Distance Matters: Biasing Mechanism, Transfer of Asymmetry, and Stereomutation in N-Annulated Perylene Bisimide Supramolecular Polymers

The synthesis of two series of N-annulated perylene bisimides (PBIs), compounds 1 and 2, is reported, and their self-assembling features are thoroughly investigated by a complete set of spectroscopic measurements and theoretical calculations. The study corroborates the enormous influence that the distance between the PBI core and the peripheral groups exerts on the chiroptical properties and the supramolecular polymerization mechanism. Compounds 1, with the peripheral groups separated from the central PBI core by two methylenes and an ester group, form J-type supramolecular polymers in a cooperative manner but exhibit negligible chiroptical properties. The lack of clear helicity, due to the staircase arrangement of the self-assembling units in the aggregate, justifies these features. In contrast, attaching the peripheral groups directly to the N-annulated PBI core drastically changes the self-assembling properties of compounds 2, which form H-type aggregates following an isodesmic mechanism. These H-type aggregates show a strong aggregation-caused quenching (ACQ) effect that leads to nonemissive aggregates. Chiral (S)-2 and (R)-2 experience an efficient transfer of asymmetry to afford P- and M-type aggregates, respectively, although no amplification of asymmetry is achieved in majority rules or “sergeants-and-soldiers” experiments. A solvent-controlled stereomutation is observed for chiral (S)-2 and (R)-2, which form helical supramolecular polymers of different handedness depending on the solvent (methylcyclohexane or toluene). The stereomutation is accounted for by considering the two possible conformations of the terminal phenyl groups, eclipsed or staggered, which lead to linear or helical self-assemblies, respectively, with different relative stabilities depending on the solvent.


Experimental section
General. All solvents were dried according to standard procedures. Reagents were used as purchased.
Analytical thin layer chromatography (TLC) was performed using aluminium-coated Merck Kieselgel 60 F254 plates. NMR spectra were recorded on a Bruker Avance 300 MHz (1H: 300 MHz; 13C: 75 MHz) spectrometer at 298 K using partially deuterated solvents as internal standards. Coupling constants (J) are quoted in Hz and chemical shifts (δ) in ppm. Multiplicities are denoted as follows: s = singlet, d = doublet, t = triplet, m = multiplet, br = broad. FT-IR spectra were recorded on a Bruker Tensor 27 (ATR device) spectrometer. UV-Vis spectra were registered on a Jasco-V630 spectrophotometer equipped with a Peltier thermoelectric temperature controller. Electronic circular dichroism (ECD) measurements were performed in a Jasco-J1500 spectrophotometer equipped with a Peltier thermoelectric temperature controller (Jasco MCB-100 model). The spectra were recorded in the continuous mode between 750 and 220 nm, with a wavelength increment of 0.2 nm, a response time of 1 s, and a bandwidth of 2 nm. 1 cm and a 1 mm path-length quartz cuvettes (Hellma) with screw cap were used. S-3

Synthetic details and characterization
Scheme S1. Synthesis of the reported N-annulated PBIs 1 and 2.
Compounds 3-14 and compound 1 were prepared according to previously reported synthetic procedures and showed identical spectroscopic properties to those reported therein. S2-S4 Synthesis of PBIs 1 and 2. General procedure.

Theoretical Calculations
The conformational space of a simplified monomeric unit of 1, where the long alkoxy chains attached to the peripheral benzene rings are removed and the aliphatic C10H21 chain of the pyrrolic central unit is substituted by a methyl group, was explored through the Conformer-Rotamer Ensemble Sampling Tool (CREST) utility using the xtb-6.3.3 program package. S5 Figure S5 displays the most stable conformers found for 1 after geometry optimization in gas phase at the semiempirical GFN2-xTB level of theory as implemented in the xtb program. (1) where Gstack corresponds to the free energy of the stacked aggregate with n monomeric units and Gmonomer to the free energy calculated for the monomer. To shed light on the supramolecular mechanism of the self-assembling process of compounds 1 and 2, interaction energy calculations were performed for regular oligomers of increasing size (from n = 1 to 50 monomers) at the GFN2-xTB level in gas phase. The intermolecular geometry parameters used for building up the ideal regular oligomers were extracted from the central part of fully optimized oligomers of 50 units. The binding energy per interacting pair (∆Ebind,n-1) was calculated in a similar way to as (2) where Estack is the total energy of the stacked aggregate with n monomeric units and Emonomer is the energy calculated for the monomer.
To understand the changes observed experimentally in the optical properties associated to the supramolecular polymers formed by compounds 1 and 2, a vibronic Hamiltonian similar in spirit to that proposed by F. Spano and co-workers was used to calculate the UV-Vis spectra of the aggregates. S8 The vibronic Hamiltonian can be decomposed as follows: . ( between the involved electronic states. S9 In our case, the HR factor between the ground state and the first electronic state was used. denotes the diagonal Hamiltonian involving charge transfer (CT) excited states and can be written as: where is a two-particle charge-transfer state with a cation (anion) located in the i (i+1) molecule. ECT is the energy of the CT excited states between vicinal monomers, where the cation (anion) is localized on monomer i and the anion (cation) on i+1.
is the frequency of an effective vibration for the cation and anion states, which are assumed to be equivalent. and correspond to the vibrational levels of the cation and anion states, respectively.
Finally, the term denotes the Frenkel/CT coupling Hamiltonian, which can be written as follows: where te and th denotes the electron and hole transfer integrals, and the Franck−Condon integrals , , and depend on the HR factors , , , and , respectively (see Table S1 for the values used).
The eigenstates of the Hamiltonian in Eq. 3, for which cyclic boundary conditions were applied, can be described as a linear combination of the molecular excited states as: where the ( 1) ,  ( 1 ) ,   program package. S10 The energy of the first bright electronic S0 → S1 transition was estimated from the optimization of the monomer at the TD-DFT level with the B3LYP functional S11,S12 and the 6-31G** basis. S13 The gas-phase energy was corrected to account for solvent effects (n-hexane) with the PCM approach. S14 The diabatic energy is not easy to be accurately predicted by B3LYP calculations and was set to be 0.2 eV above the bright electronic S0 → S1 transition according to the recent diabatic calculations by Negri and co-workers. S15 The effective frequency and the factor were derived from the experimental absorption spectrum of compound 1 (monomer) according to Spano and coworkers. S8 was assumed to be equal to and the ionic HR factors ( and ) were computed to reproduce the relaxation energy of the ionic species ( and ) at B3LYP/6-31G** according to and . The intermolecular parameters ( , , and ) were evaluated by using the central dimers extracted from the previously GFN2-xTB-optimized pentamers (1A5, 2A5, and 2B5). As GFN2-xTB level tends to slightly underestimate the intermolecular separation in π-stacks, the intermolecular separation between the Nannulated PBI cores were slightly elongated to 3.5 Å while maintaining the same orientation. On the basis of these corrected dimers, TD-DFT calculations were performed at the B3LYP/6-31G** level in n-hexane to estimate the intermolecular excitonic couplings by using the approximation developed by Curutchet and Mennucci (EET keyword in Gaussian). S16 The hole and electron transfer integrals were computed by employing the projected method proposed by Baumeier et al. using the data obtained from B3LYP/6-31G** calculations. S17 In the projection method, the dimer molecular orbitals are projected into the basis of the isolated-molecule molecular orbitals.
Circular dichroism spectra of monomers 1 and 2 and related oligomers were calculated at the B3LYP/6-31G** level by convoluting the lowest-lying singlet excited states with a Gaussian function broadening of FWHM = 0.2 eV. The spectra in Figure S23 are calculated for a dimer of the N-annulated PBI core at different values of the rotational angle θ along the growing axis. The spectra in Figure S24 are calculated for trimers of compound 2 optimized at the B3LYP-D3/6-31G** level following the aggregation pattern found for 2A5 and 2B5.