Efficient Excited‐State Symmetry Breaking in a Cationic Quadrupolar System Bearing Diphenylamino Donors

Abstract

We report a joint experimental and theoretical investigation of a quadrupolar D-π-A(+) -π-D system, the electron donors being diphenylamino groups and the electron acceptor being a methylpyridinium, in comparison with the dipolar D-π-A(+) system. The emission spectra of the two compounds overlap in all the investigated solvents. This finding could be rationalized by TD-DFT calculations: the LUMO-HOMO molecular orbitals involved in the emission transition are localized on the same branch of the quadrupolar structure that becomes the fluorescent portion, corresponding to that of the single-arm compound. Excited-state symmetry breaking has been rarely observed for quadrupolar systems showing negative solvatochromism and is here surprisingly revealed, even in low polarity solvents. Femtosecond transient absorption measurements revealed that an efficient photoinduced intramolecular charge transfer takes place in the quadrupolar chromophore, more efficient than in its dipolar analogue. This result is promising in view of the application of these compounds as novel two-photon absorbing materials.

Solvent tuning of photochemistry upon excited-state symmetry breaking

“…Their decay in polar solvents represents the first report of a TA signature of ES-SB in the visible region. No distinct feature could be observed in previous UV-Vis TA investigations of multipolar systems by us [30][31][32]34 and others 39,43,46,47 . They could be identified here not only because of the possibility to compare the TA spectra of the multipolar molecules with those of the singlebranch D in non-polar solvents, but mostly thanks to the unambiguous spectral signatures obtained in the TRIR and FLUPS experiments.…”

“…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] .…”

A simple model of solvent-induced symmetry-breaking charge transfer in excited quadrupolar molecules

“…[28][29][30][31] Although time-resolved electronic spectroscopy confirmed the dipolar nature of the excited state, it did not provide information on the dynamics of the symmetry breaking and on its magnitude. 21,23,[32][33][34][35] Real-time visualization of the symmetry-breaking upon photoexcitation of quadrupolar D-π-A-π-D and A-π-D-π-A molecules was recently reported using femtosecond timeresolved infrared spectroscopy. 36,37 The symmetry breaking dynamics is directly reflected in the IR spectral dynamics associated with vibrational modes localised in the two arms of the molecules, namely, − −C≡ ≡C− − or − −C≡ ≡N stretching vibrations.…”

Photoinduced Intramolecular Charge Transfer and Hyperpolarizability Coefficient in Push‐Pull Pyridinium Salts with Increasing Strength of the Acceptor Group

“…The solvent effect on the photobehaviour of neutral dipolar push‐pull systems showing positive solvatochromism has been investigated in our research group, leading to estimate the quadratic hyperpolarizability coefficient (β) through the solvatochromic method and, more recently, to determine the two photon absorption (a cubic NLO process) cross section through the “Two‐Photon Excited Fluorescence” technique ,. The study has lately been focused on ionic chromophores (A+‐π‐D), namely N ‐methylpyridinium or N ‐methylquinolinium (electron‐deficient portion, A+) derivatives bearing different strong donor groups (D) at the opposite site of the molecule, connected through π‐linkers and showing negative solvatochromism . The latter compounds have been found to be very interesting for their ability to bind DNA, as well …”

“…[7,8] The study has lately been focused on ionic chromophores (A +-π-D), namely N-methylpyridinium or Nmethylquinolinium (electron-deficient portion, A +) derivatives bearing different strong donor groups (D) at the opposite site of the molecule, connected through π-linkers and showing negative solvatochromism. [9][10][11][12][13][14][15] The latter compounds have been found to be very interesting for their ability to bind DNA, as well. [16][17][18] Salts are especially promising not only because the cationic chromophores can act as efficient electron-withdrawing groups (A) but also for their water solubility, which is really appealing for biological and medical applications.…”