DOI: 10.1002/cphc.200800648 Quadrupolar Chromophores for Voltage Sensing and White-Light Generation Anna Painelli and Francesca Terenziani* [a] Molecules whose properties depend on an applied electric voltage find useful applications as microscopic voltage sensors in biological environments, but can also be processed to build voltage-tunable light-emitting devices (LEDs). Dipolar push– pull chromophores are currently used as probes of transmem- brane fields: the electrical activity within a cell is monitored by exploiting the field dependence of the second-harmonic gen- eration (SHG) signal and/or the electrochromism of fluores- cence emission. [1–3] On the other hand, the luminescence of di- polar chromophores is too weakly affected by applied fields to build useful color-tunable LEDs based on dipolar dyes. [4] Quad- rupolar chromophores, characterized by a D–p–A–p–D or A–p–D–p–A motif (A and D are electron-acceptor and elec- tron-donor groups and p represents a p-conjugated bridge), are an interesting class of molecules for their large two-photon absorption cross sections, but so far they have not been thor- oughly considered for voltage-sensing or LED applications. Here we extend the essential-state model, recently proposed to describe optical spectra of quadrupolar chromophores, to account for the effects of an applied static electric field. Based on numerical simulations, we suggest that among quadrupolar chromophores very good candidates are found for the devel- opment of color-tunable LEDs and/or for white-light genera- tion, as well as particularly well suited molecules for micro- scopic recording of membrane action potentials with 3D reso- lution. Quadrupolar chromophores have a linear (or almost linear) molecular structure, with a vanishing dipole moment along the main conjugation axis. As expected for nondipolar mole- cules, quadrupolar chromophores typically show nonsolvato- chromic absorption spectra, but some of them are character- ized by impressive emission solvatochromism. [5] This puzzling behavior was recently rationalized in an essential-state model that describes the main charge-resonance processes in the molecule and their coupling with molecular vibrations and polar solvation. [5] Charge resonance in DAD (ADA) molecules leads to a quadrupolar charge distribution in the ground state, D +1/2 A 1 D +1/2 (A 1/2 D +1 A 1/2 ), where 1 is the molecular quadru- pole moment. According to the amount of charge transfer, quadrupolar chromophores can be classified in three families. Chromophores with intermediate charge transfer (class II) have stable nondipolar ground and excited states: absorption and fluorescence spectra of class II chromophores are almost unaf- fected by solvent polarity. Quadrupolar chromophores belong- ing to class I have very small quadrupole moment in the ground state. For this class of chromophores the excited state is conditionally unstable: for large enough coupling to molecu- lar vibrations, the excited state reached on absorption of a photon is bistable, that is, characterized by two minima corre- sponding to broken-symmetry configurations. If excitation is localized in one of the minima (along one of the two molecu- lar arms) symmetry is effectively broken. For chromophores be- longing to class I, polar solvents stabilize broken-symmetry configurations, so that the emitting excited state is dipolar; this is the reason why they show largely solvatochromic fluo- rescence spectra. For highly quadrupolar molecules (class III), symmetry breaking is predicted for the ground state, with con- sequent solvatochromic behavior of the absorption band. The essential-state model has been successfully applied to describe linear and nonlinear optical spectra of several quadrupolar chromophores. [5] Many molecules are known belonging to class I, a few examples of class II are available, but we are not aware of examples of class III chromophores. [5] An applied electric field (or, more precisely, its component along the molecular axis) lowers the symmetry of quadrupolar chromophores and thus leads to polar ground and excited states. The magnitude of the effects of the applied field how- ever can be very different for different chromophores. Both the ground and excited states of class II chromophores are slightly affected by this symmetry lowering, so that weakly polar ground and excited states are obtained in the presence of ap- plied fields of reasonable magnitude. The situation is different for class I dyes: the ground state is only marginally affected by an applied field, but the excited state, which is intrinsically un- stable, acquires a strong dipolar character in the presence of even a modest applied field. For quadrupolar chromophores belonging to class I, we then predict very pronounced electro- chromism of fluorescence spectra. At the same time, the effect of an applied electric field is expected to be important for those properties such as the second-order nonlinear response b that become allowed only in systems with reduced symme- try. In the following we discuss the field-dependent properties of chromophore 1 (Scheme 1), a prototypical and well-charac- terized dye belonging to class I. [5] [a] Prof. A. Painelli, Dr. F. Terenziani Dipartimento di Chimica GIAF and INSTM-UdR Parma Università di Parma Parco Area delle Scienze 17/a, 43100 Parma (Italy) Fax: (+ 39) 0521 905556 E-mail : francesca.terenziani@unipr.it Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200800648. Scheme 1. ChemPhysChem 2009, 10, 527 – 531  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 527