Structure and Dynamics of the Excited States of 1,3-Diarylisobenzofurans: An Experimental and Theoretical Study Je ´ro ˆ me Jacq 1 , Svetlana Tsekhanovich 2 , Maylis Orio* 3 , Cathy Einhorn 1 , Jacques Einhorn 1 , Bernard Bessie `res 1 , Je ´ro ˆ me Chauvin 2 , Damien Jouvenot* 2 and Fre ´de ´rique Loiseau* 2 1 Synthe `se et Re ´ activite ´ en Chimie Organique, De ´ partement de Chimie Mole ´ culaire, UMR 5250, ICMG FR-2607, Universite ´ Joseph Fourier, Grenoble Cedex 9, France 2 Chimie Inorganique Redox, De ´ partement de Chimie Mole ´ culaire, UMR 5250, ICMG FR-2607, Universite ´ Joseph Fourier, Grenoble Cedex 9, France 3 Laboratoire de Spectrochimie Infrarouge et Raman, UMR CNRS 8516, Universite ´ des Sciences et Technologies de Lille, Villeneuve d’Ascq Cedex, France Received 18 November 2011, accepted 9 January 2012, DOI: 10.1111/j.1751-1097.2012.01090.x ABSTRACT The emission properties of a series of substituted 1,3-diary- lisobenzofurans have been studied. Most compounds exhibit very intense emission in the nanosecond timescale at room temper- ature as well as at 77 K. The room temperature emission is attributed to the deactivation of a twisted intramolecular charge transfer excited state, based on its energy, shape and solvent dependence. The experimental results are strongly supported by a theoretical study on one representative compound. The DFT ⁄ TD-DFT calculations demonstrate that the initial excited state relaxes toward a twisted structure. INTRODUCTION Since their first appearance in the beginning of the last century (1), 1,3-diarylisobenzofurans (IBFs) have been widely studied. These molecules have been used as synthetic intermediates to more complex structures (2,3) as they represent ideal dienes for Diels–Alder reactions (4). Photochemists very well know such compounds, as they are very efficient probes for the presence of singlet oxygen yielding to the opening of the furan ring when detected (5–7). The electroluminescent properties of IBFs have been known for more than 50 years (8) and are still exploited (9,10). Even more promising, in the realm of dye- sensitized solar cells, is the ability of IBFs to perform singlet fission in the solid state thus generating two triplet excited states by the absorption of a single photon (11–14). The photochemical and photophysical properties of the IBFs are expected to be altered by the introduction of various sub- stituents or functional groups at selected positions of the 1,3-diarylisobenzofuran backbone. New versatile and regio- specific synthesis of functionalized IBF derivatives have been developed recently (15–17), giving a straightforward access to previously unknown compounds, and thus providing a library of diversely substituted IBFs. A thorough inspection of the events following the absorption of a photon is described hereafter. The convergence of experimental data and DFT calculations evidences that the strong fluorescence displayed by these molecules originates from a twisted intramolecular charge transfer state (TICT). MATERIALS AND METHODS Electronic spectroscopic and photophysical experiments. Electronic absorption spectra were recorded on a Cary 300 UV-visible spectro- photometer (Agilent Technologies, Santa Clara, CA). Emission spectra were recorded in dichloromethane at room temperature and in butyro- nitrile rigid glass at 77 K on a Varian Cary Eclipse fluorescence spectrophotometer. Samples were placed in 1 cm path length quartz cuvettes. Luminescence lifetimes measurements were performed after irradiation at k = 400 nm obtained by the second harmonic of a Titanium:Sapphire laser (picosecond Tsunami laser spectra physics 3950-M1BB + 39868-03 pulse picker doubler) at a 800 kHz repetition rate. Fluotime 200 from AMS technologies was used for the decay acquisition. It consists of a GaAs microchannel plate photomultiplier tube (Hamamatsu model R3809U-50) followed by a time-correlated single photon counting system from Picoquant (PicoHarp300). The ultimate time resolution of the system is close to 30 ps. Luminescence decays were analyzed with FLUOFIT software available from Picoquant. Emission quantum yields / were determined at room temperature in dichlorome- thane solutions using the optically dilute method (18). 9,10-diphenylan- thracene in air-equilibrated cyclohexane solution was used as quantum yield standard (/ = 0.9). Experimental uncertainties are as follows: absorption maxima, 2 nm; molar absorption, 20%; emission maxima, 5 nm; emission lifetimes, 10%; emission quantum yields, 20%. Computational details. Theoretical calculations were based on density functional theory (DFT) and have been performed with the ORCA program package (19). Full geometry optimizations were carried out using the GGA functional BLYP (20,21) in combination with the TZV ⁄ P (22) basis set for all atoms and by taking advantage of the resolution of identity (RI) approximation in the Split-RI-J variant (23) with the appropriate Coulomb fitting sets (24). Increased integration grids (Grid4 in ORCA convention) and tight SCF convergence criteria were used. Numerical frequency calculations were performed to ensure that each geometry optimization converged to a real minimum. Optical properties were obtained from additional single-point calculations using the same functional and basis set (20–22). Vertical electronic transitions and dipole moments were computed using time-dependent DFT (TD-DFT; 25–27) within the Tamm-Dancoff approximation (28,29). To increase computational efficiency, the RI approximation (30) was used in calculating the Coulomb term and at least 30 excited states were calculated. For clarity, only computed transitions with non-negligible oscillator *Corresponding author emails: maylis.orio@univ-lille1.fr (Maylis Orio); damien.jouvenot@ujf-grenoble.fr (Damien Jouvenot); frederique.loiseau@ujf-grenoble.fr (Fre´ de´ rique Loiseau) Ó 2012 Wiley Periodicals, Inc. Photochemistry and Photobiology Ó 2012 The American Society of Photobiology 0031-8655/12 Photochemistry and Photobiology, 2012, 88: 633–638 633