1 Introduction
Photochemistry is the study of physical and chemical processes following absorption
of photons; it aims to understand the electronic and structural characteristics of the
molecule in excited state and deduce mechanism of chemical change. Since its
formal recognition as a separate field of study that had begun in the earlier half of
the eighteenth century, photochemistry has provided fundamental insights regarding
the chemical nature of matter: structure and electronic properties of electronically
excited species [1, 2]. This has also resulted in technological advancements ranging
from photonic devices and high-density storage media to nanocircuits, synthetic
transformations, and drug delivery systems [3, 4]. As a complementary science to
ground-state reactions driven by thermal energy, photochemistry also possesses
several complementary advantages over ground-state reactions: light being a concentrated form of energy, chromophoric transparency, functional group specificity,
faster mechanistic timescales, low operating temperature, access to highly strained
structures, etc. However, the awesome potential of photochemistry was not fully
realized until the emergence of the subdiscipline of supramolecular photochemistry.
The ability of weak intermolecular interaction to control molecular structure in the
excited state has provided a new dimension of exploration and exploitation of this
field. Manipulation of electronic and structural properties of photoactive molecules
in ground and excited state has allowed photochemists to study previously
unobserved phenomena, which were predicted but were not realized.
Unlike thermal organic reactions where functional group chemistry is generally
consistent and predictable based on qualitative valence shell electron repulsion
(VSEPR) considerations, photochemical reactions are difficult to predict due to
high sensitivity to electronic structure and need for knowledge of molecular orbitals
(MOs), which cannot be deduced without rigorous mathematical treatment of
molecular systems. Supramolecular photochemistry has served as a useful means
for predicting the feasibility of reactions and phenomena on several occasions. Thus,
the influence of supramolecular chemistry on photochemical reactions has led to a
great expansion of the significance of the latter field as a science for understanding
the chemical nature of the natural world.
Supramolecular interactions have been utilized for controlling ground- [5] and
excited-state property of chemical entities [6–8]. Several supramolecular approaches
exist for dictating molecular behavior: inter and intramolecular weak interactions in
solid state [9–13], mechanical interlocking [14, 15], and host-guest chemistry
[12, 16, 17]. Among these approaches, host-guest approach to controlling structure
and directing chemical dynamics has been responsible for the most progress made in
supramolecular photochemistry [6, 18]. A molecular host is a chemical framework
that encapsulates a relatively smaller entity, wholly or partly [16, 19, 20]. As an
associative phenomenon, host-guest complexation is driven by either attractive weak
interactions between host and guest or repulsive solvophobic interactions between
host or guest and the surrounding medium. Molecular host systems such as cavitands
[21–24], micelles [25, 26], and microporous materials such as zeolites [27, 28] and
324
M. Pattabiraman and A. Natarajan
Photochemistry is the study of physical and chemical processes following absorption
of photons; it aims to understand the electronic and structural characteristics of the
molecule in excited state and deduce mechanism of chemical change. Since its
formal recognition as a separate field of study that had begun in the earlier half of
the eighteenth century, photochemistry has provided fundamental insights regarding
the chemical nature of matter: structure and electronic properties of electronically
excited species [1, 2]. This has also resulted in technological advancements ranging
from photonic devices and high-density storage media to nanocircuits, synthetic
transformations, and drug delivery systems [3, 4]. As a complementary science to
ground-state reactions driven by thermal energy, photochemistry also possesses
several complementary advantages over ground-state reactions: light being a concentrated form of energy, chromophoric transparency, functional group specificity,
faster mechanistic timescales, low operating temperature, access to highly strained
structures, etc. However, the awesome potential of photochemistry was not fully
realized until the emergence of the subdiscipline of supramolecular photochemistry.
The ability of weak intermolecular interaction to control molecular structure in the
excited state has provided a new dimension of exploration and exploitation of this
field. Manipulation of electronic and structural properties of photoactive molecules
in ground and excited state has allowed photochemists to study previously
unobserved phenomena, which were predicted but were not realized.
Unlike thermal organic reactions where functional group chemistry is generally
consistent and predictable based on qualitative valence shell electron repulsion
(VSEPR) considerations, photochemical reactions are difficult to predict due to
high sensitivity to electronic structure and need for knowledge of molecular orbitals
(MOs), which cannot be deduced without rigorous mathematical treatment of
molecular systems. Supramolecular photochemistry has served as a useful means
for predicting the feasibility of reactions and phenomena on several occasions. Thus,
the influence of supramolecular chemistry on photochemical reactions has led to a
great expansion of the significance of the latter field as a science for understanding
the chemical nature of the natural world.
Supramolecular interactions have been utilized for controlling ground- [5] and
excited-state property of chemical entities [6–8]. Several supramolecular approaches
exist for dictating molecular behavior: inter and intramolecular weak interactions in
solid state [9–13], mechanical interlocking [14, 15], and host-guest chemistry
[12, 16, 17]. Among these approaches, host-guest approach to controlling structure
and directing chemical dynamics has been responsible for the most progress made in
supramolecular photochemistry [6, 18]. A molecular host is a chemical framework
that encapsulates a relatively smaller entity, wholly or partly [16, 19, 20]. As an
associative phenomenon, host-guest complexation is driven by either attractive weak
interactions between host and guest or repulsive solvophobic interactions between
host or guest and the surrounding medium. Molecular host systems such as cavitands
[21–24], micelles [25, 26], and microporous materials such as zeolites [27, 28] and
324
M. Pattabiraman and A. Natarajan