γ-cyclodextrin cavity, with up to 35% emission observed from energy transfer
compared with exciting SQ directly. Due to the complete lack of spectral overlap
between anthracene and squaraines, such energy transfer from ANT to SQ is
unprecedented. In particular, γ-CD was used to promote energy transfer from five
common polycyclic aromatic hydrocarbons (PAHs) to near-infrared-emitting
squaraine dyes, as illustrated in Fig. 5. They demonstrated efficient energy transfer
from anthracene (ANT) to a near-infrared-emitting squaraine (SQ) fluorophore
acceptor within γ-CD. For instance, Fig. 5 shows the wavelength-dependent emission of the ANT.SQ@γ-CD ternary complex wherein direct photoexcitation of ANT
at 360 nm and SQ at 620 nm resulted in predominant emission of the respective
fluorophores. Energy transfer between ANT and SQ was ascertained through
increasing SQ emission upon photoexcitation at 360 nm; this was further validated
when increasing concentration of γ-CD increases SQ emission while that of ANT
decreases.
Similar ET fluorescence emission from SQ was observed for four other PAHs
including pyrene (Py), wherein presence of γ-CD under photoexcitation conditions
showed increase in SQ emission with concomitant decrease in excimer emission of
Py. This energy transfer setup, which occurs via the formation of ternary complexes,
is analogous to a sensor array wherein the presence of PAHs could be detected
through emission from near IR emitter [35].
Manipulating fluorescence through cavitand-controlled fluorophore rigidity was
demonstrated by Rebek’s group using their synthetic molecular host [36]. Rebek’s
host (RH, Fig. 6) is a synthetic “deep cavitand” with one opening, therefore only one
portal, through which guest molecules could enter its cavity. It consists of the
C-undecylcalix[4]resorcinarene basin with four 1,4-pyrazyl-amido pillars. The
urea (amido) units in the host, capable of hydrogen bonding, enable the host to
self-assemble into a dimeric capsule (RH•RH) in nonpolar solvents. The interior of
the capsule provides well-protected and isolated molecular spaces for guests, which
has been utilized efficiently for controlling molecular behavior [37]. RH is also
capable of forming an elongated self-assembled capsule through hydrogen-bonding
glycoluril linkers Ls (Fig. 6).
Trans-stilbenes are known to give weak to moderate fluorescence in solution due
to their high quantum efficiency for photochemical change (trans ! cis) and lack of
structural rigidity, whereas it is known to exhibit strong emission within constrained
environment such as antibody interior, yielding an intense blue fluorescence [38–
40]. Though the reason for enhanced emission within constrained media is attributed
to the reduced quantum efficiency of photoisomerization, little is known about the
photophysical nature of its excited state. Rebek’s group utilized the constraining
effect of their molecular hosts to study the factors controlling behavior of stilbenes in
a sterically constrained environment. For example, 4,4
0 -dimethyl stilbene STB
exhibits moderate fluorescence in solution (Fig. 7, black solid line); encapsulation
of the same in self-assembled RH•RH capsule resulted in a near quantitative
fluorescence quenching (red solid line). This loss in fluorescence, despite encapsulation within the capsule, was remarkable especially in the context of what was
observed within antibody environments. The loss of emission was attributed to the
Photophysicochemical Processes Directed Within Nano-Containers
329
compared with exciting SQ directly. Due to the complete lack of spectral overlap
between anthracene and squaraines, such energy transfer from ANT to SQ is
unprecedented. In particular, γ-CD was used to promote energy transfer from five
common polycyclic aromatic hydrocarbons (PAHs) to near-infrared-emitting
squaraine dyes, as illustrated in Fig. 5. They demonstrated efficient energy transfer
from anthracene (ANT) to a near-infrared-emitting squaraine (SQ) fluorophore
acceptor within γ-CD. For instance, Fig. 5 shows the wavelength-dependent emission of the ANT.SQ@γ-CD ternary complex wherein direct photoexcitation of ANT
at 360 nm and SQ at 620 nm resulted in predominant emission of the respective
fluorophores. Energy transfer between ANT and SQ was ascertained through
increasing SQ emission upon photoexcitation at 360 nm; this was further validated
when increasing concentration of γ-CD increases SQ emission while that of ANT
decreases.
Similar ET fluorescence emission from SQ was observed for four other PAHs
including pyrene (Py), wherein presence of γ-CD under photoexcitation conditions
showed increase in SQ emission with concomitant decrease in excimer emission of
Py. This energy transfer setup, which occurs via the formation of ternary complexes,
is analogous to a sensor array wherein the presence of PAHs could be detected
through emission from near IR emitter [35].
Manipulating fluorescence through cavitand-controlled fluorophore rigidity was
demonstrated by Rebek’s group using their synthetic molecular host [36]. Rebek’s
host (RH, Fig. 6) is a synthetic “deep cavitand” with one opening, therefore only one
portal, through which guest molecules could enter its cavity. It consists of the
C-undecylcalix[4]resorcinarene basin with four 1,4-pyrazyl-amido pillars. The
urea (amido) units in the host, capable of hydrogen bonding, enable the host to
self-assemble into a dimeric capsule (RH•RH) in nonpolar solvents. The interior of
the capsule provides well-protected and isolated molecular spaces for guests, which
has been utilized efficiently for controlling molecular behavior [37]. RH is also
capable of forming an elongated self-assembled capsule through hydrogen-bonding
glycoluril linkers Ls (Fig. 6).
Trans-stilbenes are known to give weak to moderate fluorescence in solution due
to their high quantum efficiency for photochemical change (trans ! cis) and lack of
structural rigidity, whereas it is known to exhibit strong emission within constrained
environment such as antibody interior, yielding an intense blue fluorescence [38–
40]. Though the reason for enhanced emission within constrained media is attributed
to the reduced quantum efficiency of photoisomerization, little is known about the
photophysical nature of its excited state. Rebek’s group utilized the constraining
effect of their molecular hosts to study the factors controlling behavior of stilbenes in
a sterically constrained environment. For example, 4,4
0 -dimethyl stilbene STB
exhibits moderate fluorescence in solution (Fig. 7, black solid line); encapsulation
of the same in self-assembled RH•RH capsule resulted in a near quantitative
fluorescence quenching (red solid line). This loss in fluorescence, despite encapsulation within the capsule, was remarkable especially in the context of what was
observed within antibody environments. The loss of emission was attributed to the
Photophysicochemical Processes Directed Within Nano-Containers
329