Impact of intramolecular twisting and exciton migration on emission efficiency of multifunctional fluorene-benzothiadiazole-carbazole compounds.

Novel donor-acceptor compounds consisting of singly bonded fluorene (Fl), benzothiadiazole (BT), and carbazole (Cz) functional units in the same molecule were investigated. Analysis of the optical spectra and fluorescence transients of the compounds revealed the domination of intramolecular charge transfer (ICT) states with high fluorescence quantum yield (72%-85%). A similar Cz-Fl-Cz compound exhibiting 100% fluorescence quantum yield and no ICT character was also studied as a reference to reveal the impact of electron-accepting BT groups. Thorough examination of the optical properties of the compounds in different media, i.e., dilute solution and polymer matrix, indicated their twisted conformations due to steric hindrance in the ground state and flattened geometry in the excited state for both reference and ICT compounds. Remarkable fluorescence efficiency losses (amounting to 70%) observed upon casting the molecular solutions into neat films were determined to originate from the low-fluorescent twisted conformers and migration-facilitated exciton quenching. The majority of emission efficiency losses (over 70%) were caused by the twisted conformers, whereas only less than 30% by exciton-migration-induced nonradiative deactivation.

[1]  Aleksander Rebane,et al.  Optimizing simultaneous two-photon absorption and transient triplet-triplet absorption in platinum acetylide chromophores. , 2010, The journal of physical chemistry. A.

[2]  Paras N. Prasad,et al.  Diphenylaminofluorene-Based Two-Photon-Absorbing Chromophores with Various π-Electron Acceptors , 2001 .

[3]  J. Zuo,et al.  Synthesis and characterization of efficient luminescent materials based on 2,1,3-benzothiadiazole with carbazole moieties , 2011 .

[4]  Mario Leclerc,et al.  A Low‐Bandgap Poly(2,7‐Carbazole) Derivative for Use in High‐Performance Solar Cells , 2007 .

[5]  Fuxi Gan,et al.  Large three-photon absorption and intramolecular charge transfer of the bis-donor fluorene-based molecules , 2005 .

[6]  Peter Strohriegl,et al.  Novel 2,7-Linked Carbazole Trimers as Model Compounds for Conjugated Carbazole Polymers , 2004 .

[7]  Daoben Zhu,et al.  Large third-order nonlinear optical response of conjugated copolymers consisting of fluorene and carbazole units , 2002 .

[8]  C. McNeill,et al.  Efficient Polythiophene/Polyfluorene Copolymer Bulk Heterojunction Photovoltaic Devices: Device Physics and Annealing Effects , 2008 .

[9]  S. Juršėnas,et al.  Multifunctional red phosphorescent bis-cyclometallated iridium complexes based on 2-phenyl-1,2,3-benzotriazole ligand and carbazolyl moieties , 2011 .

[10]  Z. Kotler,et al.  Two photon absorption properties of bis(n-carbazolyl)-poly-phenylenes , 2000 .

[11]  M. Leclerc,et al.  Absorption and emission properties of carbazole-based dyads studied from experimental and theoretical investigations , 2004 .

[12]  G. Wegner,et al.  Comparative study of hole transport in polyspirobifluorene polymers measured by the charge-generation layer time-of-flight technique , 2006 .

[13]  Xiaopeng Chen,et al.  Synthesis and fluorescence properties of carbazole and fluorene-based compounds , 2007 .

[14]  Synthesis, crystal structure and spectroscopic properties of an unsymmetrical compound with carbazole and benzothiadiazole units , 2007 .

[15]  Wei Huang,et al.  Syntheses, characterization, and energy transfer properties of benzothiadiazole-based hyperbranched polyfluorenes , 2006 .

[16]  S. Ruchirawat,et al.  Synthesis and characterization of novel N-carbazole end-capped oligothiophene-fluorenes , 2007 .

[17]  Yongsheng Chen,et al.  Selective Tuning of the HOMO―LUMO Gap of Carbazole-Based Donor―Acceptor―Donor Compounds toward Different Emission Colors , 2010 .

[18]  Klaus Meerholz,et al.  Multi-colour organic light-emitting displays by solution processing , 2003, Nature.

[19]  T. Michinobu,et al.  Band-gap tuning of carbazole-containing donor–acceptor type conjugated polymers by acceptor moieties and π-spacer groups , 2008 .

[20]  W. Rettig,et al.  Pseudo-Jahn–Teller and TICT-models: a photophysical comparison of meta- and para-DMABN derivatives , 1999 .

[21]  Wen‐Chang Chen,et al.  Synthesis and characterization of new fluorene-acceptor alternating and random copolymers for light-emitting applications , 2006 .

[22]  S. Jungsuttiwong,et al.  Synthesis and characterization of N-carbazole end-capped oligofluorenes , 2007 .

[23]  S. H. Lee,et al.  Molecular design of fluorene-based polymers and oligomers for organic light-emitting diodes , 2000 .

[24]  Shijie Ren,et al.  The synthesis and properties of novel π-conjugated 2,1,3-benzothiadiazole oligomers , 2009 .

[25]  Y. Geng,et al.  Blue light-emitting materials based on terfluorenes with carbazole terminal units , 2005 .

[26]  Shijie Ren,et al.  Synthesis and properties of poly(arylenevinylene)s comprising of an electron-donating carbazole unit and an electron-accepting 2,1,3-benzothiadiazole (or fluorenone) unit in the main chain , 2005 .

[27]  Richard H. Friend,et al.  An improved experimental determination of external photoluminescence quantum efficiency , 1997 .

[28]  R. Czerwieniec,et al.  Radiative electron transfer in planar donor–acceptor quinoxaline derivatives , 2000 .

[29]  Yuqing Xie,et al.  Saturated red light-emitting copolymers of poly(aryleneethynylene)s with narrow-band-gap (NBG) units : Synthesis and luminescent properties , 2005 .

[30]  V. Jankauskas,et al.  Multifunctional emissive material based on 1-phenyl-1,2,3,4-tetrahydroquinoline , 2009 .

[31]  S. Juršėnas,et al.  Impact of Linking Topology on the Properties of Carbazole Trimers and Dimers , 2011 .

[32]  U. Scherf,et al.  Synthesis and characterization of new carbazole/fluorene‐based derivatives for blue‐light‐emitting devices , 2006 .

[33]  Chain-Shu Hsu,et al.  Donor-acceptor polymers based on multi-fused heptacyclic structures: synthesis, characterization and photovoltaic applications. , 2010, Chemical communications.

[34]  J. Reynolds,et al.  Soluble narrow band gap and blue propylenedioxythiophene-cyanovinylene polymers as multifunctional materials for photovoltaic and electrochromic applications. , 2006, Journal of the American Chemical Society.

[35]  Eduardo T. Iamazaki,et al.  Highly efficient polymer blends from a polyfluorene derivative and PVK for LEDs , 2009 .

[36]  David Beljonne,et al.  Charge-transfer and energy-transfer processes in pi-conjugated oligomers and polymers: a molecular picture. , 2004, Chemical reviews.

[37]  Mats Andersson,et al.  High‐Performance Polymer Solar Cells of an Alternating Polyfluorene Copolymer and a Fullerene Derivative , 2003 .

[38]  David Beljonne,et al.  Electronic and optical properties of polyfluorene and fluorene-based copolymers: A quantum-chemical characterization , 2003 .

[39]  J. Gražulevičius,et al.  Synthesis and properties of poly(3,9-carbazole) and low-molar-mass glass-forming carbazole compounds , 2002 .

[40]  Bassler,et al.  Diffusion and relaxation of energy in disordered organic and inorganic materials. , 1986, Physical review. B, Condensed matter.

[41]  D. Bradley,et al.  Simultaneous optimization of charge-carrier mobility and optical gain in semiconducting polymer films. , 2008, Nature materials.

[42]  Hong Yang,et al.  Large and ultrafast third-order optical nonlinearity of novel copolymers containing fluorene and tetraphenyldiaminobiphenyl units in backbones , 2001 .

[43]  S. Jungsuttiwong,et al.  Synthesis and characterization of N-carbazole end-capped oligofluorene-thiophenes , 2007 .

[44]  S. Juršėnas,et al.  Synthesis, photophysical and photoelectrical properties of glass-forming phenothiazinyl- and carbazolyl-substituted ethylenes , 2006 .

[45]  V. Jankauskas,et al.  Carbazolyl-and diphenylamino substituted fluorenes as hole transport materials , 2007 .

[46]  Mats Andersson,et al.  The electronic states of polyfluorene copolymers with alternating donor-acceptor units. , 2004, The Journal of chemical physics.

[47]  F. Gan,et al.  Large three-photon absorption cross-section in a novel class of bis-(N-carbazolyl) fluorene derivatives , 2005 .

[48]  Vidmantas Gulbinas,et al.  Charge Transfer Induced Excited State Twisting of N,N-Dimethylaminobenzylidene-1,3-indandione in Solution , 1999 .

[49]  Wolfgang Rettig,et al.  Structural changes accompanying intramolecular electron transfer: focus on twisted intramolecular charge-transfer states and structures. , 2003, Chemical reviews.

[50]  M. J. Adams,et al.  Determination of absolute fluorescence quantum efficiency of quinine bisulfate in aqueous medium by optoacoustic spectrometry , 1977 .

[51]  K. Zachariasse.  The PICT model for dual fluorescence of aminobenzonitriles. , 2000 .

[52]  Mario Leclerc,et al.  New Well-Defined Poly(2,7-fluorene) Derivatives: Photoluminescence and Base Doping , 1997 .

[53]  Yasuhiko Shirota,et al.  Photo- and electroactive amorphous molecular materials—molecular design, syntheses, reactions, properties, and applications , 2005 .