Breaking the Symmetry in Molecular Nanorings

Because of their unique electronic properties, cyclic molecular structures ranging from benzene to natural light-harvesting complexes have received much attention. Rigid π-conjugated templated porphyrin nanorings serve as excellent model systems here because they possess well-defined structures that can readily be controlled and because they support highly delocalized excitations. In this study, we have deliberately modified a series of six-porphyrin nanorings to examine the impact of lowering the rotational symmetry on their photophysical properties. We reveal that as symmetry distortions increase in severity along the series of structures, spectral changes and an enhancement of radiative emission strength occur, which derive from a transfer of oscillator strength into the lowest (k = 0) state. We find that concomitantly, the degeneracy of the dipole-allowed first excited (k = ±1) state is lifted, leading to an ultrafast polarization switching effect in the emission from strongly symmetry-broken nanorings.

Scheme S1: Detailed chemical structures of the porphyrin oligomers used in this study. The sidechain [trihexylsilyl, Si(C 6 H 13 ) 3 or 3-((2-ethylhexyl)alkoxy)] has no significant effect on the absorption or photoluminescence spectra of the compounds, but it influences the solubility, aggregation behavior and ease of purification. TIPS = triisopropyl silyl. Si(C 6 H 13 ) 3

Porphyrin Monomer P1
Dipyrromethane (3.00 g, 20.5 mmol) and 3-((2-ethylhexyl)oxy)benzaldehyde (4.81 g, 20.5 mmol) were dissolved in oxygen-free CH 2 Cl 2 (2.5 L) in the dark. TFA (0.90 mL, 28.7 mmol) was added and the reaction was stirred in the dark for 3 h. Then, DDQ (6.81 g, 30.0 mmol) was added and the mixture stirred for 20 mins before Et 3 N (7.5 mL, 54 mmol) was added. The volume was reduced and the mixture passed through a short silica gel column (CH 2 Cl 2 ) to partially remove the side products. The product was carried forward to the next step without        Figure S6 shows extinction coefficient and PL emission spectra at room temperature for all porphyrin nanorings under investigation. Extinction coefficient spectra were measured over the range of 350-1000 nm and the emission spectra were measured in the range of 800-1500 nm at an excitation wavelength of 500nm corresponding to the Soret band.

Time-Resolved Photoluminescence Spectroscopy
The photoluminescence (PL) upconversion technique was engaged to investigate the PL dynamics of sample solutions held in quartz cuvettes. An excitation pulse was generated by a mode-locked Ti:Sapphire laser with pulse duration of 100 fs and a repetition rate of 80 MHz. PL is collected and optically gated in a beta-barium-borate (BBO) crystal by a vertically polarized time-delayed gate beam. The upconverted signal, which consists of sum-frequency photons from the gate pulse and the vertical component of the PL, was collected, dispersed in a monochromator and detected using a nitrogen-cooled CCD. Using a combination of a half-wave plate and a Glan-Thompson polarizer, the polarization of the excitation pulse was varied and the PL intensity dynamics were recorded separately for components polarized parallel ( || ) and perpendicular ( ! ) to the excitation pulse polarization as shown in Figure S7a) for l-P6·T6 as an example. The PL anisotropy is defined using = and calculated from the measured components. 2,5,6 The full-width-half-maximum of the instrumental response function was measured to be ~270 fs, which gives the time-resolution limit of the system. The anisotropy dynamics are constant within at least 15ps after excitation, as shown for l-P6·T6 in Figure S7b) as an example for such traces. To obtain a representative value for the PL anisotropy as a function of excitation wavelength, eight PL intensity values were independently recorded at 5 ps delay for each polarization direction, averaged and the equation above was applied. The concentration of the sample solutions was on the order of 10 -4 mol/L. To achieve a good signalto-noise ratio and comparability, the detection wavelength was chosen to be near the emission peak for each sample and kept constant for all excitation wavelengths. To investigate PL decay dynamics at longer delay time after excitation electronic gating through time-correlated single-photon counting (TCSPC) technique was used (Becker&Hickl module). Here, the emission was detected with a silicon single-photon avalanche diode, yielding a temporal resolution of around 40 ps. By fitting the PL intensity (I) decay to a single exponential decay model, = !!/! , the lifetime of the excitation was extracted.

Quantum Yield and Radiative Rate
As quantum yield measurements had been carried out on porphyrin nanorings in previous studies, 7 a relative approach was adopted in this study using l-P6 as a reference: where I is the integrated area of PL emission spectrum at the excitation wavelength !"# and !"# is the absorbance at !"# . The QY of the reference compound l-P6 is 28%. The quantum yields for each compound was independently measured three times and the average was taken.
Steady-state absorption and time-integrated PL spectra at room temperature were recorded using a PerkinElmer Lambda 1050 UV/VIS/NIR spectrometer and a Horiba FluoroLog fluorimeter respectively.
Using the obtained quantum yield and the overall decay rate Γ !"!#$ extracted from the PL transients (Γ !"!#$ = 1/ ), the radiative (Γ ! ) and non-radiative (Γ !" ) rate were extracted using: The non-radiative rate is approximately a factor of ten larger than the radiative rate for all compounds except the symmetry-broken l-P6·T6, where Γ !" is only larger by a factor of four.
The observed PL transients are dominated by the contribution from the non-radiative decay.