Excited-state symmetry breaking in 9,10-dicyanoanthracene-based quadrupolar molecules: the effect of donor–acceptor branch length†

Excited-state symmetry breaking is investigated in a series of symmetric 9,10-dicyanoanthracenes linked to electron-donating groups on the 2 and 6 positions via different spacers, allowing for a tuning of the length of the donor–acceptor branches. The excited-state properties of these compounds are compared with their dipolar single-branch analogues. The changes in electronic structure upon their optical excitation are monitored by transient electronic spectroscopy in the visible and near-infrared regions as well as by transient vibrational spectroscopy in the mid-infrared. Our results reveal that, with the shortest branches, electronic excitation remains distributed almost symmetrically over the molecule even in polar environments. Upon increasing the donor–acceptor distance, excitation becomes unevenly distributed and, with the longest one, it fully localises on one branch in polar solvents. The influence of the branch length on the propensity of quadrupolar dyes to undergo excited-state symmetry breaking is rationalised in terms of the balance between interbranch coupling and solvation energy.


S1.2.1 Stationary measurements
Electronic absorption spectra were recorded on a Cary 50 spectrometer. Stationary fluorescence spectra were measured using a Horiba FluoroMax-4 spectrofluorometer and corrected using a set of secondary emissive standards. [2] Stationary IR absorption spectra were recorded with a Bio-Rad Excalibur FTIR spectrometer in a NaCl cell using THF as solvent.

S1.2.2 Time-resolved infrared spectroscopy
Time-resolved infrared (TRIR) spectroscopy measurements were performed using a setup described in detail elsewhere. [3,4] Briefly, the pump pulses at 450 nm were generated by doubling the 900 nm output of an optical parametric amplifier (TOPAS C, Light Conversion) pumped by a Ti-Sapphire amplified laser system (Spectra-Physics, Solstice amplifier, 1 kHz). The pump pulses at 400 nm were produced by doubling the 800 nm output of the same amplifier. The IR probe pulses were generated from another part of the output of the same laser system using an optical parametric amplifier (TOPAS C, Light Conversion) and a non-collinear difference frequency mixing module (NDFG, Light Conversion). The polarisations of the pump and probe pulses were controlled and the pump pulses were set at magic angle relative to the probe. The probe pulses were dispersed in a Triax 190 spectrograph (Horiba, 150 lines/mm) and detected with an MCT array (2 × 64 pixels, Infrared Sytems Development). All TRIR data were measured using a flow-through cell made of CaF 2 windows with a 500 µm spacer. The absorbance of the samples at 450 nm was less than 0.3 in the cell.

S1.2.3 Electronic transient absorption spectroscopy
-UV-Vis transient absorption spectroscopy UV-Vis transient absorption spectra were recorded on a setup described in detail elsewhere. [5,6] In brief, the pump pulses at 460 nm were generated using a TOPAS-Prime combined with a frequency mixer (Light Conversion) and the TOPAS was pumped by a Ti-Sapphire amplified system (Spectra-Physics, Solstice Ace amplifier, 1 kHz). The pump pulses at 400 nm were generated by doubling the 800 nm output of the same amplifier. The white light continuum probe pulses were produced by focusing the 800 nm pulses of the amplifier in a CaF 2 plate. The polarization of both, pump and probe pulses, was controlled and the pump was set at magic angle relative to the probe. The signal was recorded applying a referenced detection system of two spectrographs. Pixel to wavelength conversion was performed using a NIST 2065 standard. The measured data were chirp-corrected using the optical Kerr effect measured in the neat solvent samples. All the samples were measured in 1 mm quartz cuvettes and bubbled with N 2 . A wavelength dependent instrument response function (IRF) was observed and an IRF of 200-350 fs was achieved. The absorbance of the sample at the pump wavelength was less then 0.3 in the 1 mm quartz cuvette.
-Near-IR transient absorption spectroscopy Near-IR transient absorption spectroscopy was performed using a setup similar to the UV-Vis transient absorption setup and the same amplifier was used to generate the pump and probe pulses. The main differences are as follows, all the samples were excited at 400 nm. The probe pulse were generated by focusing the 800 nm output of the amplifier in a YAG plate. The detection was done by a referenced home-built prism spectrometer consisting of two InGaAs detectors. For balancing the white light continuum spectrum, apodizing neutral density filters were placed front of both detectors. [6] -Merging the UV-Vis and Near-IR spectra In order to reconstruct the electronic transient absorption spectra from 6500 cm −1 to 30000 cm −1 , the chirp corrected UV-Vis and near-IR spectra were then merged by comparing the overlapping ∼710-770 nm spectral region. The near-IR spectra were multiplied by a constant factor for correcting for the difference of pump power, as well as the differences of sample absorbance at the pump wavelength.

S1.3 Quantum-chemical calculations
Quantum-chemical calculations were performed using the Gaussian16 package. [7] The ground-state and excited-state geometries were optimised without symmetry restriction at the density functional theory (DFT) level and time-dependent DFT (TD-DFT) level using the ωB97X-D long-range corrected hybrid density functional, [8] with the standard 6-31+G* basis set in vacuum. Frequency S5 calculations were performed on the optimised structures at the DFT level and no imaginary frequencies were found, indicating that the observed stationary points are real minima of the potential energy surface (PES).

S2.1 Global analysis
Global analysis was performed on the merged electronic TA spectra and TRIR spectra assuming a series of consecutive exponential steps with increasing time constants. The resulting evolution associated difference spectra (EADS) are shown in the main text figures.

S2.3 Polarity dependent down-shift of the symmetric -C≡N stretching mode in D2
The absence of the symmetric -C≡N stretching mode in D2 in BCN was investigated by measuring the TRIR spectra in solvents of varying polarity. The resulting spectra are shown in Figure S31. They point to a polarity dependent redshift of the symmetric stretching mode. When measuring in DBE, the symmetric -C≡N stretching mode red-shifts by ∼10 cm −1 compared to the spectra obtained in CHXene, while in BCN and NMF, it is further red-shifts and completely overlaps with the more intense antisymmetric mode. Figure S31: TRIR spectra measured with D2 in cyclohexene, dibutyl ether, benzonitrile and Nmethylformamide S18 S3 Solvation energy of a quadrupole vs. a dipole The magnitude of a quadupole moment, Q, consisting of two local dipoles of magnitude µ/2 is ( Figure S32A): where d is the centre-to-centre distance between the two (point) local dipoles. Figure S32: Definition of the cavity radius for the quadupolar state (A, B) and the dipolar state (C) According to the continuum model, the dipolar solvation energy of a quadrupole is: [9] E s,Q = − Q 2 8π 0 a 5 where a Q is the cavity radius ( Figure 1A), and ∆f = f ( s ) − f (n 2 ), with f (x) = 2(x − 1)/(2x + 1).
With the assumption that d ∼ a Q , eq.S2 becomes: The dipolar solvation energy of a dipole of size µ in a cavity of radius a D is ( Figure S32C): [9] E s,D = − µ 2 8π 0 a 3 D ∆f. (S4) Comparison of eq.S3 and S4 shows that, because the cavity radius of the quadrupole is larger than that of the dipole, the solvation energy of the dipolar state is larger than that of the quadrupolar state.
Assumption that a Q ∼ 2a D gives: This approach probably underestimates E s,Q because of the large cavity radius.
Alternatively, if the quadrupole is described as two local dipoles, each of radius a D ( Figure  S32B), the solvation energy is: Here, the solvation energy is probably overestimated. Nevertheless, both eq.S5 and S6 indicate that dipolar solvation favours the purely dipolar state D rather than the symmetric quadrupolar state Q.