out numerous computational and experimental studies to
explore the origins of the red shifts and demonstrated that
the nonplanar deformation is the primary factor for the shift,
rather than the early in-plane nuclear reorganization (IPNR).9
Schweitzer-Stenner13 et al. subsequently offered a clearer
explanation using the static normal coordinate deformations
(SNCDs) theory. Their results implied that the spectral shifts
of the tetrapyrrole macrocycle are potentially useful indica-
tors of the unique changes in the ring nonplanarity.
To clarify the relationship between the spectral shifts and
the distortions to the porphyrin rings, we have (i) developed
an efficient approach to synthesize a series of nonplanar
porphyrins 5,15-meso,meso-strapped with different degrees
of distortion as a model system (see the Supporting Informa-
tion, S2) and (ii) augmented the previous “deformation”
theories and experimentally demonstrated that the spectral
red-shift of the nonplanar porphyrins is primarily due to the
hybrid orbital deformation (HOD) effect of the distortion in
the tetrapyrrole macrocycle.
Figure 1. Strapped porphyringen (top) and strapped porphyrin
(bottom). Take the 1,6-hexanediyl strap as the example.
and characterized according to the above mentality. Strapped
materials 6-10 were obtained from the condensation of
dialdehyde compounds 11-15 with a substituted dipyr-
romethane 16 under a molar ratio of 1:224 and then
complexed with zinc(II) to afford compounds 1-5 (Scheme
1). The dialdehyde compounds 11-15 and the dipyr-
romethane 16 were prepared according to the methods of
Whitlock25 and Lindsey,26 respectively. These types of
strapped structures effectively avoid the disturbances due to
substituent effects and the exchange of conformations in
periphery crowded porphyrins that represent the most popular
distorted porphyrin samples prepared.10,27 Direct measure-
ment of the compounds was achieved using NMR and
UV-vis spectral methods and a high-resolution mass
spectrometry (HR-MS) technique (see the Supporting Infor-
mation).
Shorter alkyl straps induce a stronger degree of distortion
to the porphyrin, and the length of the straps can be flexibly
adjusted by their carbon atom number (nC: from 7 to 3). The
1H NMR spectra of compounds 1-5 indicated a distorted
handbasket-type conformation (Scheme 1). All proton signals
in the straps showed a distinct upfield shift resonating at <3.0
ppm (Figure 2). This observation is because the straps
vertically locate to one side of the large π-system planarity28
and their protons are shielded by the porphyrin π-system.29
The distortive degree of these porphyrins can be judged
by the chemical shift of straps’ protons, e.g., the chemical
shift of ether methylene proton R decrease as the alkyl straps
shortened. For compound 5, the signal R appeared at 2.8
The field of strapped-metalloporphyrin functional assembly
has seen tremendous growth, with continuous emergence of
exciting applications in selective guest inclusion,14 photo-
induced electron transfer,15 molecular device,16,17 and recep-
tor model,18 and so on. Strapped metalloporphyrins also show
good potential in the fields of catalysis19,20 and asymmetric
catalysis.21
Porphyrin skeletons are known to originate from their
corresponding porphyrinogen, a nonplanar calyx[4]pyrrole
macrocycle,22 by using the condensation of aldehyde and
pyrrole, thus allowing the meso substituents A and B (or C
and D) to approach each other (Figure 1). If the two
substituents were bridged by a shorter strap (e.g., a 1,6-
hexandiyl group or shorter) before formation of porphyrino-
gen, the resulting strapped porphyrin would exhibit a ruffling
distortion10 after its oxidation due to the competetion between
the binding of the alkyl strap and the coplanarity of the
porphyrin π-system.23
A series of 5,15-meso,meso-strapped porphyrins with
different degrees of ruffling distortion have been synthesized
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