Organic Letters
Letter
belonged to the monoclinic system with a P21/c space group,
whereas those of 4d were triclinic with a P-1 space group.
Details of the crystallographic data are included in Tables S1−
S4. The inset of Figure 2 shows the overall change in the
tricyclic core as 3d transforms to 4d. The central carbon
retained sp3 geometry with minimum compromise from typical
bond angles (Table S3), whereas the atoms situated along the
outer rim were found to reach a near-planar arrangement to
facilitate delocalization of electrons! Hyperconjugative inter-
action of the central C−CH3 σ-bond with suitably aligned
antibonding orbitals may also be contributing to the overall
stability. Notable changes were seen in bond angles and bond
lengths around the carbon that carried the tert-hydroxyl group.
Flattening of the structure is associated with widening of angle
∠ijk from 113.10° (3d) to 140.30° (4d) as well as ∠cbg from
107.2° (3d) to 114.2° (4d). A small decrease in bond lengths
of Cb−Cj (1.549 to 1.451 Å); Nc−Cl (1.398 to 1.368 Å), Ck−
Cj (1.529 to 1.396 Å), Cj−Ci (1.530 to 1.421 Å) and an
increase in Ck−Cl bond length (1.354 to 1.425 Å) are also
tricyclic moiety. Therefore, the electronic transitions appear to
involve an intramolecular charge transfer from the 1,2-diaryl
fragment to the aza-tricyclic core.
The emission spectra of these compounds were subse-
quently recorded in different solvents (Figure S5). Compound
4a exhibited dual emission in DCM with λem at 457 and 520
nm. An identical emission pattern with λem at 442 and 584 nm
was observed in compound 4b. Their behavior in other
solvents such as toluene, ethyl acetate, and acetonitrile were
also similar (Table S5). On the contrary, the emission of 4c
having an −OMe group on the C-3 aryl ring displayed a broad
emission band in all of the solvents studied; in DCM its λem
was centered around 600 nm. The compound 4d having
−OMe group on C-2 and C-3 aryl rings had an emission at 588
nm in DCM. The emission spectra of these compounds in
DCM are shown in Figure 3c, and the data from other solvents
The red-shift in emission on introducing −OMe illustrates
the tunable photophysical behavior of these cyclazines. The
quantum yields of 4a, 4b, 4c, and 4d in DCM were estimated
as 0.3%, 1.24%, 1.82%, and 2.33%, respectively (Table S6).
Overall, these observations suggest that the aryl rings at C-2
and C-3 positions exert different electronic effects on the aza-
tricyclic core and also show the potential of this unit to act as
an efficient acceptor in charge-transfer-mediated photophysical
responses.
Impressive colors of 4a−d in the solid and solution states
(Figure 3) and their attractive emission behavior prompted us
Conventional flat aromatic molecules exhibit excellent
emission in solution, but their solid-state emissions are
generally mediocre, which limits their application poten-
tial.37,38 The peripherally conjugated heterocyclic framework
discussed here resembles a “Vietnamese hat” with a shallow
cone, and could be an ideal framework for developing solid-
emissive materials. The structure and lattice arrangement of 4d
presented in Figure 4 shows a number of features which favor
this.
The lattice is formed from enantiomers (E1 and E2, Figure
4a) of this molecule arranged in separate rows. The unique
geometry of the core along with its substitution pattern
prevented them from establishing any significant secondary
interaction with one another; the only proximity was seen
between C-3 aryl ring of one enantiomer with that of another.
They existed in a “parallelly displaced” arrangement, with an
offset distance of about 2.7 Å and centroid−centroid distance
Figure 3. (a) Solutions of 4a−d in DCM when viewed under 352 nm
light. (b) Solid samples of 4a−d viewed under 352 nm light. (c)
Normalized emission spectra of 3.33 × 10−5 M solutions of 4a−d in
DCM. (d) Normalized solid-state emission spectra of 4a−d.
of 3.17 Å (Figure 4b).39 At the same time, BF4 counterions
−
and cocrystallized water molecules were found to form
independent channels and bridge these enantiomers efficiently
(Figure S1). On one hand, absence of efficient π-stacking of
chromophores can be expected to thwart the quenching
process after excitation, and on the other, their interactions
involving counterions and the water molecules could decrease
rotational possibilities. To know whether these factors would
favor good emission in the solid state, a systematic study was
carried out. The solid-state emission behavior and spectra of
these compounds are shown in Figure 3b,d. The parent
molecule 4a had λem at 503 nm, whereas the compounds 4b,
4c, and 4d showed emissions at 546, 595, and 601 nm
respectively.
The red-shift in emission wavelength with substitution
parallels their behavior in solution and provides additional
support in favor of the tunable photophysical features of the
aza-tricyclic scaffold. The absolute quantum yields of 4a, 4b,
and 4d were recorded using an integrating sphere and were
found to be 2.85, 4.10, and 11.4, respectively. These values
to investigate their photophysical characteristics in a greater
detail. Those containing NO2 group (4e, 4g, and 4f) were
brown gummy solids and were not included in the study.
Absorption spectra of these compounds (4a−d) were
recorded in toluene, dichloromethane (DCM), ethyl acetate,
DCM showed three absorption bands at 330, 402, and 490 nm.
Similar behavior was observed in toluene and acetonitrile, with
slight variation in the wavelengths of absorption. Compounds
4b and 4c having an −OMe group at the para-position of the
aryl ring at either C-2 or C-3 also exhibited similar absorption
behavior (Table S5). The compound 4d with an −OMe group
on both these rings displayed four absorption bands in most of
the solvents. The nature of these electronic transitions was
analyzed by TD-DFT calculations using the B3LYP/6-31G(d)
Table S7). In this compound, the occupied orbitals were found
to have their major contribution from the 1,2-diaryl fragment,
while the unoccupied orbitals are mainly located on the aza-
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Org. Lett. 2021, 23, 3354−3358
Chakkumkumarath, Lakshmi
