extended for this population indicates that the ΔE 1 barrier is actually that accessing the
singlet in character MLCT states and the observed emission decay results from
relaxation from both the lowest-energy
3
MLCT manifold and higher-energy
1/
3
MLCT excited states. In addition, the k 0 value is also nearly identical to that observed
for RuBpy in solution indicating a relaxation from the
3
MLCT is relatively
unperturbed.
4 Conclusions and Future Perspectives
The ability to encapsulate photoactive guests, particularly Ru(II) polyimines, into
porous MOF frameworks is an important advance in the development of new
photoactive/photoresponsive materials. The increasing number of photoactive
guest-MOFs is also providing a unique opportunity to examine the mechanisms
through which encapsulation within varying cavity types can modulate the
photophysics of the photoactive guest. The summary presented here of RuBpy
encapsulation within Zn-carboxylate-based MOFs provides insights into such mechanisms. Several general features are evident. First, the energy associated with the
emitting
3 MLCT depends on both the accessibility of the RuBpy to solvent within
the cavities and to the overall framework charge. Second, in nearly all cases,
encapsulation increases the barrier to access the
3 LF, which should lead to
stabilization of the RuBpy ligands. In addition, other MLCT states, including a
fourth
3 MLCT state above the emitting
3 MLCT state manifold and other high-energy
1/3 MLCT states, become accessible and modulate the rate of emission decay. Finally,
in nearly all cases, the emission decays could be fit to a biexponential function
indicating two populations of encapsulated RuBpy with a population of short
lifetime RuBpy cations that appear to have quenched
3 MLCT manifolds. The longer
lifetimes are associated with populations of RuBpy in structurally confined cavities
that are not accessible to either exogenous quenchers or neighboring RuBpy
allowing for self-quenching. These observations provide an important framework
from which a more detailed analysis of cavity structure-photophysical modulation
relationships can be performed.
The results summarized in this chapter provide a foundation for understanding the
effects of encapsulation of transition metal cations on their corresponding
photophysics. With the present catalog of MOFs with well-resolved RuBpy cations,
it is now possible to examine the molecular details that modulate the electronic states
of the encapsulated guest. Specifically, the relationship between structural distortions associated with RuBpy encapsulation and the observed photophysics can now
be compared with intermolecular interactions between the RuBpy and elements of
the framework. The overall intent is to utilize cavity-RuBpy photophysical relationships in order to produce light-responsive materials “by design.” These studies also
provide a platform through which to modulate the photophysical properties of other
transition metal complexes through encapsulation within MOF-type materials.
180
R. W. Larsen et al.
singlet in character MLCT states and the observed emission decay results from
relaxation from both the lowest-energy
3
MLCT manifold and higher-energy
1/
3
MLCT excited states. In addition, the k 0 value is also nearly identical to that observed
for RuBpy in solution indicating a relaxation from the
3
MLCT is relatively
unperturbed.
4 Conclusions and Future Perspectives
The ability to encapsulate photoactive guests, particularly Ru(II) polyimines, into
porous MOF frameworks is an important advance in the development of new
photoactive/photoresponsive materials. The increasing number of photoactive
guest-MOFs is also providing a unique opportunity to examine the mechanisms
through which encapsulation within varying cavity types can modulate the
photophysics of the photoactive guest. The summary presented here of RuBpy
encapsulation within Zn-carboxylate-based MOFs provides insights into such mechanisms. Several general features are evident. First, the energy associated with the
emitting
3 MLCT depends on both the accessibility of the RuBpy to solvent within
the cavities and to the overall framework charge. Second, in nearly all cases,
encapsulation increases the barrier to access the
3 LF, which should lead to
stabilization of the RuBpy ligands. In addition, other MLCT states, including a
fourth
3 MLCT state above the emitting
3 MLCT state manifold and other high-energy
1/3 MLCT states, become accessible and modulate the rate of emission decay. Finally,
in nearly all cases, the emission decays could be fit to a biexponential function
indicating two populations of encapsulated RuBpy with a population of short
lifetime RuBpy cations that appear to have quenched
3 MLCT manifolds. The longer
lifetimes are associated with populations of RuBpy in structurally confined cavities
that are not accessible to either exogenous quenchers or neighboring RuBpy
allowing for self-quenching. These observations provide an important framework
from which a more detailed analysis of cavity structure-photophysical modulation
relationships can be performed.
The results summarized in this chapter provide a foundation for understanding the
effects of encapsulation of transition metal cations on their corresponding
photophysics. With the present catalog of MOFs with well-resolved RuBpy cations,
it is now possible to examine the molecular details that modulate the electronic states
of the encapsulated guest. Specifically, the relationship between structural distortions associated with RuBpy encapsulation and the observed photophysics can now
be compared with intermolecular interactions between the RuBpy and elements of
the framework. The overall intent is to utilize cavity-RuBpy photophysical relationships in order to produce light-responsive materials “by design.” These studies also
provide a platform through which to modulate the photophysical properties of other
transition metal complexes through encapsulation within MOF-type materials.
180
R. W. Larsen et al.