Specific Monitoring of Excited-State Symmetry Breaking by Femtosecond Broadband Fluorescence Upconversion Spectroscopy

Abstract

Most quadrupolar molecules designed for large two-photon absorption cross section have been shown to undergo symmetry breaking upon excitation to the S state. This was originally deduced from their strong fluorescence solvatochromism and later visualized in real time using transient infrared spectroscopy. For molecules not containing clear IR marker modes, however, a specific real-time observation of the symmetry breaking process remains lacking. Here we show that this process can be resolved using broadband fluorescence upconversion spectroscopy by monitoring the instantaneous emission transition dipole moment. This approach is illustrated with measurements performed on two quadrupolar molecules, with only one of them undergoing excited-state symmetry breaking in polar solvents.

Extended Alkenyl and Alkynyl Benzotriazoles with Enhanced Two‐Photon Absorption Properties as a Promising Alternative to Benzothiadiazoles

“…In molecules with ES‐SB, the dependence of the emission on the polarity function shows a slope that increases with Δ f . Compound 2 d also shows this behavior, but this increment is not significantly augmented upon moving to highly polar solvents (Figure S3 b in the Supporting Information).…”

“…In molecules with ES-SB, the dependence of the emission on the polarity function shows as lopet hat increases with Df. [36,39] Compound 2d also shows this behavior,b ut this increment is not significantly augmented upon moving to highly polar solvents ( Figure S3 bi nt he Supporting Information). This could suggest that this behavior cannot be unambiguously explained as ES-SB behavior and other factorsc ould contribute to stabilize the excited state.…”

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.…”

Dynamic exciton localisation in a pyrene–BODIPY–pyrene dye conjugate

“…Thus, excitation of Py2B may be best described by neither IC nor energy transfer but by what we would like to call "dynamic exciton localisation" which clearly represents the bridge between the both extreme scenarios 51 and also may involve symmetry breaking. [52][53][54][55]…”