Bimolecular Sandwich Aggregates of Porphyrin Nanorings

Abstract

Extended π-systems often form supramolecular aggregates, drastically changing their optical and electronic properties. However, aggregation processes can be difficult to characterize or predict. Here, we show that butadiyne-linked 8- and 12-porphyrin nanorings form stable and well-defined bimolecular aggregates with remarkably sharp NMR spectra, despite their dynamic structures and high molecular weights (12.7 to 26.0 kDa). Pyridine breaks up the aggregates into their constituent rings, which are in slow exchange with the aggregates on the NMR time scale. All the aggregates have the same general two-layer sandwich structure, as deduced from NMR spectroscopy experiments, including ¹H DOSY, ¹H-¹H COSY, TOCSY, NOESY, and ¹H-¹³C HSQC. This structure was confirmed by analysis of residual dipolar couplings from ¹³C-coupled ¹H-¹³C HSQC experiments on one of the 12-ring aggregates. Variable-temperature NMR spectroscopy revealed an internal ring-on-ring rotation process by which two π-π stacked conformers interconvert via a staggered conformation. A slower dynamic process, involving rotation of individual porphyrin units, was also detected by exchange spectroscopy in the 8-ring aggregates, implying partial disaggregation and reassociation. Molecular dynamics simulations indicate that the 8-ring aggregates are bowl-shaped and highly fluxional, compared to the 12-ring aggregates, which are cylindrical. This work demonstrates that large π-systems can form surprisingly well-defined aggregates and may inspire the design of other noncovalent assemblies.

Figure 1

Removal of pyridine from porphyrin nanorings (c-PN) leads to self-assembly into discrete dimer ring aggregates if the ring size (N = 8 or 12, rather than 6) and side chain type are right (OOct or t-Bu, rather than THS). Ar groups correspond to 3,5-bis(octyloxy)phenyl (OOct), 3,5-bis(tert-butyl)phenyl (t-Bu), and 3,5-bis(trihexylsilyl)phenyl (THS).

Figure 2

Absorption spectra of 12-porphyrin nanorings c-P12_THS, c-P12_OOct, and c-P12_t-Bu in the absence and presence of pyridine (1% by volume of solvent) recorded in CDCl₃ at 25 °C (concentration ca. 1 μM).

Figure 3

Model of the (c-P12_t-Bu)₂ aggregate. Teal and purple coding of carbon and nitrogen atoms have been used to highlight the two separate rings of the aggregate, while zinc atoms are in red. Hydrogen atoms have been omitted for clarity.

Figure 4

a) General structures with proton labels of free nanorings and their bimolecular aggregates. b–e) ¹H and ¹H DOSY spectra (500 MHz, CDCl₃, 25 °C) of c-P12_t-Bu, c-P8_t-Bu, c-P12_OOct, and c-P8_OOct in the presence and absence of pyridine. * = residual solvent signal, Py = pyridine.

Figure 5

Comparative plots of HSQC cross-peaks for c-P12_t-Bu, c-P8_t-Bu, c-P12_OOct, and c-P8_OOct in the presence (pink) and absence of pyridine (green). The grouping of cross-peaks measured for aggregates (green) around cross-peaks measured for their free rings in the presence of pyridine (pink) was used to correlate between different types of protons in the free rings and their aggregates. ¹H signals of “a” type β-pyrrole protons in all aggregates are broadened out beyond the limit of detection; thus, only the point for the free rings in the presence of pyridine is shown. Similarly, “b” type resonances are not observable in (c-P8_t-Bu)₂, only in the corresponding free ring.

Figure 6

¹H–¹H NOESY analysis of (c-P12_t-Bu)₂. a) A 1/12th fragment of the model with the key calculated intermolecular distances used to distinguish between the exterior (e)/interior (i) and up (u)/down (d) proton environments. Hydrogen atoms of the t-Bu groups are omitted for clarity. b) Region of the NOESY spectrum highlighting key differences in NOE magnitudes for the assignment. c) Calculated versus experimental distances for the (c-P12_t-Bu)₂ aggregate.

Figure 7

Variable-temperature ¹H NMR spectra of the bimolecular aggregate of c-P12_t-Bu. a) Structure of (c-P12_t-Bu)₂ with proton labeling at −50 °C and ¹H NMR spectra from +50 °C to −50 °C, recorded at 500 MHz in CDCl₃. * = residual solvent signal; x = CDCl₃ satellite signals; Ex = exchange, detected by ¹H–¹H EXSY (ROESY) spectroscopy. b) Cartoon representation of the ring-on-ring rotation process in (c-P12_t-Bu)₂ leading to fast exchange of β-pyrrole protons at +50 °C, and slow exchange at −50 °C.

Figure 8

a) ¹H–¹H EXSY (NOESY) spectrum of a 2.2:1.0 mixture of (c-P8_OOct)₂ and c-P8_OOct·Py₈ showing direct exchange correlations between the two states (500 MHz, CDCl₃, 25 °C, t_mix = 200 ms). b) Cartoon representation of the equilibrium between c-P8_OOct·Py₈ and (c-P8_OOct)₂ giving rise to exchange correlations in 2D EXSY. Key exchange correlations between disaggregated ring and aggregate include: a ↔ a_i, a ↔ a_e, o ↔ o_iu, o ↔ o_eu, o ↔ o_id, o ↔ o_ed, p ↔ p_i, and p ↔ p_e.

Figure 9

a) Structure of (c-P8_OOct)₂ with arrows indicating three internal rotation processes: interior aryl rotation (blue arrow), porphyrin rotation (green arrow), and exterior aryl rotation (red arrow), identified based on characteristic proton exchanges listed below. b) From top to bottom: selective ¹H EXSY NMR spectra (CDCl₃, 500 MHz, 25 °C) at different mixing times for targeted protons p_i (t_mix 2.4–200 ms), o_iu (t_mix 2.4–220 ms), c_iu (t_mix 2.4–220 ms), and ¹H NMR spectrum of (c-P8_OOct)₂.

Figure 10

Magnetic field dependence of the aryl side chain ¹H–¹³C total splittings (¹T_CH) for the (c-P12_t-Bu)₂ bimolecular aggregate. a) ¹³C-coupled HSQC spectra for ortho-aryl resonances o_ed and o_id, showing a field-dependence of ¹T_CH resulting from a dipolar coupling contribution. b) Dependence of the observed ¹T_CH on the square of the magnetic field strength, plotted for both ortho- and para-aryl side chain protons. c) Root-mean-square deviation between experimental and calculated RDCs for model geometries of (c-P12_t-Bu)₂ with varying degrees of porphyrin rotation, given as the angle θ between the plane of the ring (mean plane of 12 Zn atoms) and the plane of the porphyrin units (24-atom mean plane).

Figure 11

Molecular dynamics simulations of (c-P12_t-Bu)₂ and (c-P8_OOct)₂. a) Distribution of angles (as defined in Figure 10c) between porphyrin units and the plane of the nanoring defined by all zinc atoms, together with representative geometries for some of the most populated angles. Side chains and hydrogen atoms are omitted for clarity. b) Distribution of porphyrin zinc atom positions in one ring projected onto the plane of the other ring of the aggregate.