The magic of biaryl linkers: the electronic coupling through them defines the propensity for excited-state symmetry breaking in quadrupolar acceptor–donor–acceptor fluorophores

Charge transfer (CT) is key for molecular photonics, governing the optical properties of chromophores comprising electron-rich and electron-deficient components. In photoexcited dyes with an acceptor–donor–acceptor or donor–acceptor–donor architecture, CT breaks their quadrupolar symmetry and yields dipolar structures manifesting pronounced solvatochromism. Herein, we explore the effects of electronic coupling through biaryl linkers on the excited-state symmetry breaking of such hybrid dyes composed of an electron-rich core, i.e., 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP), and pyrene substituents that can act as electron acceptors. Experimental and theoretical studies reveal that strengthening the donor–acceptor electronic coupling decreases the CT rates and the propensity for symmetry breaking. We ascribe this unexpected result to effects of electronic coupling on the CT thermodynamics, which in its turn affects the CT kinetics. In cases of intermediate electronic coupling, the pyrene-DHPP conjugates produce fluorescence spectra, spreading over the whole visible range, that in addition to the broad CT emission, show bands from the radiative deactivation of the locally excited states of the donor and the acceptors. Because the radiative deactivation of the low-lying CT states is distinctly slow, fluorescence from upper locally excited states emerge leading to the observed anti-Kasha behaviour. As a result, these dyes exhibit white fluorescence. In addition to demonstrating the multifaceted nature of the effects of electronic coupling on CT dynamics, these chromophores can act as broad-band light sources with practical importance for imaging and photonics.


Steady-state optical spectroscopy and emission decays
Spectrophotometric-grade solvents were used for all optical-spectroscopy studies.Steady-state absorption spectra are recorded in a transmission mode using a JASCO V-670 spectrophotometer (Tokyo, Japan).The steady-state emission spectra and the time-correlated single-photon counting (TCSPC) fluorescence decays are measured, using a FluoroLog-3 spectrofluorometer (Horiba-Jobin-Yvon, Edison, NJ, USA), equipped with a pulsed diode laser (λ = 406 nm, 200 ps pulse width) and a pulsed light emitting diode (λ = 278 nm, 1 ns pulse width).
The fluorescence quantum yields, Φf (Table 1), are determined by comparing the integrated emission intensities of the samples with the integrated fluorescence of a reference sample with a known fluorescence quantum yield, Φf0: (eq.S1) Where F(λ) is the fluorescence intensity at wavelength λ; A(λex) is the absorbance at the excitation wavelength; n is the refractive index of the media; and the subscript "0" indicates the quantities for the reference sample used.The reference sample was an ethanol solution of coumarin 151, Φf0 = 0.53.
The TCSPC emission decays were recorded at the fluorescence maxima.Signals from the Rayleigh scattering of the excitation pulse from MQ water was used for the instrument-response function (IRF).The emission decays were fitted to exponential functions using deconvolution algorithm employing the IRF signals.Table 1 reports the from single-exponential fits and the intensity-average lifetimes from multi-exponential fits.

Transient-absorption spectroscopy
Spectrophotometric-grade solvents were used for all transient-absorption (TA) measurements.The TA spectra at each time point, ΔA(λ, t), are recorded in a transmission mode with 2-mm quartz cuvettes using Helios pump-probe spectrometer (Ultrafast systems, LLC, Florida, USA) equipped with a 3.2-ns delay stage allowing 7-fs temporal step resolution.The laser source for the Helios is a SpitFire Pro 35F regenerative amplifier (Spectra Physics, Newport, CA, USA) generating 800nm pulses (≥35 fs, 4.0 mJ, at 1 kHz).The SpitFire amplifier is pumped with of an Empower 30 Qswitched laser ran at 20 W at the 2 nd harmonic; and a MaiTai SP oscillator provides the seed beam (55 nm bandwidth). 2 nd -harmonic and 3 rd -harmonic generators of the 800-nm pulses provided 400-nm and 266-nm pump, respectively; and attenuated 800-nm pulses, passed through a Ti:Sapph white-light generator, provided the probe.Global-fit (GF) analysis of the TA data, ΔA(λ, t), yields the evolution associated difference spectra (EADS) with the corresponding time constants, τ, of the sequential transitions.

Dependence of the emission on the excitation wavelength
These N-substituted pyrene-pyrrolopyrroles conjugates tend to manifest fluorescence from different locally excited and CT states.Therefore, we examine the effects of excitation wavelength, λex, on the emission spectra of N1-Pyr/tBu and N2-Pyr that manifest dual fluorescence, i.e., long-wavelength and short-wavelength emission bands, for all solvents (figure S28 and S29).Shifting λex from 310 nm to 400 nm in increments 10, 15, and 20 nm, results in huge variations of the ratio between the amplitudes of the short-wavelength and long-wavelength fluorescence.
Upon excitation at 310 nm, the broad long-wavelength fluorescence exhibits strongest contribution to the emission spectra of N1-Pyr/tBu and N2-Pyr for all solvents (Figure S28, S29).It suggests that the upper excited states, accessible at this short wavelength, relax preferentially to an emissive CT state.One cannot preclude also a possibility for a direct transition to an upper CT state, i.e., S0→CT (FC) , with high oscillator strength.
Increasing λex to about 340 nm leads to a growth of the relative intensity of the sharp 390-nm peaks, ascribed to fluorescence from the pyrene locally excited state in the spectra of N1-Pyr/tBu (Figure S28).A further increase in λex to 400 mn induces a sharp growth of a mission band ascribed to fluorescence for an excited state localized on the pyrrolopyrrole.For toluene and DCM, when λex exceeds 400 mn, only weak long-wavelength emission emerges consistent with direct photoexcitation to a CT state with small osculator strength.For DMF, long excitation wavelengths result in emission spectra dominated by the pyrrolopyrrole fluorescence and lacking the CT band (Figure S28c).Lowering the energy level of the CT state in the polar DMF medium enhances the rates of non-radiative deactivation.
In the case of N1-Pyr, the largest contributions from the short-wavelength fluorescence emerges as increases from 310 nm to about 340 nm.This wavelength range covers the distinct absorption peaks ascribed to S0→S2 pyrene transition.Nevertheless, the emission spectra do not reveal such sharp features corresponding to pyrene S1→S0 radiative deactivation (Figure S29).The short-wavelength emission of N2-Pyr, however, exhibits an unusually long 40-ns decay components (Figure S22b).

Electrochemistry and spectroelectrochemistry
Anhydrous aprotic solvents were used for all electrochemical measurements and tetrabutylammonium hexafluorophosphate (electrochemistry grade, ≥99.0%) served as a supporting electrolyte.Cyclic voltammetry is conducted using Reference 600 TM Potentiostat/Galvanostat/ZRA (Gamry Instruments, PA, U.S.A.), connected to a three-electrode cell.Using the conditions that provide optimal reversibility of the oxidation of the pyrolopyrrole samples, we assembled a three-electrode cell in a 2 mm x 10 mm quartz cuvette placed in a UV/Vis spectrometer.The light was transmited through a platinum mesh that served as a working electrode.The spectra of the intensity of the transmitted light, I(λ, t), were recorded at 100-mV increments through the potential sweeps.The differential absorption spectra, ΔA(λ, t), at different time points were obtained by using the initially recorded intensity (at time 0) as a baseline, i.

S29
Theoretical analysis: We have performed the DFT and TD-DFT calculations with Gaussian 16. [1] For all systems we kept the actual structures w/o simplification.Default Gaussian16 thresholds and algorithms were used but for an improved optimization threshold (10 -5 au on average residual forces), a stricter self-consistent field convergence criterion (10 -10 a.u.) and the use of the ultrafine DFT integration grid.
Firstly, the S0 geometries have been optimized with DFT and the vibrational frequencies have been analytically determined, using the M06-2X meta-GGA hybrid exchange-correlation functional. [2]We build the molecules in the Ci point group and this led to stable ground-state minima.These calculations were performed with the 6-31+G(d) atomic basis set and account for solvent effects through the linear-response PCM approach considering toluene and acetonitrile as solvents. [3]Secondly, starting from the optimal ground-state geometries, we have used TD-DFT with the same functional and basis set to optimize the S1 geometry and compute the vibrational frequencies.These calculations sometimes led to unstable results in the Ci point group and symmetry was lowered.Alternatively, we build molecules with a CT character starting from optimized excited-state and ground-state fragments "plugged" together and checked the relative stabilities of these structures after full excited-state optimization.All optimized structures used in the main text correspond to true minima of the potential energy surface.Thirdly, the vertical transition energies were determined with TD-DFT and the same functional, but a larger basis set, namely 6-311+G(2d,p) using the cLR 2 variant of the PCM, [4] in its non-equilibrium limit.From this, we obtained 0-0 energies using a procedure detailed elswehre. [5]The EDD were computed from the relaxed TD-DFT densities, and CT parameters used the well-known Le Bahers approach. [6]30

Figure S31 .
Figure S31.Differential absorption spectra corresponding to radical cations recorded at the first anodic waves during positive cyclic-voltammetry sweeps of (a) C1-Pyr, (b) N1-Pyr/tBu, (c) N1-Pyr/CF3, and (d) N1-Pyr/CN in DMF in the presence of 100 mM N(C4H9)4PF6.The close similarity between these spectra of the pyrene-pyrrolopyrrole conjugates and that of TAPP indicate that the radical cation is localized on the pyrrolopyrrole core.

Figure S32 .
Figure S32.Jablonski-like diagram obtained with PCM(DMF)-TD-DFT for TAPP.For both absorption ane emission, we report electron density difference plots (EDDs).Blue and red lobes correspond to decrease and increase of electron density upon transition.Contour: 1 × 10 -3 a.u.