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were identified. Owing to the asymmetric conformation, the
absorption peak at labs = 534 nm (e = 189000 Lmolꢁ1 cmꢁ1)
dihedral angles between the BODIPY planes are 60 and 858
for 4b, while the corresponding angles for 4c are almost
identical (ca. 768). Their symmetry considerations could be
relevant to the individual molecular dipoles of the BODIPY
units to cancel it out. Interestingly, a nano-tubular assembly
was observed along the b-axis of the packing structure of 4c
(Supporting Information, Figure S15): Each core unit in the
individual column is separated by approximately 14 ꢁ and
unidirectional stacking occurs owing to the nonbonding
interaction between the fluorine and b-pyrrolic hydrogen
atoms.
and a shoulder band tailing up to ca. 650 nm (Figure 2b) are
seen. The DFT (B3LYP/6-31 G(d,p)) calculation indicates the
through-bond interactions between the molecular orbitals of
the BODIPY units for both 4b and 4c (Supporting Informa-
tion, Figures S16–S18 and Tables S4 and S5).
Accordingly, the emission wavelengths were ranged in the
far-red (lem = 609 and 641 nm for 4b and 4c in toluene,
respectively), so that the apparent Stokes shift (Dnmax
)
amounted to 2997 and 3126 cmꢁ1, respectively (Supporting
Information, Table S2). The absolute fluorescence quantum
yield (FFL) was determined to be 0.19 and 0.42 for 4b and 4c,
respectively. The fluorescence lifetime (tFL) was investigated
by means of time-correlated single photon counting
(TCSPC). Assuming a single exponential decay, the fluores-
cence lifetime of 4b was measured as 14 ns, which is
remarkably longer than that of the monomer 1 (tFL = 4 ns)
and the noncyclic BODIPY oligomers (Figure 2c).[11] In the
case of 4c, the fluorescence decay profile could be fitted with
a double exponential function, giving rise two decay constants
with 8 ns (60%) and 27 ns (40%). This suggests the presence
of two excited-state components (Figure 2d), which may arise
from the interconversion between the potential open–close
box conformers owing to the pronounced, inherent molecular
flexibility.[12] According to the Strickler–Berg law, the fluo-
rescence lifetime must be inversely proportional to the
integrated molar absorption coefficient.[13] This is consistent
with the fact that the cyclization of BODIPY fluorophores
present in 4b and 4c allows long-lived emission.[14]
Enforced arrangement of the BODIPY chromophores in
4b and 4c in solution gives rise to intriguing photophysical
properties. The UV/Vis absorption of 4b in toluene showed
an intense band that goes through a maximum at labs
=
515 nm (e = 149000 Lmolꢁ1 cmꢁ1) with a weak shoulder on
the lower energy side (> 550 nm; Figure 2a). Considering the
photophysical properties of the monomeric BODIPY 1 (labs
=
520 nm; S0!S1 transition), the split in the lowest band of 4b
could be explained by the presence of complicated inter-
chromophore exciton coupling between the moderately
angular-aligned BODIPY units.[10] For 4c, the redshifted
Interestingly, remarkable solvent effects were found in the
photophysical properties of 4b and 4c. First, the spectral
shape of absorption remained unchanged irrespective of the
solvent media, such as acetonitrile, THF, CH2Cl2, and
cyclohexane (Supporting Information, Figure S19 and
Table S2). In contrast, an increased solvent polarity was
found to bring a decrease in both the quantum yield and the
singlet state lifetimes (for example, FFL ꢀ 0.001 and tS = 35 ps
estimated by femtosecond transient absorption (TA) spec-
troscopy in acetonitrile), and the vibrational features in
fluorescence were even lost (Supporting Information, Figur-
es S20 and S21). The kinetic parameters of the radiative and
nonradiative decay rates (kr and knr) are summarized in the
Supporting Information, Table S2. In view of these, it is
inferred that the excited-state structures of 4b and 4c are
altered by the intramolecular charge transfer (ICT) in the
excited states. The electronic redistribution in the chromo-
phores that follows possibly accompanies geometric relaxa-
tion involving S0 and S1 states.
In fact, the structural relaxation of 4b in the singlet
excited-state (S1) was found to be significant in comparison
with that of 4c as inferred from the excited-state energies
(Supporting Information, Figures S22 and S23). Since the
S0!S1 (HOMO–LUMO) transitions in 4b and 4c evidently
involve ICT character, the S1!S0 transition becomes virtually
forbidden (f = 0.0003 ꢀ 0.0005; Supporting Information, Fig-
Figure 2. Spectra of UV/Vis absorption (black line) and fluorescence
(blue line) taken from a) 4b and b) 4c in toluene. The excitation
wavelength was set at the absorption maxima. Inset: comparison of
images of the solution under ambient light (left) and a UV lamp
(right). c),d) Fluorescence decay profiles of c) 4b and d) 4c. Also
shown are theoretical fits (red lines).
ure S17, Tables S4 and S5). The smaller energy gap (DES0-S1
=
1.76 eV) for 4b than that for 4c (2.10 eV) may cause more
significant structural relaxation in the excited states. These
energy level considerations may explain the lower fluores-
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3
These are not the final page numbers!
Ishida, Masatoshi
