Methylviologen (MV2+) has been adsorbed in a series
of zeolites including alkaline ion faujasites (HNaY,
LiNaY, NaY, KNaY, RbNaY, CsNaY, and NaX), Naβ, NaMor, and NaZSM-5 by
ion exchange. Extra large
pore aluminosilicate MCM-41 was also used. The resulting
MV2+-doped zeolites have been characterized
by an array of analytical and spectroscopic techniques such as chemical
analyses, X-ray photoelectron
spectroscopy, thermogravimetry−differential scanning calorimetry,
diffuse reflectance spectroscopy, and
fluorescence, FT-IR, FT-Raman, and MAS 13C NMR
spectroscopies. Influence of the confinement and
nature
of the charge-balancing cation on the molecular properties of
MV2+ resulted in shifts of the λmax of
the
absorption band in the diffuse reflectance spectra from 270 to 280 nm
for MV−NaX and 290 nm for MV−NaZSM-5 samples. In addition, changes of the relative intensities
of the emission bands at 340 and 420 nm
were also noted. The formation of the
MV•+ cation radical as a persistent species
by thermal treatment from
the MV2+ ground state was not observed in any of the
samples. In contrast, laser flash photolysis of these
samples allowed in all cases the detection of
MV•+ as a long-lived transient on the
microsecond time scale.
Formation of MV•+ was also observed by
X-ray photoelectron spectroscopy. This constitutes the first
firm
experimental evidence that zeolites can behave as single electron
donors. For MV−RbNaY and MV−CsNaY
samples in outgassed sealed quartz cells, the photogenerated
MV•+ is so long-lived that its diffuse
reflectance
spectrum and disappearance kinetics can be obtained by conventional
diffuse reflectance spectroscopy. A
relationship between the basic strength of the zeolite framework and
the stability of photogenerated
MV•+
has been established.