Cu/
Al ) n) with n ) 1 × 10-3, 1 × 10-2, 1 × 10-1, and 1 with
irradiation time. The photoyield is also dependent on the amount
of metal content in the UHM-3 materials. From Figure 8 it is
seen that the highest photoyield is obtained for PC1/CuUHM-3
(Si/Al ) 0.4 and Cu/Al ) 1 × 10-3). The calculated rate
+
stability toward PC1 cation radicals.
The sample PC10/UHM-3 (Si/Al ) 2.4) shows only weak ESR
signals before irradiation (Figure 9). This indicates that some
decylphenothiazine cation radicals are generated during the

9666 J. Phys. Chem. B, Vol. 104, No. 41, 2000
Bae et al.
Figure 12. Room-temperature photoyields based on ESR intensities
of PCn/UHM3 (Si/Al ) 2.4) (where n ) 1, 3, 6, 10, and 16) measured
after 60 min photoirradiation at room temperature versus the alkyl chain
length of the N-alkylphenothiazines.
for PC1/UHM-3 (Si/Al ) 2.4). The photoyield of the photo-
produced PCn+ cation radicals was evaluated and it was found
that PC10/UHM-3 (Si/Al ) 2.4) exhibited the highest photoyield
+
toward PC10 cation radicals. In the case of decay of PCn/
UHM-3 (Si/Al ) 2.4) with n ) 1, 3, 6, 10, and 16 the ESR
intensity was essentially the same for several days. The half-
life times (t1/2) for PCn/UHM-3 (Si/Al ) 2.4) with n ) 1, 3, 6,
10, and 16 were estimated to be t1/2 ) 150 days for PC10 and
PC6, t1/2 ) 114 days for PC3, t1/2 ) 93 days for PC1, and t1/2
)
32 days for PC16. The stability of the photoproduced PCn+ cation
radicals was evaluated and it was found that PC10/UHM-3 (Si/
Al ) 2.4) and PC6/UHM-3 (Si/Al ) 2.4) exhibited the highest
Figure 10. ESR spectra at room temperature of PCn/UHM-3 (Si/Al )
2.4) (where n ) 1, 3, 6, 10, and 16) samples after 60 min room
temperature irradiation at 320 nm.
+
+
stability toward PC10 and PC6 cation radicals. The net
photoionization yields of PCn/UHM-3 (Si/Al ) 2.4) are plotted
as a function of the pendent alkyl chain length in Figure 12.
An irradiation time of 60 min was selected for comparative
yields.
Discussion
Improved thermal stability of mesoporous UHM-3 and
CuUHM-3 molecular sieves was obtained by the modified
synthesis to survive calcination at 700 °C with an average pore
diameter of 1.7 nm and a 1.3 nm thick pore wall (Figures 2
and 3). The photoionization of N-alkylphenothiazines with alkyl
chain lengths of 0.6-2.0 nm (Figure 1) was studied for these
materials.
A little dark reaction occurs during sample preparation. This
is not completely understood although it is not uncommon in
oxide materials. Defect sites in UHM-3 or residual light
exposure may produce some oxidation of PC1. However, the
photooxidation of PC1 is much greater than any thermal
oxidation.
Figure 11. Increase of ESR intensities at room temperature of PCn/
UHM-3 (Si/Al ) 2.4) (where n ) 1, 3, 6, 10, and 16) versus
photoirradiation time.
sample preparation process. The samples with decylphenothi-
azine introduced into PCn/UHM-3 (Si/Al ) 2.4) (where n ) 1,
3, 6, 10, and 16) show strong ESR signals after 60 min room-
temperature irradiation at 320 nm (Figure 10).
To understand the influence of the Si/Al ratio on the
photoyield, several mesoporous UHM-3 molecular sieves were
+
prepared. The strong ESR signals of PC1 cation radicals in
Figure 11 shows the increase in the ESR signal intensities of
PCn-containing samples PCn/UHM-3 (Si/Al ) 2.4) with n ) 1,
3, 6, 10, and 16 with irradiation time. From Figure 11 it is seen
that the highest photoyield is obtained for PC10/UHM-3 (Si/Al
PC1/UHM-3 (Si/Al ) 0.4) after irradiation by 320 nm at room
temperature in Figure 4 show a sextet with g ) 2.0055. The
hyperfine structure depends on the interaction of the unpaired
+
electron with nitrogen and protons. The ESR spectrum of PC1
+
) 2.4). The calculated rate constants for the formation of PCn
cation radicals exhibits a sextet with relative intensities of 1:4:
7:7:4:1 showing interaction with nitrogen and three protons of
a methyl group.84-87
cation radicals are k ) 203 × 10-4 s- 1 for PC10/UHM-3 (Si/
Al ) 2.4), k ) 170 × 10-4 s-1 for PC6/UHM-3 (Si/Al ) 2.4),
k ) 143 × 10-4 s-1 for PC16/UHM-3 (Si/Al ) 2.4), k ) 77 ×
10-4 s-1 for PC3/UHM-3 (Si/Al ) 2.4), and k ) 27 × 10-4 s-1
+
The ESR spectra of PC1 cation radicals photoproduced at
room temperature in PC1/UHM-3 with different Si/Al ratios have

Photoionization of N-Alkylphenothiazines
J. Phys. Chem. B, Vol. 104, No. 41, 2000 9667
the same line shapes as that of chemically produced PC1+ cation
To study the influence of the metal concentration on the
photoyield, several mesoporous CuUHM-3 molecular sieves
with different Cu(II) concentrations were prepared. From Figure
8, one can conclude that the photoionization efficiency depends
on the metal concentration in ion-exchange sites. It is observed
that the photoyield increases as the Cu concentration in the
mesoporous UHM-3 molecular sieves decreases. The optimum
concentration of Cu(II) in the present study occurs for Cu/Al
radicals in UHM-3 with Si/Al ) 0.4. But the intensities of the
+
photoproduced PC1 cation radicals vary with different Si/Al
+
ratios (Figure 5). The ESR signal intensities of PC1 cation
radicals of Figure 5 clearly confirm the photooxidation of
+
methylphenothiazine molecules into PC1 cation radicals in
UHM-3 with different Si/Al ratios by 320 nm irradiation at room
temperature. The UHM-3 framework is the likely electron
acceptor. Surface hydroxyl groups may serve as electron
acceptors at low Si/Al ratio with production of a hydrogen atom.
However, this seems relatively inefficient since the photoion-
ization yields are small for high Si/Al ratio. Mesoporous UHM-3
molecular sieves of low Si/Al ratio possess more surface
hydroxyl groups than UHM-3 molecular sieves of high Si/Al
ratio because the surface hydroxyl groups are bonded to
aluminums. The photoyield increases as the Si/Al ratio de-
creases.
) 1 × 10-3
.
To understand the influence of the alkyl chain length on the
photoyield, several mesoporous PCn/UHM-3 molecular sieves
with different n values were prepared. The photoproduced ESR
spectra (Figures 9 and 10) for samples containing PCn (n > 3)
+
are similar to the ESR spectrum of PC3 cation radicals in
UHM-3 (Si/Al ) 2.4). The photoproduced ESR signal in
samples containing PCn is reasonably assigned to photoproduced
PCn+ cation radicals. The relative mobilities of the PCn+ cation
radicals are suggested by the breadth and resolution of the ESR
spectra in Figures 9 and 10. The broad ESR spectra (PCn with
n > 1) indicate that the radicals are largely immobilized at room
temperature. The partially resolved ESR spectra with the relative
To understand the influence of metal type, various metal ion-
exchanged mesoporous MH-UHM-3 molecular sieves were
prepared with a Si/Al ratio of 0.4. The variable valence metal
ions can act as electron acceptors. Experiments with transition
metal ions (Cu, Ni, Cr, and Mn) in mesoporous UHM-3 show
strong ESR signals at room temperature. Figure 6 shows the
increase in the intensities of the ESR signals due to PC1+ cation
radicals with irradiation time for MH-UHM-3, with transition
metal ions in ion-exchange sites. As one can observe from
Figure 6, the presence of Cu in mesoporous UHM-3 enhances
the photoyield more compared to Ni, Cr, and Mn. The
photoionization efficiency of transition metal ions in UHM-3
decreases in the order, CuH-UHM-3 > NiH-UHM-3 > CrH-
UHM-3 > MnH-UHM-3. Thus the photoionization efficiency
can be controlled by the nature of the metal ion in ion-exchange
sites.
+
intensities of 1:4:7:7:4:1 (PCn with n ) 1) for PC1 cation
radical show that PC1+ has some mobility in these materials at
room temperature. The results suggest a greater mobility for
+
PCn with smaller alkyl chain length which correlates with
greater probability for decay by back electron transfer from
+
UHM-3 or by some other process. The lifetime of the PCn
cation radicals in UHM-3 is a few months at room temperature,
depending on the alkyl chain length.
Figures 11 and 12 show a remarkable change in the
photoyield of alkylphenothiazine cation radicals with the alkyl
chain length. The alkylphenothiazine cation radical photoyield
and stability increase from methyl to decyl, and as the alkyl
chain length further increases from decyl to hexadecyl the
photoyield decreases. A possible explanation for this trend is
as follows. When the PCn alkyl chain length is short, some of
the PCn molecules react with atmospheric oxygen during sample
preparation, oxidize, and turn light pink. Increasing the alkyl
chain length of the PCn molecules reduces the extent of this
dark reaction with atmospheric oxygen. If the PCn molecules
are distributed uniformly on the surface of UHM-3 independent
of alkyl chain length, then the photooxidation yields are expected
to increase monotonically. Thus the decreased photooxidation
yields for PC10/UHM-3 to PC16/UHM-3 may suggest that PC10/
UHM-3 to PC16/UHM-3 are bound nonuniformly on mesopo-
rous UHM-3 molecular sieves which makes oxidation more
diffcult.37,90 In other work, N-alkylphenothiazines with longer
alkyl chain length are suggested to form clusters and change
their reactivity toward oxidizing agents.91
The higher photoyield and stability of the methylphenothi-
azine cation radicals can be accounted for on the basis of higher
electron affinity of Cu2+ (E° Cu2+/Cu1+ ) 0.153) as compared
to Cr3+ (E° Cr3+/Cr2+ ) -0.407) and Mn2+ (E° Mn2+/Mn1+
) -3.0) and of Cu2+ (E° Cu2+/Cuo ) 0.342) as compared to
Ni2+ (E° Ni2+/Nio ) -0.257) and Cr3+ (E° Cr3+/Cr1+
)
-0.744). Incorporation of a metal ion electron acceptor into
the framework or into ion-exchange sites creates a static system
in which light energy can be converted into chemical energy
and stored for extended periods of time until diffusion of the
photoproducts releases it.
Powder ESR spectra (Figure 7) of as-synthesized and calcined
CuUHM-3 show Cu(II) species with A ) 147 × 10-4 cm-1
.
||
||
Based on a number of single-crystal ESR studies the A
hyperfine component of Cu(II) gives a guide to its overall
coordination symmetry. A increases from 70 × 10-4 cm-1 for
||
The alkyl chain length of PCn in mesoporous UHM-3
molecular sieves is a critical parameter affecting the yield of
photoproducts. This is related to the control of the spatial
separation between the electron donor and the acceptor by the
alkyl chain length of PCn. TGA suggests that mesoporous
UHM-3 molecular sieves can accommodate molecules such as
N-alkylphenothiazines inside their channels. The mobility of the
molecule depends on its alkyl chain length. Figure 13 shows
TGA results obtained from the incorporation of N-alkylphe-
nothiazines into UHM-3 samples. All the three curves typically
show three weight losses, the first near 50-100 oC is attributed
to water desorption, the second near 200 °C (for PC16), 220 °C
(for PC6) or 250 °C (for PC10) is attributed to N-alkylphenothi-
azine desorption, and the third broad peak centered around 470
oC is assigned to the decomposition of N-alkylphenothiazines
tetrahedral coordination, through square pyramidal, octahedral,
trigonal bipyramidal to 170 × 10-4 cm-1 for square planar
coordination.88 For methanol and acetone adsorbate A ) 130
||
× 10-4 cm-1 seems indicative of distorted octahedral coordina-
tion as has also been suggested for water adsorbate.89 Thus we
suggest that Cu(II) is coordinated to four lattice oxgens and
two oxygens of adsorbate molecules in a distorted octahedral
environment.
The spin Hamiltonian parameters of PC1/CuUHM-3 (Figure
7) are g ) 2.3554, g ) 2.0716, and A ) 147 × 10-4 cm-1
||
||
from Cu(II) and g ) 2.0055 for PC1+. The copper ions show
that incorporation of the transition metal ion into the framework
can change the fraction of the light energy that can be converted
into chemical energy.

9668 J. Phys. Chem. B, Vol. 104, No. 41, 2000
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