Two-Photon Transitions in Quadrupolar and Branched Chromophores: Experiment and Theory
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- 作者：Claudine Katan ; Sergei Tretiak ; Martinus H. V. Werts ; Angus J. Bain ; Richard J. Marsh ; Nicholas Leonczek ; Nicholas Nicolaou ; Ekaterina Badaeva ; Olivier Mongin ; Mireille Blanchard-Desce
- 刊名：Journal of Physical Chemistry B
- 出版年：2007
- 出版时间：August 16, 2007
- 年：2007
- 卷：111
- 期：32
- 页码：9468 - 9483
- 全文大小：1077K
- 年卷期：v.111,no.32(August 16, 2007)
- ISSN：1520-5207
文摘
A combined experimental and theoretical study is conducted on a series of model compounds in order toassess the combined role of branching and charge symmetry on absorption, photoluminescence, and two-photon absorption (TPA) properties. The main issue of this study is to examine how branching of quadrupolarchomophores can lead to different consequences as compared to branching of dipolar chromophores. Hence,three structurally related 
hars/pi.gif" BORDER=0 >-conjugated quadrupolar chromophores symmetrically substituted with donor endgroups and one branched structure built from the assembly of three quadrupolar branches via a commondonor moiety are used as model compounds. Their photophysical properties are studied using UV-visspectroscopy, and the TPA spectra are determined through two-photon excited fluorescence experiments usingfemtosecond pulses in the 500-1000 nm range. Experimental studies are complemented by theoreticalcalculations. The applied theoretical methodology is based on time-dependent density functional theory, theFrenkel 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 themiddle of the molecule takes place in all quadrupolar chromophores upon photoexcitation. In contrast, branchingvia a central electron-donating triphenylamine moiety breaks the quadrupolar symmetry of the branches.Consequently, all Frank-Condon excited states have significant asymmetric multidimensional charge-transfercharacter upon excitation. Subsequent vibrational relaxation of the branched chromophore in the excited stateleads to a localization of the excitation and fluorescence stemming from a single branch. As opposed to whatwas 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 toan additive contribution of its quadrupolar branches. In contrast, substantial modifications of the spectralshape are observed. This is attributed to the subtle interplay of interbranch electronic coupling and asymmetrycaused by branching.
hars/pi.gif" BORDER=0 >-conjugated quadrupolar chromophores symmetrically substituted with donor endgroups and one branched structure built from the assembly of three quadrupolar branches via a commondonor moiety are used as model compounds. Their photophysical properties are studied using UV-visspectroscopy, and the TPA spectra are determined through two-photon excited fluorescence experiments usingfemtosecond pulses in the 500-1000 nm range. Experimental studies are complemented by theoreticalcalculations. The applied theoretical methodology is based on time-dependent density functional theory, theFrenkel 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 themiddle of the molecule takes place in all quadrupolar chromophores upon photoexcitation. In contrast, branchingvia a central electron-donating triphenylamine moiety breaks the quadrupolar symmetry of the branches.Consequently, all Frank-Condon excited states have significant asymmetric multidimensional charge-transfercharacter upon excitation. Subsequent vibrational relaxation of the branched chromophore in the excited stateleads to a localization of the excitation and fluorescence stemming from a single branch. As opposed to whatwas 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 toan additive contribution of its quadrupolar branches. In contrast, substantial modifications of the spectralshape are observed. This is attributed to the subtle interplay of interbranch electronic coupling and asymmetrycaused by branching.