Two-Photon Transitions in Quadrupolar and Branched Chromophores:  Experiment and Theory

Abstract

A combined experimental and theoretical study is conducted on a series of model compounds in order to assess the combined role of branching and charge symmetry on absorption, photoluminescence, and twophoton absorption (TPA) properties. The main issue of this study is to examine how branching of quadrupolar chomophores can lead to different consequences as compared to branching of dipolar chromophores. Hence, three structurally related π-conjugated quadrupolar chromophores symmetrically substituted with donor end groups and one branched structure built from the assembly of three quadrupolar branches via a common donor moiety are used as model compounds. Their photophysical properties are studied using UV-vis spectroscopy, and the TPA spectra are determined through two-photon excited fluorescence experiments using femtosecond pulses in the 500-1000 nm range. Experimental studies are complemented by theoretical calculations. The applied theoretical methodology is based on time-dependent density functional theory, the Frenkel exciton model, and analysis in terms of the natural transition orbitals of relevant electronic states. Theory reveals that a symmetrical intramolecular charge transfer from the terminal donating groups to the middle of the molecule takes place in all quadrupolar chromophores upon photoexcitation. In contrast, branching via a central electron-donating triphenylamine moiety breaks the quadrupolar symmetry of the branches. Consequently, all Frank-Condon excited states have significant asymmetric multidimensional charge-transfer character upon excitation. Subsequent vibrational relaxation of the branched chromophore in the excited state leads to a localization of the excitation and fluorescence stemming from a single branch. As opposed to what was earlier observed when dipolar chromophores are branched via the same common electron-donating moiety, we find only a slight enhancement of the maximum TPA response of the branched compound with respect to an additive contribution of its quadrupolar branches. In contrast, substantial modifications of the spectral shape are observed. This is attributed to the subtle interplay of interbranch electronic coupling and asymmetry caused by branching.

Localized Emitting State and Energy Transfer Properties of Quadrupolar Chromophores and (Multi)Branched Derivatives

“…25−27 In that case, significant cooperative enhancement of the twophoton cross-section of triple-branched structures could not be observed, in comparison with their counterpart linear moiety. 9,19,20,25 However, considering the short spatial distance between the branches and the identical transition moment of these moieties, it is still not likely that the branches are fully independent of each other. Energy transfer and interactions among the branches in the excited state should still exist within these compounds.…”

“…Both symmetry-breaking phenomena in excited state (caused by vibrational relaxation and solvation) and disorder-induced symmetry lowering (caused by ground state solute−solvent interaction) have been reported to contribute to the excitation localization. [25][26][27][42][43][44]64,65 In order to track down the excited state energy redistribution and the interbranch interaction of the multibranched molecules, it is instructive to determine the direction of molecular transition dipole moment and polarization of the spectrum.…”

Enhanced Two‐Photon Absorption of Organic Chromophores: Theoretical and Experimental Assessments

“…[33,36,57] In turn, exploitation of intermolecular interactions through branching strategies and the supramolecular approaches offers even more possibilities to tune or enhance TPA properties. This has already been demonstrated for branched chromophores built from the gathering of either dipolar [108][109][110][111][112][113][114][115][116][117][118][119][120][121][122][123] or quadrupolar [97,102,[124][125][126] sub-chromophores via common conjugated core moieties and multichromophore structures in which subchromophores interact only via electrostatic interactions. [127] Alternative routes such as those based on porphyrins, [128][129][130][131][132][133][134][135][136] oligomers and polymers [103,[137][138][139] have been explored as well.…”

Stimulated emission depletion and fluorescence correlation spectroscopy of a branched quadrupolar chromophore

“…The branched fluorescent probe (OM77) used in this study is shown in figure 1, designed for optimal near infra-red twophoton absorption and single photon fluorescence yield 9 . It has a substantial two-photon cross-section and a fast ground state vibrational relaxation time 10 which makes it an ideal candidate for single molecule STED 8 .…”

“…Furthermore, OM77 is an efficient two-photon absorbing system in the visible where we have recently identified the onset of a giant two-photon resonance (ca. 12000GM at 570nm) 9 and the probability of DUMP excited fluorescence from ground state molecules needs to be minimized. To limit these effects, the DUMP pulses were temporally stretched using a grating pair to greater than 20 picoseconds.…”