Angewandte
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alkynylated positions of pentacene). According to the frontier
introduced on the monomer structure instead of PBu3 units in
molecular orbital (FMO) theory, the second-order perturba-
1. Two amides were attached on both sides of s-platinated
pentacene, which reinforced packing capability via the
participation of intermolecular hydrogen bonds. In apolar
methylcyclohexane (MCH, c = 0.02 mM), the maximal p!p*
absorbance of 2 located at 685 nm, with the presence of four
isosbestic points (588 nm, 636 nm, 656 nm, and 698 nm) upon
varying the temperature (Figure S14). The results indicated
the transition from monomeric to self-assembled states upon
lowering the temperature.
tion energy (DE) reflects orbital overlapping between the
1
“termini” of acene rings and O2.[5d,e] To achieve the highest
DE value [Eq. (S1)], 1O2 adducted to aromatic rings with the
largest coefficients for the atom orbitals.[5b,d,e,7a] For 1, the
largest orbital coefficients located at the innermost ring (0.410
and ꢀ0.404 for 6,13-positions, Figure 3a), which were larger
those of 3 (0.397 and ꢀ0.397, Figure 3a).
As widely documented, circular dichroism (CD) spectros-
copy is a powerful tool to study supramolecular polymeri-
zation properties, since supramolecular chirality signals are
present for long-range ordered chiral assemblies yet absent
for monomers and oligomers.[14] To induce helical bias for 2,
optically active (S)-3,7-dimethyloctyl groups were introduced
to the peripheral side chains. At 608C, no bisignate CD signal
was detected for 2 in MCH, indicating the dominance of
molecularly dissolved state (Figure 4a). In stark contrast,
strong bisignate Cotton effect existed at 58C (Figure 4a), with
the positive maximum at 320 nm (De = 203 Lmolꢀ1 cmꢀ1) and
the negative one at 332 nm (De = ꢀ112 Lmolꢀ1 cmꢀ1). It
demonstrated the formation of helical supramolecular poly-
mers at low temperature. Supramolecular chirality signals
also emerged in the NIR region (De = ꢀ18.2 Lmolꢀ1 cmꢀ1 at
705 nm, Figure 4a, inset), ascribed to chirality transfer from
the peripheral alkyl chains to the inner s-platinated penta-
cene core. Such low-energy CD signals have been seldom
encountered in the previous helical supramolecular sys-
tems.[15]
By monitoring CD intensities at 319 nm versus temper-
ature (melting rate: 608Chꢀ1), a non-sigmoidal melting curve
was obtained for 2, with the critical temperature point (Te) at
258C (Figure 4b). Hence, 2 assembled into supramolecular
polymers in MCH via the cooperative nucleation-elongation
mechanism.[16] The thermodynamic parameters for supra-
molecular polymerization processes were acquired via the
vanꢀt Hoff plot (Figure 4c, and Figure S16). The enthalpy
(DH) and entropy (DS) values were determined to be
ꢀ62.8 kJmolꢀ1 and ꢀ121 Jmolꢀ1 Kꢀ1, respectively, suggesting
the enthalpy-driven process for supramolecular polymeri-
zation of 2.
Figure 3. Calculated atom orbital coefficients for the HOMOs (isoval-
ue=0.02) of a) pentacene-based compounds 1 and 3, together with
b) TIPS-substituted tetracene 4 and s-platinated tetracene 5. All hydro-
gen atoms in FMOs are omitted for clarity.
For tetracene derivatives, poor [4+2] photo-oxygenation
regioselectivity existed.[7a,8c] For example, two regioisomers
(5,12- and 6,11- positions) were obtained in 1:2 mole ratio
upon light irradiation of TIPS-substituted tetracene 4 (Fig-
ure 3b and Figure S9a). We sought to investigate s-platina-
tion effect on [4+2] endoperoxidation regioselectivity of
tetracenes. The s-platinated tetracene 5 (Figure 3b) favored
for the formation of 5,12-regioisomer (mole ratio of 5,12- and
6,11-regioisomers: 8.5:1, Figures S9,S10). The improved
[4+2] photo-oxygenation regioselectivity for 5 over 4 was
elucidated via the FMO theory (Figure 3b). For 4, the orbital
coefficients of 5,12-positions were 0.401 and ꢀ0.401, while
comparable values were obtained for the 6,11-positions (0.369
and ꢀ0.369, respectively). Upon s-platination, the orbital
coefficients for 5,12-positions displayed negligible changes
(0.409 and ꢀ0.403 for 5). In contrast, the 6,11-positions were
greatly perturbed, as manifested by the decrease of orbital
coefficients (0.310 and ꢀ0.313 for 5 versus 0.369 and ꢀ0.369
for 4). Both experimental and computational results demon-
strated the improvement of [4+2] photo-oxygenation regio-
selectivity via s-platination of high-order acenes.
The supramolecular polymerization mechanism was clari-
fied via DFT computations.[12] In the optimized geometry of
the dimeric stacks 22 (Figure 4d), the dihedral angles between
PEt3 ligands and pentacene unit are reduced (33.4–44.78)
compared to those of 2 (dihedral angel: 92.48 and 105.08,
Figure S5c). Pt(PEt3)2 moieties on the same side of 22 adopt
the identical twisting directions, giving rise to the rotation
angle of 4.48 for the neighbouring monomers. As a result,
steric hindrance is decreased for the dimeric stacked structure
ꢀ
22. The N H—O distances are 1.85 ꢂ and 1.91 ꢂ for the
neighbouring amides (Figure 4d), suggesting that intermo-
lecular hydrogen bonds constitute the primary driving forces
for supramolecular polymerization of 2. The conclusion is
demonstrated by 1H NMR measurements, with downfield
shifts of the amide resonances upon increasing the monomer
concentration (from 7.65 ppm at 2.00 mM to 7.81 ppm at
43.5 mM in CDCl3, Figure S17). The pentacene-pentacene
Supramolecular polymerization of s-platinated pentacene
2. NIR photo-responsiveness was further amplified at the
supramolecular level, by incorporating s-platinated penta-
cene into the structure of 2. To ensure supramolecular
polymerization of 2, less bulky PEt3 units (Figure S13) were
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
ꢀ 2021 Wiley-VCH GmbH
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Han, Yifei
