Unusual Photophysical Properties of Porphyrin-Based Supramolecular Polymers Unveiled: The Role of Metal Ligands and Side Group Amide Connectivity

Supramolecular polymers based on porphyrins are an interesting model system, since the self-assembly and thus the photophysics can be modified by the chemical structure of the porphyrins, e.g., by a metal inserted in the ligand or by different (solubilizing) side groups. Here, we investigate the photophysical properties of supramolecular polymers based on free-base and Zn-centered porphyrins, each with different amide connectivity in the side chains, by absorption and (time-resolved) photoluminescence spectroscopy on solutions. We find that for all porphyrin derivatives the B-band absorption of supramolecular polymers is a superposition of H- and J-type aggregate spectra, while the Q-band absorption indicates only J-type aggregation. The emission of supramolecular polymers stems exclusively from the Q-band and shows only J-type behavior. For supramolecular polymers based on the free-base porphyrins, we identify only a single aggregate species, whereas for Zn-centered porphyrins, two distinct species coexist in solution, each with a (slightly) different arrangement of monomers. We rationalize this complex behavior by a slip-stacking of porphyrins along the direction of one of the two B-band transition dipole moments, resulting in simultaneous H- and J-type intermolecular coupling in the B-band. In the Q-band, with its transition dipole moments oriented 45° relative to the corresponding B-band moments, only J-type coupling is thus present. Our results demonstrate that the self-assembly and the photophysics of supramolecular polymers based on porphyrins can only be fully understood if spectral information from all bands is considered.

Section S1: Q-band absorption of free-base porphyrins

Section S3: Non-radiative decay rates and (average) relative PL quantum yields
Since the PL quantum yield (PLQY) is very low for all compounds and close to the detection limit of our integrating sphere (~1 %), we report in Table S1 only relative PLQYs, i.e., the PLQY of the monomer relative to that of the supramolecular aggregate (see Materials and Methods section in the main text).Based on the definition of the PLQY as the ratio of the radiative decay rate and the total decay rate (sum of radiative and non-radiative decay rates, which we measured, Table 1 of the main text) and the measured relative PLQY, we can calculate the ratio of non-radiative rates between aggregate and monomer k nr agg / k nr mon i.e., the change of the non-radiative rate k nr upon aggregation.For the freebase porphyrin aggregates, we show in Fig. S9 the ratio of non-radiative rates as a function of the PLQY of the monomers, since we do not know exactly the latter.It is clear that for realistic values of the PLQY of around (or below) 1%, the non-radiative rate in the aggregates is significantly reduced compared to that of the free-base monomers.Since for the N-H (C=O) centred free-base porphyrin aggregates the relative PLQY is increased (decreased), the ratio of the non-radiative rates show the opposite trend with increasing monomer PLQY.
For the Zn-centred porphyrin aggregates we cannot discuss such changes in rates, since we only determine average relative PLQY due to averaging over both co-existing species.It is, however, conceivable that the species with the longer lifetime and smaller linewidths (J 1 ) features a similar reduction in non-radiative rate.For the short lifetime species (J 2 ), this is less pronounced, in S10 agreement with the more disordered aggregates that are formed with this species (see the linewidths in Table 2 of the main text).Fig. S11 compares the spectrally integrated PL decay curves (from 550 to 800 nm) of the supramolecular polymers for the two excitation wavelengths.Again we find that those are very similar, except for C=O Zn based supramolecular polymers.As outlined above, for that latter system the short-lived J 2 -species (see Table 1) is excited with higher probability, and hence the decay at short times after excitation is more pronounced.
To summarise, both the spectral and lifetime data upon excitation at 389 nm and at 413 nm are consistent with PL exclusively from supramolecular polymers.There is no detectable monomer signal after self assembly.Importantly, those spectral and lifetime features are also inconsistent with null-or X-type aggregates discussed by Spano 1 and Gierschner, 2 i.e., a specific assembly of monomers within a supramolecular polymer so that the electronic interactions cancel out (in that case no changes in lifetime and spectral shapes are observed compared to monomers).

Figure
Figure S1: Q-band absorption of the free-base porphyrin derivatives with C=O centred (top) and N-H centred side groups (bottom).The dotted lines represent the absorption spectra of the molecularly dissolved monomers in chloroform, and the dashed lines depict the absorption spectra of the supramolecular polymers in MCH.The (relative) changes of the 0-0 peaks of the Q x -and Q yabsorptions upon supramolecular polymerisation are highlighted by arrows and are indicative of Jtype aggregation with the slipped stacking of the Q-band transition dipole moments.

Figure S3 :
Figure S3: Streak camera data with transient spectra (top) and PL decay curves (right) for supramolecular polymers based on C=O FB porphyrins.

Figure S4 :
Figure S4: Streak camera data with transient spectra (top) and PL decay curves (right) for supramolecular polymers based on N-H FB porphyrins.

Figure S5 :
Figure S5: Lifetimes of the J 1 -aggregates (dots) and J 2 -aggregates (crosses) as a function of the fluence of the excitation pulses and of the repetition rate for C=O Zn based supramolecular polymers.

Figure S6 :
Figure S6: Lifetimes of the J 1 -aggregates (dots) and J 2 -aggregates (crosses) as a function of the fluence of the excitation pulses and of the repetition rate for N-H Zn based supramolecular polymers.

Figure S7 :
Figure S7: Lifetimes of the J-aggregates as a function of the fluence of the excitation pulses and of the repetition rate for C=O FB based supramolecular polymers.

Figure S8 :
Figure S8: Lifetimes of the J-aggregates as a function of the fluence of the excitation pulses and of the repetition rate for N-H FB based supramolecular polymers.

Figure S9 :
Figure S9: Ratio of non-radiative rates of aggregate and monomer as a function of the (unknown) PLQY of the corresponding monomers for the free-base porphyrins.

Figure S11 :
Figure S11: Spectrally integrated PL decays of supramolecular polymers extracted from Streak data using excitation wavelengths of 413 nm (red) and 389 nm (blue).

Table S1 :
Lifetimes of monomers in chloroform and aggregates in MCH, as well as the (average) relative PLQY for all porphyrin compounds studies here.