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

Abstract

Symmetry-breaking charge transfer upon photoexcitation of al inear A-p-D-p-A molecule (D and Ab eing electron donating and accepting groups) could be visualized using ultrafast time-resolved infrared spectroscopyb ym onitoring the CN stretching modes on the Aunits.W hereas in apolar solvents,the S 1 state remains symmetric and quadrupolar,s ymmetry breaking occurs within ca. 100 fs in polar solvents as shown by the presence of two CN bands,instead of one in apolar solvents,w ith as plitting that increases with polarity.I np rotic solvents,s ymmetry breaking is significantly amplified by H-bonding interactions,which are the strongest at the CN group with the highest basicity.I ns trongly protic solvents,the two CN bands transform in about 20 ps into new bands with al arger splitting,a nd the lifetime of the S 1 state is substantially reduced. This is attributed to the formation of an excited asymmetric tight H-bond complex.Scheme 1. Structure of ADA.Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under http://dx.

Solvent tuning of photochemistry upon excited-state symmetry breaking

“…This model predicts complete exciton localization even in weakly polar media (D = 0.985 in benzene and D = 0.993 in chloroform, with D = 1 being a full localization on a single branch). This result differs from previous investigations, where complete localization was only achieved in the most polar solvents 30,34 , or could not be realized at all 32,33 . It is possible here because of the weak electronic coupling between the branches (~850-900 cm −1 ) and the large charge transfer in each arm upon photoexcitation.…”

“…The most direct real-time visualization of ES-SB can be achieved by monitoring vibrational modes localized at the ends or within the D-π-A branches of these systems, using ultrafast timeresolved infrared spectroscopy (TRIR). These studies revealed how the occurrence and extent of ES-SB depend on properties of the dye itself 30,32,34,35 and of its environment [30][31][32][33] . Until now, such TRIR investigations were limited to quadrupolar molecules.…”

“…Photoexcitation of these molecules leads to a delocalized and symmetric multipolar exciton that can undergo symmetry breaking due to surrounding solvent fluctuations [30][31][32] . Excitedstate symmetry breaking (ES-SB) can be viewed as the decoherence of a multipolar exciton evenly delocalized over all branches of the molecule that eventually results in its confinement on a single branch.…”

“…Excitedstate symmetry breaking (ES-SB) can be viewed as the decoherence of a multipolar exciton evenly delocalized over all branches of the molecule that eventually results in its confinement on a single branch. This phenomenon has seen a growing interest over the past few years, and was extensively investigated in linear quadrupolar rod-like molecules [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] . Attempts to identify a spectroscopic signature of ES-SB using electronic transient absorption in the UV-Vis region failed, because localized and delocalized states exhibit similar spectra 39,[45][46][47][48] .…”

Vector Form of Symmetry Degree

“…The application of the symmetry especially the spontaneous symmetry breaking has inspired many fields of physics, such as particle physics 3–8 , condensed matter physics 9–12 , physical chemistry 13–16 , biological physics 17,18 and quantum information 19,20 . Mathematically, symmetry refers to that a system is invariant under some specific transformations.…”