Symmetry‐Breaking Charge Transfer and Hydrogen Bonding: Toward Asymmetrical Photochemistry

Abstract

Symmetry-breaking charge transfer upon photoexcitation of a linear A-π-D-π-A molecule (D and A being electron donating and accepting groups) could be visualized using ultrafast time-resolved infrared spectroscopy by monitoring the CN stretching modes on the A units. Whereas in apolar solvents, the S state remains symmetric and quadrupolar, symmetry breaking occurs within ca. 100 fs in polar solvents as shown by the presence of two CN bands, instead of one in apolar solvents, with a splitting that increases with polarity. In protic solvents, symmetry breaking is significantly amplified by H-bonding interactions, which are the strongest at the CN group with the highest basicity. In strongly protic solvents, the two CN bands transform in about 20 ps into new bands with a larger splitting, and the lifetime of the S state is substantially reduced. This is attributed to the formation of an excited asymmetric tight H-bond complex.

Electron‐Deficient Bipyrrole Boomerangs: Bright Fluorophores Obtained via Double C−H Bond Activation

“…Importantly, a similar but faster evolution of TA spectra was recorded for cNMI n EE in dichloromethane ( τ 1 ≈1.3–1.9 ps). In both solvents, the τ 1 time constants are in good agreement with those reported for the solvation dynamics of coumarin 153, indicating that the corresponding processes are driven by solvation . Because of the rigidity of the boomerang chromophores, ESA signals corresponding to a completely charge‐separated state are not observed in the TA spectra.…”

Excited‐State Symmetry Breaking in an Aza‐Nanographene Dye

“…However, positive solvatochromic shifts in emission have been reported for many quadrupolar dyes, indicating the existence of a polar emissive state . Recently, symmetry breaking effects leading to an asymmetrical distribution of the electronic excitation on the two molecular arms have been monitored in real time through transient spectroscopy …”

“…[13][14][15][16][17][18][19] Recently,s ymmetry breaking effects leading to an asymmetricald istribution of the electronic excitation on the two molecular arms have been monitoredi nr eal time through transient spectroscopy. [20][21][22][23][24][25][26][27] Solvatochromic effects in quadrupolar chromophores were naturallyr ationalized by essential-state models,a sr elated to chargei nstabilities of either the ground or the excited state. [28] In chromophores with weak Da nd Ag roups (class Ic hromophores), for which the ground state has as malld egree of charget ransfer,t hat is, the quadrupolarm oment is small, the first excited state is bistable:t he vertical excited state reached upon absorption, characterized by as ymmetric charged istribution, relaxes in ab roken-symmetry structure,c onferring ad ipolar character to the relaxed excited state responsible for fluorescence.…”

“…The pyrrolo [3,2-b]pyrrole (PP), which is an electron rich heterocyclic unit, [1,29,30] behavesa sa ne nergy donor moiety in Ap-D-p-A-type chromophores, when coupledt os trong electron withdrawing moieties like cyano, [23,24] nitro [31,32] and carbonyl groups. [33] PP-derivative 1 (Figure 1) has aq ualitativelyd ifferent structure from previously investigated PP-basedq uadrupolar dyes.…”

Transient Glass Formation around a Quadrupolar Photoexcited Dye in a Strongly H‐Bonding Liquid Observed by Transient 2D‐IR Spectroscopy

“…has no permanent dipole moment. [7] As ac onsequence,i np rotic solvents,H -bond interactions between the solvent and the CN group located on the most excited side of ADA strengthen, whereas those at the other side weaken, amplifying symmetry breaking. However,i np olar environments,s olvent fluctuations break the symmetry,l eading to an uneven distribution of the electronic density on the two cyano-capped ends.…”