6.6 Emission Properties of 1 and 2
169
weak as compared to those in the solid state (Figs. 6.22 and 6.23), as expected for a
long-lived triplet emission that is highly susceptible to quenching [10]. The complex
emission lifetimes in the microsecond regime described below confirmed that the
crystalline state emission can be assigned as phosphorescence. Crystal 1 showed
reversible visual changes in emission from green to yellow as the temperature changes
from 298 to 193 K (Fig. 6.6a). The temperature range of these changes matches well
the temperature range where the thermally activated molecular rotator varies from
highly dynamic to nearly static. The emission spectrum of 1 consists of a sharp, well
resolved peak at 498 nm, followed by a several less resolved peaks and shoulders
with a λ max = 543 nm, suggesting a relatively rich vibronic structure. Lowering the
temperature from 298 to 193 K resulted in the relative increase in intensity of the
lower energy bands, which is responsible for the visual changes in emission going
from green to yellow (Fig. 6.6b).
The solid state emission of the static molecular rotor 2 shown in Fig. 6.7 does
not have the vibrational resolution of the unsubstituted compound, and there are
no visual or spectral changes as a function of temperature from 298 to 193 K. The
spectrum has a λ max = 545 nm with an envelop that spans from ca. 490 to 700 nm,
covering the same energy range as 1, but lacking a strong 0-0 transition and displaying
significant broadening of the lower energy bands. To suggest an explanation for the
observations shown in Figs. 6.6 and 6.7, we propose assignments for the emissions
of 1 and 2, consider the photophysical properties of conjugated arylene-ethynylenes
as a function of rotation, and explore the role of the gold(I) centers in the electronic
communication among the diethynylphenylene rotors in the crystal.
We first note that the emission spectrum of 1 is consistent with the phosphorescence of an isolated π-conjugated diethynyl arene chromophore [10, 11]. This
includes a vibronic structure that includes a strong 0-0 transition and a rich vibrational progression that arises from several C-H stretching and bending modes [10,
12]. The spectrum of 2, by contrast, is consistent with a similar diethynyl arene chromophore under conditions of co-facial aggregation [12, 13], which tends decrease
the intensity of the 0-0 band while broadening the rest of the spectrum [12, 13].
These assignments are also consistent with structural parameters available from the
corresponding crystal structures, which show the central aromatic ring to have a Tshaped C-H-π interaction with neighboring molecules in the case of 1 and co-facial
π-π interactions in the case of 2.
Spectral changes as a function of rotational angle have been well documented
for π-conjugated chromophores that are linked by triple bonds, such as tolane [14],
bis(phenylethynyl)benzene (Scheme 6.2) [12, 13], and numerous structures where
electronic interactions are determined by metal centers [10b, c, 15]. While rotation of
aromatic groups relative to each other does not affect the ground state potential energy
profile, electronic excitation causes changes in bond order with single and triple bonds
acquiring cumulene character, which makes the excited state energies sensitive to
twist angles of arene groups [12, 13]. Thus, coplanar structures are fully delocalized in
the ground and excited states (shown in red in the top frame of Scheme 6.2), such that
they tend to absorb and emit at lower energies as compared to the analogous twisted
structures. In fact, twisting restricts conjugation in the excited state (Scheme 6.2,
169
weak as compared to those in the solid state (Figs. 6.22 and 6.23), as expected for a
long-lived triplet emission that is highly susceptible to quenching [10]. The complex
emission lifetimes in the microsecond regime described below confirmed that the
crystalline state emission can be assigned as phosphorescence. Crystal 1 showed
reversible visual changes in emission from green to yellow as the temperature changes
from 298 to 193 K (Fig. 6.6a). The temperature range of these changes matches well
the temperature range where the thermally activated molecular rotator varies from
highly dynamic to nearly static. The emission spectrum of 1 consists of a sharp, well
resolved peak at 498 nm, followed by a several less resolved peaks and shoulders
with a λ max = 543 nm, suggesting a relatively rich vibronic structure. Lowering the
temperature from 298 to 193 K resulted in the relative increase in intensity of the
lower energy bands, which is responsible for the visual changes in emission going
from green to yellow (Fig. 6.6b).
The solid state emission of the static molecular rotor 2 shown in Fig. 6.7 does
not have the vibrational resolution of the unsubstituted compound, and there are
no visual or spectral changes as a function of temperature from 298 to 193 K. The
spectrum has a λ max = 545 nm with an envelop that spans from ca. 490 to 700 nm,
covering the same energy range as 1, but lacking a strong 0-0 transition and displaying
significant broadening of the lower energy bands. To suggest an explanation for the
observations shown in Figs. 6.6 and 6.7, we propose assignments for the emissions
of 1 and 2, consider the photophysical properties of conjugated arylene-ethynylenes
as a function of rotation, and explore the role of the gold(I) centers in the electronic
communication among the diethynylphenylene rotors in the crystal.
We first note that the emission spectrum of 1 is consistent with the phosphorescence of an isolated π-conjugated diethynyl arene chromophore [10, 11]. This
includes a vibronic structure that includes a strong 0-0 transition and a rich vibrational progression that arises from several C-H stretching and bending modes [10,
12]. The spectrum of 2, by contrast, is consistent with a similar diethynyl arene chromophore under conditions of co-facial aggregation [12, 13], which tends decrease
the intensity of the 0-0 band while broadening the rest of the spectrum [12, 13].
These assignments are also consistent with structural parameters available from the
corresponding crystal structures, which show the central aromatic ring to have a Tshaped C-H-π interaction with neighboring molecules in the case of 1 and co-facial
π-π interactions in the case of 2.
Spectral changes as a function of rotational angle have been well documented
for π-conjugated chromophores that are linked by triple bonds, such as tolane [14],
bis(phenylethynyl)benzene (Scheme 6.2) [12, 13], and numerous structures where
electronic interactions are determined by metal centers [10b, c, 15]. While rotation of
aromatic groups relative to each other does not affect the ground state potential energy
profile, electronic excitation causes changes in bond order with single and triple bonds
acquiring cumulene character, which makes the excited state energies sensitive to
twist angles of arene groups [12, 13]. Thus, coplanar structures are fully delocalized in
the ground and excited states (shown in red in the top frame of Scheme 6.2), such that
they tend to absorb and emit at lower energies as compared to the analogous twisted
structures. In fact, twisting restricts conjugation in the excited state (Scheme 6.2,