Solid-State Fluorescence DOI: 10.1002/anie.200501781 Molecular Mechanism of the Solid-State Fluores- cence Behavior of the Organic Pigment Yellow 101 and Its Derivatives** Andreas Dreuw,* Jürgen Plötner, Lisa Lorenz, Josef Wachtveitl, JusteE. Djanhan, Jürgen Brüning, Thomas Metz, Michael Bolte, and MartinU. Schmidt* Many organic colorants that fluoresce in solution (e.g. fluoresceine) lose this property when they are crystallized. PigmentYellow101(P.Y.101, 2 ;R 1 = H,R 2 = OH)istheonly commercial organic yellow pigment [1,2] that exhibits fluores- cence also in the crystalline state. P.Y.101 has been known since1899andhasbeenproducedindustriallyfordecades.It isused,forexample,asapigmentforviscose,forprintinginks, and for artists) applications. Analysis of the fluorescence propertiesofthederivatives 1 (R 1 = R 2 = H) 3 (R 1 = H, R 2 = OCH 3 ), and 4 (R 1 = CH 3 , R 2 = OH) reveals an unexpected behavior: Pigments 1 and 3 donotfluoresce,neitherin solution nor in the solid state. However, pigment 4 does exhibit fluorescence like 2 both in solution and in the solid state (Table1). This is remarkable for two reasons: 1)Compounds 2 and 4 are fluorescent in contradictiontoanempiricalrulethatstates that pigments with OH groups usually do notfluoresce.Here,onlythepigments with OH groups exhibit fluorescence. 2)The fluorescent pigments remain fluorescent upon crystallization, although compound 4 displays short intermoleculardistancesinthecrystal, which should lead to fluorescence quenching. [3] To understand the influ- enceoftheOHgroupsonthefluores- cence of the pigments we performed extensive quantum-chemical calcula- tions employing methods based on time-dependent density functionaltheory(TDDFT).Inthefollowingpresentationof our theoretical study, we focus on the pigments 1 and 2,but analogousresultshavebeenfoundfor 3 and 4. The calculated geometrical parameters of the ground statesoftheindividualmolecules 1 and 2 agreeverywellwith theX-raycrystalstructure(seetheSupportingInformation). Forallthemolecules,excitationintothelowest-lyingoptically allowedstateleadstobond-lengthequilibrationoftheNN, C =N, and C C bonds of the bisazomethine substructure, in otherwords,toatransitionoftheC = NN = CtoaCPNPNPC fragment (see the Supporting Information). The absorption and fluorescence spectra of the individual molecules have been calculated and compared with experimental spectra as shown in Figure1 for 2. The calculated excited states are shiftedabout77nmtosmallerwavelengths(higherenergy)to match the 0 !0 transition of the strongly allowed and thus experimentally observed absorption band. The good agree- mentbetweentheoryandexperimentinthiscaseisstriking. The experimentally observed absorption band at 413nm (l max ) exhibits a pronounced vibrational fine structure that points to a mode with a frequency of about 1400cm 1 (Figure1), which is excited upon optical transition from the groundstate.Thisfrequencycorrespondstoatypicalvaluefor symmetric stretching vibrations n s involving the bisazometh- inesubunit.Thisisalsoinagreementwiththechangeofthe equilibriumgeometriesfromthegroundstatetotheoptically allowed excited state S 1 inthecaseof 2 (see the Supporting Information). To answer the question why pigments 2 and 4 fluoresce while 1 and 3 do not, their excited states were calculated (Table2).Therelevantstatesandthecorrespondingorbitals are displayed in Figure2. In the case of 1 and 3, the energetically lowest excited state S 1 corresponds to a for- Table 1: Crystal structure data and fluorescence properties of the pigments 1–4. 1 2 3 4 R 1 H H H CH 3 R 2 H OH OCH 3 OH Fluorescence in solution no yes no yes Fluorescence in crystal no strong no strong Space group P2 1 /n P2 1 /n P2 1 /n C2/c Z (molecules/unit cell) 2 2 2 4 Center-to-center distance [] 6.085 6.114 7.007 3.863 [*] Dr. A. Dreuw, Dipl.-Chem. J. Plötner, Dipl.-Chem. L. Lorenz, Prof. Dr. J. Wachtveitl Institut für Physikalische und Theoretische Chemie Johann Wolfgang Goethe-Universität Frankfurt Marie Curie-Strasse 11, 60439 Frankfurt/Main (Germany) Fax: (+ 49)69-798-29709 E-mail: andreas.dreuw@theochem.uni-frankfurt.de Dipl.-Chem. J. E. Djanhan, J. Brüning, Dr. M. Bolte, Prof. Dr. M. U. Schmidt Institut für Anorganische und Analytische Chemie Johann Wolfgang Goethe-Universität Frankfurt Marie Curie-Strasse 11, 60439 Frankfurt/Main (Germany) Fax: (+ 49)69-798-29235 E-mail: m.schmidt@chemie.uni-frankfurt.de Dr. T. Metz Clariant GmbH Div. Pigments & Additives Pigments Research, G 834 65926 Frankfurt/Main (Germany) [**] A.D. is supported by the Deutsche Forschungsgemeinschaft as an Emmy Noether fellow. The authors thank Antje Lemanczyk and Uwe Jobstmann (Clariant GmbH) for syntheses, Dr. Jürgen Glinnemann and Dr. Chunhua Hu (Universität Frankfurt) for recrystallizations, and Edith Alig (Universität Frankfurt) for X-ray powder diffraction diagrams. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Angewandte Chemie 7783 Angew. Chem. Int. Ed. 2005, 44, 7783–7786 # 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim