Photoinduced charge separation at room temperature of N-alkylpyrenes in transition metal ion containing mesoporous SiMCM-41 and MeMCM-41 silica molecular sieves where Me = Ti(IV), V(V), Ni(II), Co(II) and Cu(II)

Article abstract of DOI:10.1039/b107371k

The photoinduced charge separation of N-alkylpyrenes in transition metal ion containing mesoporous SiMCM-41 and MeMCM-41 materials [Me = Ti(IV), V(V), Ni(II), Co(II) and Cu(II)] with ultraviolet irradiation at room temperature is investigated. N-alkylpyrene cation radicals (PyC_n⁺) are generated in SiMCM-41 and MeMCM-41 mesoporous silica materials and are characterized by electron spin resonance (ESR) spectroscopy. Mesoporous SiMCM-41 and MeMCM-41 materials are shown to be effective heterogeneous hosts for the photoinduced formation and stabilization of PyC_n⁺ cation radicals at room temperature indicating stable photoinduced charge separation for several hours. The long lifetimes for the cation radicals indicate the utility of the environment of these MCM-41 materials for controlling back electron transfer. A series of TiMCM-41 materials are synthesized that increase the PyC_n⁺ photoionization efficiency in comparison with VMCM-41, NiMCM-41, CoMCM-41 and CuMCM-41. The photoyield efficiency is decreased significantly when the concentration of pyrene is increased above about 10^-2 M. The photoionization efficiency depends on the type and concentration of the metal ions in the mesoporous MeMCM-41 materials which suggests that certain metal ions can act as electron acceptors. Also, as the N-alkylpyrene alkyl chain length increases from methyl to hexadecyl the photoionization yield decreases. The photoionization efficiency increases in the order TiMCM-41 > NiMCM-41 > VMCM-41 > SiMCM-41 > CoMCM-41 > CuMCM-41. The photoyield is higher by about 1.5 times at 77 K compared to room temperature. The observed photoyields of PyC_n (n = 1, 4, 8, 12, or 16) in mesoporous MeMCM-41 materials suggest that these materials are useful models for long-term photoinduced charge separation at room temperature.

Full text of DOI:10.1039/b107371k

View Article Online / Journal Homepage / Table of Contents for this issue
Photoinduced charge separation at room temperature of
N-alkylpyrenes in transition metal ion containing mesoporous
SiMCM-41 and MeMCM-41 silica molecular sieves where
Me ¼ Ti(IV), V(V), Ni(II), Co(II) and Cu(II)
R. M. Krishna and L. Kevan*
Department of Chemistry, University of Houston, Houston, TX 77204-5003, USA
Received 14th August 2001, Accepted 1st October 2001
First published as an Advance Article on the web
The photoinduced charge separation of N-alkylpyrenes in transition metal ion containing mesoporous
SiMCM-41 and MeMCM-41 materials [Me ¼ Ti(IV), V(V), Ni(II), Co(II) and Cu(II)] with ultraviolet irradiation
at room temperature is investigated. N-alkylpyrene cation radicals (PyCnþ) are generated in SiMCM-41 and
MeMCM-41 mesoporous silica materials and are characterized by electron spin resonance (ESR) spectroscopy.
Mesoporous SiMCM-41 and MeMCM-41 materiaþls are shown to be effective heterogeneous hosts for the
photoinduced formation and stabilization of PyCn cation radicals at room temperature indicating stable
photoinduced charge separation for several hours. The long lifetimes for the cation radicals indicate the utility
of the environment of these MCM-41 materials þfor controlling back electron transfer. A series of TiMCM-41
materials are synthesized that increase the PyCn photoionization efficiency in comparison with VMCM-41,
NiMCM-41, CoMCM-41 and CuMCM-41. The photoyield efficiency is decreased significantly when the
2
concentration of pyrene is increased above about 10 M. The photoionization efficiency depends on the type
and concentration of the metal ions in the mesoporous MeMCM-41 materials which suggests that certain metal
ions can act as electron acceptors. Also, as the N-alkylpyrene alkyl chain length increases from methyl to
hexadecyl the photoionization yield decreases. The photoionization efficiency increases in the order TiMCM-
41 > NiMCM-41 > VMCM-41 > SiMCM-41 > CoMCM-41 > CuMCM-41. The photoyield is higher by about
1.5 times at 77 K compared to room temperature. The observed photoyields of PyCn (n ¼ 1, 4, 8, 12, or 16) in
mesoporous MeMCM-41 materials suggest that these materials are useful models for long-term photoinduced
charge separation at room temperature.
years as hosts to perform photoinduced electron transfer
reactions with larger photoactive molecules are mesoporous
Introduction
M41S molecular sieves.17 Mesoporous SiMCM-41 materials
have uniform pore sizes from 15 to 100 A, which can be
The design of artificial efficient photomolecular devices for use
in energy storage and conversion has been an important
+
4
goal.1 A fundamental issue that limits the development of
controlled during synthesis,18 are effective catalysis for oxida-
tive reactions19,20 and are promising host systems for photo-
solar energy conversion is the thermal back electron transfer
between the photochemically produced redox pairs prior to the
accomplishment of any useful chemistry.5 Substantial effort is
25
redox reactions.21
Earlier studies showed that MCM-41 materials can achieve
stable photoinduced charge separation.21,23 Incorporation of
reducible transition metal ions into porous silica materials
retards back electron transfer by acting as more stable electron
being spent in developing potential molecular assemblies that
Systems
lead to stable and long-lived charge separation.6
such as micelles, vesicles, zeolites, silica gels and molecular
11
acceptors.21,23,26 Tetrahedral Si(IV) in the SiMCM-41 frame-
sieves have been examined as host materials to improve the net
14
Such systems provide appropriate
energy conversion.12
33
work can be replaced by other transition metal ions27
such
spatial and electronic organization of the donor and acceptor
molecules to retard rapid back electron transfer. Thus, one can
achieve a charge-separated state for a longer period of time by
appropriately tuning the spatial and electronic properties of
the host system.
Pyrene and its derivatives are selected because these poly-
cyclic hydrocarbons are known to undergo oxidative as well as
reductive photoinduced electron transfer reactions.15 These
can be easily photooxidized by ultraviolet irradiation to form
N-alkylpyrene cation radicals (PyCnþ) which can be char-
acterized by electron spin resonance (ESR). The design,
synthesis and study of molecular models have been instru-
mental in advancing the knowledge of electron transfer
dynamics involved in photosynthesis.16 Good heterogeneous
systems which have attracted considerable attention in recent
as Ti(IV), V(V), Al(III), Zr(IV), Ni(II) and Mn(II) to modify the
SiMCM-41 framework to enhance the photoionization effi-
ciency of incorporated photoactive molecules such as N-
alkylpyrenes (PyCn), or N-alkylphenothiazines (PCn).
Recently, we have reported successful photoinduced charge
separation of N-alkylphenothiazines in SiMCM-41, AlMCM-
41 and TiMCM-41 materials.21 The photoyield and stability of
photoinduced alkylphenothiazine cation radicals (PyCnþ) were
found to be significantly increased in TiMCM-41 materials
where Ti(IV) acts as an electron acceptor. In this work, we test
the generality of efficient photoinduced charge separation of
photoionizable molecules in metal ion containing MCM-41
materials to the alkylpyrene (PyCn , n ¼ 1, 4, 8, 12, or 16) class
of molecules. The photoionization efficiency of PyCn incor-
porated into SiMCM-41 and MeMCM-41, where Me ¼ Ti(IV),
5348
Phys. Chem. Chem. Phys., 2001, 3, 5348–5353
This journal is # The Owner Societies 2001
DOI: 10.1039/b107371k

View Article Online
V(V), Ni(II), Co(II), or Cu(II), was monitored by ESR. The
experimental results show that the photoyields depend on the
nature of the metal ion, with Ti(IV) being the most efficient,
and on the metal ion concentration in MeMCM-41 materials.
The photoionization of PyCn with different alkyl chain lengths
is also assessed.
Photoionization samples
N-alkylpyrenes were incorporated into SiMCM-41 and
MeMCM-41 by impregnation. For impregnation, 0.3 g of the
2
sample was immersed in 1 mL of 10 M PyCn in C6H6
overnight and the solvent was then removed by flowing
nitrogen through the samples for about 1 h in the dark. Thus
all samples contained the same amount of PyCn . For ESR
measurements the solid powders were introduced into 2 mm id
 3 mm od Suprasil quartz tubes (ꢁ20 mm in height), evac-
uated below 1 Torr for about 5 h at room temperature and
sealed. It was observed that the photoyields of all the samples
subjected to evacuation to remove possible remaining traces of
solvent were similar to the photoyields obtained for samples
that were not evacuated. Hence, further photoionization
experiments were carried out without evacuation.
Experimental section
The following chemicals were used without further purifica-
tion: tetramethylammonium hydroxide (TMAOH, Aldrich,
25%), tetraethylorthosilicate (TEOS, Aldrich), dodecyl-
trimethylammonium bromide (DTMABr, TCI). SiMCM-41
molecular sieve was prepared hydrothermally using dodecyl-
trimethylammonium bromide as the organic template follow-
ing an earlier procedure.32
In brief, 10.4 g of TEOS was mixed with 11.6 g of DTMABr
in 2-propanol under vigorous stirring at room temperature and
then 4.8 g of TMAOH and 76 mL of deionized water were
added to the resultant clear solution and stirring was con-
tinued for about 2 h. The pH of the mixture was adjusted to
11.5 using 2 M H2SO4 . After 2 h of stirring the mixture was
heated at 358 K for about 4 h to remove 2-propanol and
ethanol produced by the hydrolysis of tetraethylorthosilicate.
The resulting gel was crystallized hydrothermally in Teflon-
lined autoclaves for 7 days at 373 K. After crystallization the
solid product was filtered off, washed with deionized water,
and dried in air at 350 K overnight. The solid powder was
calcined in air at 820 K for about 6 h to remove water and
organic templates.
Photoionization procedures
Each powder was photoirradiated at room temperature as well
as at 77 K with a Cermax 300 W xenon lamp (ILC-LX 300
UV). The light was passed through a 10 cm water filter to
prevent infrared light and also through a Corning glass filter
7 51 with 90% transparency at 240 and 400 nm with a
maximum at 320 nm. The samples were rotated during the
photoirradiation for uniform exposure to the light. ESR
was used to detect the photoproduced N-alkylpyrene cation
radicals.
Results
For synthesis of TiMCM-41, initially TEOS and the
required amount of tetrabutylorthotitanate (TBOT, Fluka),
were vigorously mixed for 30 min and the following synthesis
steps were exactly the same as described above for the synth-
esis of SiMCM-41. These samples are termed TiMCM-41-(x),
where x is the Si=Ti ratio in the gel. Similarly for the synthesis
of VMCM-41, NiMCM-41, CoMCM-41 and CuMCM-41, for
each metal ion preparation, 10.4 g of TEOS was added with
11.6 g of DTMABr in 2-propanol under vigorous stirring and
then 4.8 g of TMAOH with 76 mL of deionized water were
added to the resultant clear solutions and stirred for 2 h. When
the above solutions become cloudy, the required amounts of
vanadyl sulfate, nickel sulfate, cobalt sulfate or copper sulfate
dissolved in 5 mL water were added respectively and the fol-
lowing synthesis steps were the same as described above for the
synthesis of SiMCM-41.
The XRD patterns for the synthesized SiMCM-41 and
MeMCM-41 materials match well with the literature.21,27,32,33
The XRD patterns of as-synthesized MeMCM-41 and calcined
MeMCM-41 show no significant differences. The pore sizes of
the MeMCM-41 samples prepared range from 24 to 27 A,
+
which are bigger than the molecular dimensions of 6.9 to 19.8
A for N-alkylpyrenes (PyC1 to PyC16). Fig. 1 shows the
+
molecular dimensions of N-alkylpyrenes which were calculated
using a PCMODEL program.34 Thus, the alkylpyrenes should
incorporate into these MeMCM-41 materials. But, thermal
gravimetric analysis (TGA) results show that the larger PyCn
molecules do not completely penetrate into the MCM-41
channels. TGA results obtained in flowing oxygen for N-
alkylpyrenes incorporated into SiMCM-41 are shown in Fig. 2.
The curves show two or three weight loss regions. The first
Commercial pyrene termed PyC1 was received from Lan-
caster Chemical Co and used as received without further
purification. N-alkylpyrene molecules termed PyCn (where
n ¼ 1, 4, 8, 12, or 16) were generous gifts from Prof. Y. S.
Kang.
Characterization
Powder X-ray diffraction (XRD) patterns were collected with a
Phillips 1840 diffractometer using Cu Ka radiation of wave-
+
length 1.541 A over the range 1.5ꢀ < 2y < 15ꢀ. Chemical
analysis was measured by
a JEOL JXA-8600 electron
microprobe with averaging over several defocused areas to give
the bulk composition. Calibration was done with known
standards. The ESR spectra were recorded at room tempera-
ture and 77 K at X-band frequency using a Bruker ESP 300
spectrometer with 100 kHz field modulation and microwave
power low enough to avoid saturation and distortion of the
spectrum. The photoproduced PyCnþ, n ¼ 1, 4, 8, 12 or 16,
cation radical yields were determined by double integration of
the ESR spectra using the ESP 300 software. Themogravi-
metric analysis (TGA) of the samples were performed using a
TGA 2050 analyzer from TA Instruments.
Fig. 1 Structures of N-alkylpyrenes with different alkyl chain
lengths.
Phys. Chem. Chem. Phys., 2001, 3, 5348–5353
5349

View Article Online
Fig. 2 TGA curves in flowing oxygen of SiMCM-41=PyCn (n ¼ 1, 4,
12 and 16) materials.
near 80 ꢀC is assigned to water desorption, the second near
200 ꢀC is assigned to alkylpyrene desorption from the external
surface of SiMCM-41 and the third peak centered around
500 ꢀC is assigned to the oxidative decomposition of alkyl-
pyrenes within the SiMCM-41 channels.24 The peak at 500 ꢀC
assigned to decomposition of alkylpyrenes is absent when
TGA is performed in flowing nitrogen. The smaller molecules
of PyC1 and PyC4 only show peaks near 500 ꢀC and not at
200 ꢀC which indicates incorporation into the SiMCM-41
channels and no adsorption on the external surface of
SiMCM-41. However, PyC12 shows a small peak near 200 ꢀC
so some PyC12 is adsorbed on the external surface of SiMCM-
41 and the peak near 500 ꢀC for PyC12 shows that some PyC12
is also incorporated into the channels of MCM-41. PyC16
shows a larger peak near 200 ꢀC and no peak near 500 ꢀC so
most of the PyC16 does not seem to penetrate into the MCM-
41 channels.
þ
Fig. 3 ESR spectra of PyC1 in SiMCM-41=PyC1 and TiMCM-
41=PyC1 at 0 (a e) and 60 min (A E) photoirradiation time with 320
nm light at room temperature.
Pyrene and its derivatives are known to undergo oxidative
photoelectron transfer reactions.15 Photoirradiated samples of
SiMCM-41 and MeMCM-41 [Me ¼ Ti(IV), V(V), Ni(II) and
Co(II)] without alkylpyrenes showed no ESR signals at room
temperature. Samples of SiMCM-41 and MeMCM-41 incor-
porated with alkylpyrenes turn light brown before irradiation
and produce very weak ESR signals of alkylpyrene cation
radicals (PyCnþ). After being photoirradiated by 320 nm light
at room temperature for 60 min, the samples turn dark brown
and show strong ESR signals. The brown color is character-
Fig. 4 ESR intensity of PyC1þ in SiMCM-41 and TiMCM-41 versus
320 nm different photoirradiation times at room temperature.
istic of PyCn cation radicals.35 This means that some PyCn
þ
þ
cation radicals are produced during sample preparation in a
‘dark’ reaction and that alkylpyrenes are the only photo-
sensitive molecules in these systems. The relative dark reaction
yields for MeMCM-41=PyC1 are 3.8 for SiMCM-41, 3.6 for
TiMCM-41, 4.1 for VMCM-41, 4.2 for CoMCM-41 and 4.4
for CuMCM-41.
Fig. 3 shows the ESR signal of pyrene cation radicals
(PyC1þ) in SiMCM-41 and TiMCM-41 samples before irra-
diation (a e) and after being irradiated (A E) by 320 nm UV
light at room temperature for 60 mþin3.4The singlet ESR signal
with g ¼ 2.006 is assigned to PyC1
.
This indicates photo-
induced formation of relatively stable PyC1þ cation radicals at
room temperature. From Fig. 4 and 5 it is clear that the
þ
intensities of the ESR signals of PyC1 cation radical ions
increase during the first 10 min and reach a plateau after about
30 min photoirradiation. CuMCM-41 and CoMCM-41 pro-
duce the highest ‘dark’ reaction compared to TiMCM-41,
NiMCM-41 and SiMCM-41. Note that the dark reaction must
be subtracted to obtain the net photoyield. The highest photo-
yield is obtained for TiMCM-41 followed by NiMCM-41,
VMCM-41, SiMCM-41, CoMCM-41 and CuMCM-41. The
photoyield in TiMCM-41 increases with increasing Ti content
þ
Fig. 5 ESR intensity of PyC1 in M-MCM-41=PyC1 [M ¼ Ti(IV),
Ni(II), Co(II) and Cu(II)] versus 320 nm different photoirradiation times
at room temperature.
5350
Phys. Chem. Chem. Phys., 2001, 3, 5348–5353

View Article Online
þ
Fig. 6 ESR intensity of PyC1 in TiMCM-41-(80)=PyC1 with dif-
ferent concentrations of PyC1 .
in TiMCM-41 up to Si=Ti ¼ 80 in the gel and then decreases
for Si=Ti ¼ 40 as shown in Fig. 4.
þ
Fig. 6 shows the photoyield of PyCn in TiMCM-41 versus
PyCn concentration for 60 min photoirradiation times. The
0.01 M solution concentration of PyCn corresponds to 2%
coverage of the surface area of the MeMCM-41 channels. At
higher solution concentrations, the photoyield decreases as
shown in Fig. 6. The photoyield at 77 K is about 1.5 times
higher than at room temperature.
Fig. 8 ESR spectra of PyC1 and VO2þ in VMCM-41=PyC1 at 0, 3,
20 and 60 min photoirradiation time with 320 nm light at room tem-
perature.
þ
Fig. 7 shows the ESR signal intensities of PyCnþversus alkyl
chain length for TiMCM-41=PyC1 samples. The relative dark
reaction yields become less as the alkyl chain length increases
and are 3.6 for PyC1 , 3.1 for PyC4 2.8 for PyC8 , 2.3 for PyC12
and 1.8 for PyC16 As one can see from Fig. 7, the photoyields
decrease with inþcreasing alkyl chain length. The rates of for-
mation of PyC1 cation radicals in MeMCM-41 [M ¼ Ti(IV),
V(V), Ni(II), Co(II) and Cu(II)] are evaluated from the initial
slopes in Fig. 4 and 5 assuming first-order kinetics. The cal-
The ESR spectra from phtoinduced PyC1þ cation radicals in
VCM-41 after several photoirradiation times at room tem-
perature are shown in Fig. 8. In addition to the line at
þ
g ¼ 2.006 due to PyC1 an eight line anisotropic spectrum due
to VO2þ is observed with the parameters g ¼ 2.024,
k
þ
4
1
4
1
culated rate constants for the formation of the PyC1 cation
2
g ¼ 2.052, A ¼ 191 Â 10 cm and A ¼ 54 Â 10 cm
?
k
?
1
radicals on this basis are 5 Â 10
s
for TiMCM-41,
for VMCM-41,
2
3
1
2
1
2.2 Â 10
2.8 Â 10
s
for NiMCM-41, 1.3 Â 10
s
1
4
1
s
for CoMCM-41 and 5.3 Â 10
s
for
Discussion
CuMCM-41. This order of the rate constants is consistent with
the order of the photoyields.
XRD and TGA experiments confirm that the pore size of
MCM-41 is large enough to contain N-alkylpyrenes (PyCn).
The ESR results clearly confirm the photoionization of N-
alkylpyrene molecules into N-alkylpyrene cation radical ions
in SiMCM-41 and MeMCM-41 [M ¼ Ti(IV), V(V), Ni(II), Co(II)
and Cu(II)] materials by 320 nm photoirradiation at room
temperature.þThe increase in the intensity of the ESR signal
due to PyC1 radical cations with time in MeMCM-41 mate-
rials (Fig. 5) indicates that the transition metal ions enhance
the photoionization of PyC1 . Other evidence for this is the
photoyield increase with Ti content up to Si=Ti ¼ 80 in
TiMMC-41=PyC1 (Fig. 4). The amount of photoyield
enhancement depends on the nature of the transition metal
ion. The net photoionization efficiency of MeMCM-41
decreases in the order of TiMCM-41=PyCn > NiMCM-41=
PyCn > VMCM-41=PyCn > SiMCM-41=PyCn > CoMCM-41=
PyCn > CuMCM-41=PyCn .
The stability of the photoinduced PyC1þ cation radical ions,
as monitored by ESR, was also studied (not shown). Assuming
þ
first-order kinetics, the initial decay rates of PyC1 cation
radical ions for MeMMCM-41 at room temperature were
3
1
3
1
evaluated to be 2.5 Â 10
s
for TiMCM-41, 4.2 Â 10
s
for NiMCM-41, 5 Â 10 3 s 1 for VMCM-41, 6.3 Â 10 3 s 1 for
4
1
CoMCM -41 and 3.8 Â 10
s
for CuMCM-41.
The observed higher photoyields at 77 K can be explained
þ
by less PyCn cation radical mobility at 77 K, which retards
back electron transfer to PyCn molecules. In earlier
work,23,36,37 methylphenothiazine molecules incorporated into
different host lattices also showed higher photoyields at 77 K
than at room temperature.
Samples of VMCMþ-41 impregnated with PyC1 show a weak
ESR signal of PyC1 cation radical because V(V) ions in
VMCM-41 oxidise some PyC1 to form V(IV) ions, which is
consistent with V(IV) observation by ESR after photoirradia-
tion, as shown in Fig. 8. This supports that V(V) ions in
VMCM-41 do accept electrons from PyC1 to enhance the
þ
Fig. 7 ESR intensity of PyCn in TiMCM-41=PyCn (n ¼ 1, 4, 8, 12
or 16; Si=Ti ¼ 80) at 60 min photoirradiation time with 320 nm light at
room temperature.
Phys. Chem. Chem. Phys., 2001, 3, 5348–5353
5351

View Article Online
þ
photoyield of PyC1 cation radicals. The ESR of the para-
This work shows that the photoionization with net long-
lived charge separation can be achieved for alkylpyrenes in
TiMCM-41 materials and that the photoyield is dependent on
the Ti(IV) concentration. This extends similar previous work on
alkylphenothiazines in TiMCM-41 and thus demonstrates the
generality of photoionization enhancement of incorporated
molecules in TiMCM-41 materials. The photoyield efficency
depends mainly on the electron acceptor side of the reaction
and is enhanced not only by Ti(IV) but also by Ni(II) and V(V)
relative to SiMCM-41.
magnetic valence state of V(IV) in VO2þ in VMCM-41=PyC1 is
clearly observed (Fig. 8) as expected for capture of photo-
induced electrons by V(V), but the expected ESR of Ti(III) in
TiMCM-41=PyC1 and Ni(I) in NiMCM-41=PyC1 are not
observed. The expected ‘g’ positions are about g ¼ 1.91,
k
g ¼ 1.97 for Ti(III) in TiMCM-41=PyC1 and g ¼ 2.49,
?
?
k
þ
g ¼ 2.11 for Ni(I) in TiMCM-41=PyC1 .38 In general, PyC1
cation radicals occur at g ¼ 2.006 at room temperature with a
derivative peak to peak linewidth of about 10 G. Simulations
indicate partial overlap of the PyC1þ line with the g features of
Ti(III) and Ni(I) and the g features may be too weak to observe.
?
k
The ESR intensities of both V(IV) and PyC1 cation radical
increase with irradiation time. The increase of PyC1 spin
þ
Acknowledgement
þ
concentration (SPyC1 ¼ 0.98 Â 107 spins cm 3) in VMCM-
þ
This research was supported by the Division of Chemical Sci-
ences, Office of Basic Energy Sciences, Office of Energy
Research, US Department of Energy, The Texas Advanced
Research Program and the Environmental Institute of Houston.
41=PyC1 is the same as that of the V(IV) spin concentration
(SVO ¼ 0.93 Â 107 spins cm 3) during 30 min irradiation time
2þ
which supports that V(V) is an electron acceptor. At longer
irradiation time (60þmin), the increase of PyC1 spin con-
þ
centration is (SPyC1 ¼ 1.02 Â 107 spins cm 3) smaller than
that 2þof the increase in V(IV) spin concentration
References
(SVO ¼ 1.42 Â 107 spins cm 3), probably due to secondary
þ 25,36
1
V. Balzani and F. Scandola, Supramolecular Photochemistry, Ellis
Harwood, Chichester, 1991.
reactions of PyC1
.
þ
The decrease of the PyCn photoyield with alkyl chain
length (Fig. 7) can be partially due to size exclusion from the
MCM-41 channels based on the TGA data of Fig. 2 which
shows that PyC12 only partially penetrates the MCM-41
channels while PyC16 penetrates even less. The decrease in
photoyield for larger PyCnþ with longer alkyl chain length may
also be partly due to a lower diffusion rate of PyCn into the
MeMCM-41 channels. A longer alkyl chain length makes the
molecules more bulky and more difficult to penetrate into thþe
MeMCM-41 channels. The decrease in photoyield of PyCn
with increasing alkyl chain length may also be related to
molecular aggregation.26,39,40 Longer chain PyCn are more
likely to generate molecular aggregates. The formation of such
aggregates will further lower the rate of diffusion of PyCn into
MeMCM-41 andþlead to diminished photoyield. However, the
stability of PyCn increases from þPyC1 to PyC16 due to an
increased inductive effect in PyCn as a function of longer
alkyl chain lengths.39,41,42
2
3
4
D. Gust and T. A. Moore, Adv. Photochem., 1991, 16, 1.
H. Imahori and Y. Sakata, Eur. J. Org. Chem., 1999, 2445.
C. Luo, D. M. Guldi, H. Imahori, K. Tamaki and Y. Sakata,
J. Am. Chem. Soc., 2000, 122, 6535.
A. J. Bard and M. A. Fox, Acc. Chem. Res., 1995, 28, 141.
D. Gust, T. A. Moore and L. A. Moore, Acc. Chem. Res., 1993,
26, 198.
Z. Chang, K. T. Ranjit, R. M. Krishna and L. Kevan, J. Phys.
Chem., 2000, 26, 198.
V. Ramamurthy, P. Lakshminarasimhan, C. P. Grey and L.
Johnston, Chem. Commun., 1998, 2411.
J. Abraham and V. Ramamurthy, Chem. Eur. J., 2000, 6, 1287.
5
6
7
8
9
10 A. G. Panov, R. G. Larsen, N. I. Totach, S. C. Larsen and V. H.
Grassian, J. Phys. Chem. B, 2000, 104, 5706.
11 Y.-K. Gong, T. Miyamoto and K. Nakashima, J. Phys. Chem. B,
2000, 104, 5772.
12 L. Kevan, in Photoinduced Electron Transfer, Part B, ed. M. A. Fox
and M. Chanon, Elsevier, Amsterdam, 1988, p. 329.
13 T. Nakato, K. Kazuyuki and C. Koto, Chem. Mater, 1992, 4, 128.
14 L. A. Vermeulen and M. E. Thompson, Nature, 1992, 358, 656.
15 M. Julliard, in Photoinduced Electron Transfer, Part B, ed. M. A.
Fox and M. Chanon, Elsevier, Amsterdam, 1988, p. 216 and
references therein.
16 M. R. Wasielewski, Chem. Rev., 1992, 92, 435.
17 C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S.
Beck, Nature, 1992, 359, 710.
A semiquantitative measure of the efficiency of the photo-
reaction is obtained from the amount of alkylpyrene cation
radicals produced relative to the number of alkylpyrene
molecules present prior to photoirradiation. The estimated
photoconversion is ꢁ15%, which is relatively high.
18 J.-Y. Ying, C. P. Mehnert and M. S. Wong, Angew. Chem., Int.
Ed., 1999, 38, 873.
19 R. Q. Long and R. T. Yang, Ind. Eng. Chem. Res., 1999, 38, 873.
20 K. Kageyama, S. Ogino, T. Aida and T. Tatsumi, Macro-
molecules, 1998, 31, 4069.
21 H. M. Sung-Suh, Z. Luan and L. Kevan, J. Phys. Chem. B, 1997,
101, 10 455.
22 A. Corma, F. L. Cozens, H. Garcia, M. A. Miranda and A. Sa-
bater, J. Am. Chem. Soc., 1994, 116, 9767.
23 R. M. Krishna, A. M. Prakash and L. Kevan, J. Phys. Chem. B,
2000, 104, 1796.
24 V. Kurshev, A. M. Prakash, R. M. Krishna and L. Kevan,
Microporous Mesoporous Mater, 2000, 34, 9.
25 Z. Chang, R. M. Krishna, J. Xu, R. Koodali and L. Kevan, Phys.
Chem. Chem. Phys., 2001, 3, 1699.
26 S. Sinlapadech, R. M. Krishna, Z. Luan and L. Kevan, J. Phys.
Chem. B, 2001, 105, 4350.
27 M. D. Alba, Z. Luan and J. Klinowski, J. Phys. Chem., 1996, 100,
2179.
28 M. Hartmann, S. Racouchot and C. Bischof, Microporous Meso-
porous Mater., 1999, 27, 309.
29 W. Zhang and T. J. Pinnavaia, Catal. L ett., 1996, 38, 261.
30 A. A. Romero, M. D. Alba and J. Klinoski, J. Phys. Chem. B,
1998, 102, 123.
31 J. Xu, Z. Luan, M. Hartmann and L. Kevan, Chem. Mater, 1999,
11, 2928.
32 K. A. Koyano and T. Tatsumi, Microporous Mater., 1997, 10, 259.
33 Z. Chang, Z. Zhu and L. Kevan, J. Phys. Chem. B, 1999, 103, 9442.
Conclusions
Mesoporous transition metal ions containing SiMCM-41
materials are potential candidates for stable photoinduceþd
charge separation of N-alkylpyrene molecules. The PyCn
cation radical photoyields depend on the incorporation of
metal ions into MeMCM-41. The results indicate that back
electron transfer is retarded in MeMCM-41 materials and that
the lifetime of photoinduced cation radical ions is increased to
hours or even weeks at room temperature. It is clear that V(V)
is an electron acceptor during the photoirradiation of VMCM-
41=PyCn samples since V(IV) is observed. The results indicate
that Ti(IV) is the most efficient electron acceptor followed by
Ni(II) and V(V), all of which enhance the photoyield compared
to SiMCM-41. The incorporation of Cu(II) and Cu(II) givþe
photoyields that are less than in SiMCM-41. The PyCn
photoyield and stability are also dependent on the length of the
alkyl chain. The photoyield is about one and half times higher
at 77 K than at room temperature. New physical insights are
shown by the dependence of the photoyield on the nature and
concentration of the transition metal ion present in MeMCM-
41, and also on the size of the electron donor molecules.
5352
Phys. Chem. Chem. Phys., 2001, 3, 5348–5353

View Article Online
34 PCMODEL , Serena Software, Box 3076, Bloomington, Indiana,
4740-3076, USA, 1993.
39 R. M. Krishna, Z. Chang, H. Choo, K. T. Ranjit and L. Kevan,
Phys. Chem. Chem. Phys., 2000, 2, 3335.
35 J.-A. Jung, D. R. Shin, J.-S. Kim, Y. S. Kang and L. Kevan,
J. Chem. Soc., Faraday T rans., 1998, 94, 1619.
40 J. Cavanaugh, J. Am. Chem. Soc., 1959, 81, 2507.
41 L. S. Levitt and H. F. Widing, in Progress in Physical Organic
Chemistry, ed. R. W. Taft, Wiley, New York, 1976, vol. 12,
ch. 12.
42 R. M. Krishna, A. M. Prakash, V. Kurshev and L. Kevan, Phys.
Chem. Chem. Phys., 1999, 1, 4119.
36 B. Xiang and L. Kevan, L angmuir, 1994, 10, 2688.
37 B. Xiang and L. Kevan, J. Phys. Chem., 1994, 98, 5120.
38 (a) A. M. Prakash, V. Kurshev and L. Kevan, J. Phys. Chem. B,
1997, 101, 9794; (b) A. M. Prakash, T. Wasowicz and L. Kevan,
J. Phys. Chem., 1996, 100, 15 947.
Phys. Chem. Chem. Phys., 2001, 3, 5348–5353
5353