Stimulated Resonance Raman and Excited-State Dynamics in an Excitonically Coupled Bodipy Dimer: A Test for TD-DFT and the Polarizable Continuum Model

Bodipy is one of the most versatile and studied functional dyes due to its myriad applications and tunable spectral properties. One of the strategies to adjust their properties is the formation of Bodipy dimers and oligomers whose properties differ significantly from the corresponding monomer. Recently, we have developed a novel strategy for synthesizing α,α-ethylene-bridged Bodipy dimers; however, their excited-state dynamics was heretofore unknown. This work presents the ultrafast excited-state dynamics of a novel α,α-ethylene-bridge Bodipy dimer and its monomeric parent. The dimer’s steady-state absorption and fluorescence suggest a Coulombic interaction between the monomeric units’ transition dipole moments (TDMs), forming what is often termed a “J-dimer”. The excited-state properties of the dimer were studied using molecular excitonic theory and time-dependent density functional theory (TD-DFT). We chose the M06 exchange–correlation functional (XCF) based on its ability to reproduce the experimental oscillator strength and resonance Raman spectra. Ultrafast laser spectroscopy reveals symmetry-breaking charge separation (SB-CS) in the dimer in polar solvents and the subsequent population of the charge-separated ion-pair state. The charge separation rate falls into the normal regime, while the charge recombination is in the inverted regime. Conversely, in nonpolar solvents, the charge separation is thermodynamically not feasible. In contrast, the monomer’s excited-state dynamics shows no dependence on the solvent polarity. Furthermore, we found no evidence of significant structural rearrangement upon photoexcitation, regardless of the deactivation pathway. After an extensive analysis of the electronic transitions, we concluded that the solvent fluctuations in the local environment around the dimer create an asymmetry that drives and stabilizes the charge separation. This work sheds light on the charge-transfer process in this new set of molecular systems and how excited-state dynamics can be modeled by combining the experiment and theory.


TD-DFT Electronic Transition Properties
Table S1.Monomer first five electronic transitions calculated with the labeled functional in benzene using the ground state optimized geometry.
Table S2.Monomer first five electronic transitions calculated with the M06 functional in benzene using the S1 optimized geometry.
Table S3.Dimer first ten electronic transitions calculated with the labeled functional in benzene using the (left) ground state and (right) S1 state optimized geometry.
Table S4.Dimer first ten electronic transitions calculated with the labeled functional in acetonitrile using the (left) ground state and (right) S1 state optimized geometry.

Cyclic Voltammetry
We used a Princeton Applied Research Model 263A potentiostat/galvanostat to control the potential during the experiment.The working and counter electrode were platinum, and the reference electrode was saturated calomel electrode (SCE) (+0.242V vs NHE) soaked in KCl.We used (TBA)PF6 as supporting electrolyte.The instrumentation was controlled with PowerSuite 2.60.The sample had a concentration of 1 mM in acetonitrile.The sample was purged with argon for 15 min prior the scan and the experiment was carried out under argon atmosphere.The scan rate was 100mV/s.

Calculation of Marcus Parameters
The solvent reorganization energy for charge-separation is calculated using the continuum dielectric theory (Nitzan, 2006) as follows: where the radii of the donor (  ) and acceptor (  ) are taken as half of donor-acceptor distance (  ).  is the optical response and   the dielectric constant (static response) of the solvent.The inner sphere reorganization energy is calculated using the four-point method (Wu & Van Voorhis, 2006): Where    * and    + are the equilibrium geometries of the excited and cationic states, respectively.Similarly,    and    − are the equilibrium geometries of the ground and anionic states, respectively.The level of theory was M06 / 6-311G* in acetonitrile using the polarizable continuum model.The geometry convergence was set to default and the integration grid was set to ultra-fine.
The total reorganization energy for the charge-separation is given by the sum of internal and solvent reorganization energies: The energy of the Ion-pair (  ) is calculated via the Rehm-Weller equation (Weller, 1982) Where   () −   (), are the oxidation and reduction potential for the donor (D) and acceptor (A), both being the monomer in this case.  the dielectric constant of acetonitrile and   the dielectric constant for another solvent (Methanol or benzene).The free energy gap (Δ  ) for charge-separation can then be estimated as follows: Δ  =   −  00 (), where  00 is the "0-0" energy gap, calculated as the crossing point between the normalized absorption and emission spectra.
The electronic coupling is calculated from the classical expression of Marcus electron transfer theory (Marcus, 1956), Here,   is the donor-acceptor electronic coupling,   is the Boltzman constant,  the temperature and  is the rate constant for charge-separation.The rate constant can be related to the charge-transfer time constant ( = 1/  ), obtained from fsTA and global analysis, and so we can calculate   from the Marcus equation with all other parameters defined by DFT and fsTA.

Figure S4 .
Figure S4.Monomer's transition dipole moment (TDM) vector from two different perspectives using M06 XCF and the ground state optimized geometry.In atomic units, the magnitude of the vector is 3.16, which corresponds to an oscillator strength of 0.696.

Figure S5 .
Figure S5.Monomers' TDM vector superimposed on the dimer's optimized ground state geometry from two different perspectives using M06 XCF.

Figure S6 .
Figure S6.Transient absorption and (b) () spectra of the monomer in acetonitrile pumped at 524 nm.

Figure S7 .
Figure S7.(a) Transient absorption and (b) () spectra of the dimer in methanol pumped at 540 nm.

Figure S9 .
Figure S9.orbitals for the dimer.Calculated using the M06 exchange-correlation functional at the excited state optimized geometry.

Figure S13 :
Figure S13: Natural transition orbitals of the first two electronic transitions of the Bodipy monomer in benzene using the ground state optimized geometry and the M06 XCF.The oscillator strength (), excitation energy (), and the associated eigenvalue are presented for each transition.

Figure S14 :
Figure S14: Natural transition orbitals of the first two electronic transitions of the Bodipy monomer in benzene using the excited state optimized geometry and the M06 XCF.The oscillator strength (), excitation energy (), and the associated eigenvalue are presented for each transition.

Figure S15 :
Figure S15: Natural transition orbitals of the first ten electronic transitions of the Bodipy dimer in MeCN using the S1 state-optimized geometry and the M06 XCF.The oscillator strength (), excitation energy (), and the associated eigenvalue are presented for each transition.

Figure S16 :
Figure S16: Natural transition orbitals of the first ten electronic transitions of the Bodipy dimer in MeCN using the S1 state-optimized geometry and the M11 XCF.The oscillator strength (), excitation energy (), and the associated eigenvalue are presented for each transition.

Figure S17 :
Figure S17: Natural transition orbitals of the first ten electronic transitions of the Bodipy dimer in benzene using the S1 state-optimized geometry and the M11 XCF.The oscillator strength (), excitation energy (), and the associated eigenvalue are presented for each transition.