Excited-State Symmetry Breaking and the Laporte Rule

Abstract

Excited-state symmetry breaking (ES-SB) is common to a large number of multibranched electron donor−acceptor (DA) molecules in polar environments. During this process, the electronic excitation, originally evenly distributed over the molecule, localizes, at least partially, on one branch. Due to the absence of an unambiguous spectroscopic signature in the UV−vis region, electronic transient absorption (TA) has not been the method of choice for real-time observation of this phenomenon. Herein, we demonstrate that the Laporte rule, which states that one-photon transitions conserving parity are forbidden in centrosymmetric molecules, provides such clear signature of ES-SB in electronic TA spectroscopy. Using a dicyanoanthracene-based D−A−D dye, we show that transitions from the S 1 state of this molecule, which are initially Laporte forbidden, become allowed upon ES-SB. This leads to the rise of new TA bands, whose intensity provides a direct measure of the extent of asymmetry in the excited state.

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

“…Based on quantum chemical calculations, this band was assigned to the symmetry-allowed S 3 ← S 1 transition. 42 These calculations also predicted Laporte-forbidden S 2 ← S 1 and S 4 ← S 1 transitions at 6170 and 10 900 cm −1 , respectively. Comparatively, the TA spectra measured with D3 show an intense ESA band around 7000 cm −1 , that most probably corresponds to the S 2 ← S 1 transition.…”

“…The appearance of this band, present in the spectrum of D2 , is a clear indication of ES-SB and originates from the loss of the centre of inversion of the excited state wavefunction. 42 The presence of this band in the early spectra in HFP can be explained by ES-SB occurring through inertial solvent motion already.…”

“…However, we recently demonstrated that ES-SB can be visualised by monitoring the appearance of excited-state absorption (ESA) bands that are Laporte forbidden in the symmetric quadrupolar state. 42 Therefore, the excited-state dynamics of all 6 dyes and DCA were investigated using both TRIR and TA spectroscopies in solvents of varying polarity. The transient data (TRIR-TA) collected in the same solvent were merged, and analysed globally assuming a series of successive exponential steps.…”

“…The excited-state dynamics of Q3 and D3 was already partially discussed. 42 The EADS obtained from the global analysis of the TRIR and electronic data measured with these dyes in various solvents are presented in Fig. 6 .…”

“…However, ES-SB can also be detected by monitoring the appearance of Laporte-forbidden transitions in the transient absorption electronic spectrum. 42 TRIR is also used here to probe the –C≡ N stretching modes of the DCA sub-units and to understand how the symmetry of the core itself is affected by the presence of one or two D substituents. Our results reveal that the electronic distribution of the DCA core is very sensitive to the environment and to the donor.…”