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PCCP Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: D. J. Stewart, M. J. Dalton, S. L. Long, R. Kannan, Z. Yu, T. M. Cooper, J. E. Haley and L. Tan, Phys. Chem. Chem. Phys., 2016, DOI: 10.1039/C5CP07716H.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
www.rsc.org/pccp
Page 1 of 23
Physical Chemistry Chemical Physics View Article Online
Steric Hindrance Inhibits Excited-State Relaxation and Lowers the Extent of Intramolecular Charge Transfer in Two-Photon Absorbing Dyes David J. Stewart,*,a,b Matthew J. Dalton,a Stephanie L. Long,a,c Ramamurthi Kannan, a,d Zhenning Yu,a,d Thomas M. Cooper,a Joy E. Haley,*,a and Loon-Seng Tana a
Air Force Research Laboratory, Materials and Manufacturing Directorate, Functional Materials Division, Wright-Patterson AFB, Ohio 45433-7750, United States b
General Dynamics Information Technology, 5100 Springfield Pike, Dayton, Ohio 45431, United States c
Southwestern Ohio Council for Higher Education, Dayton, Ohio 45420, United States d
UES, Inc., 4401 Dayton-Xenia Road, Dayton, Ohio 45432, United States
1
Physical Chemistry Chemical Physics Accepted Manuscript
Published on 27 January 2016. Downloaded by Middle East Technical University (Orta Dogu Teknik U) on 30/01/2016 02:56:13.
DOI: 10.1039/C5CP07716H
Physical Chemistry Chemical Physics
Page 2 of 23 View Article Online
DOI: 10.1039/C5CP07716H
2-yl)diphenylamine)] is modified by adding bulky alkyl groups to the diphenylamino moiety. Three new compounds are synthesized which have ethyl groups in both ortho positions of each phenyl ring (2), t-butyl groups in one ortho position of each phenyl ring (3), and t-butyl groups in the para position of each phenyl ring (4). The dyes are examined in several aprotic solvents with varying polarity to observe the effects of the sterically hindering bulky groups on the ground and excited-state photophysical properties. While the ground state shows minimal solvent dependence, there is significant dependence on the fluorescence quantum yield and lifetime, as well as the excited-state energy levels. This effect is caused by the formation of an intramolecular charge-transfer (ICT) state, which is observed in the solvents more polar than nhexane and supported by TD-DFT calculations. Electronic effects of ortho or para alkyl substitution should be similar, yet drastic differences are observed. A red shift in the fluorescence maximum is observed in 4 relative to 1, yet a blue shift occurs in 2 and 3 because the substituents at the sterically sensitive ortho-positions inhibit excited-state geometric relaxation and result in less ICT character than 1. Coupled with theoretical calculations, the data support a planar ICT (PICT) excited state where the diphenylamino nitrogen in an sp2-like geometry is integral with the plane containing the fluorene and benzothiazole moieties. Ultrafast transient absorption experiments show that ICT occurs rapidly (
PCCP Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: D. J. Stewart, M. J. Dalton, S. L. Long, R. Kannan, Z. Yu, T. M. Cooper, J. E. Haley and L. Tan, Phys. Chem. Chem. Phys., 2016, DOI: 10.1039/C5CP07716H.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
www.rsc.org/pccp
Page 1 of 23
Physical Chemistry Chemical Physics View Article Online
Steric Hindrance Inhibits Excited-State Relaxation and Lowers the Extent of Intramolecular Charge Transfer in Two-Photon Absorbing Dyes David J. Stewart,*,a,b Matthew J. Dalton,a Stephanie L. Long,a,c Ramamurthi Kannan, a,d Zhenning Yu,a,d Thomas M. Cooper,a Joy E. Haley,*,a and Loon-Seng Tana a
Air Force Research Laboratory, Materials and Manufacturing Directorate, Functional Materials Division, Wright-Patterson AFB, Ohio 45433-7750, United States b
General Dynamics Information Technology, 5100 Springfield Pike, Dayton, Ohio 45431, United States c
Southwestern Ohio Council for Higher Education, Dayton, Ohio 45420, United States d
UES, Inc., 4401 Dayton-Xenia Road, Dayton, Ohio 45432, United States
1
Physical Chemistry Chemical Physics Accepted Manuscript
Published on 27 January 2016. Downloaded by Middle East Technical University (Orta Dogu Teknik U) on 30/01/2016 02:56:13.
DOI: 10.1039/C5CP07716H
Physical Chemistry Chemical Physics
Page 2 of 23 View Article Online
DOI: 10.1039/C5CP07716H
2-yl)diphenylamine)] is modified by adding bulky alkyl groups to the diphenylamino moiety. Three new compounds are synthesized which have ethyl groups in both ortho positions of each phenyl ring (2), t-butyl groups in one ortho position of each phenyl ring (3), and t-butyl groups in the para position of each phenyl ring (4). The dyes are examined in several aprotic solvents with varying polarity to observe the effects of the sterically hindering bulky groups on the ground and excited-state photophysical properties. While the ground state shows minimal solvent dependence, there is significant dependence on the fluorescence quantum yield and lifetime, as well as the excited-state energy levels. This effect is caused by the formation of an intramolecular charge-transfer (ICT) state, which is observed in the solvents more polar than nhexane and supported by TD-DFT calculations. Electronic effects of ortho or para alkyl substitution should be similar, yet drastic differences are observed. A red shift in the fluorescence maximum is observed in 4 relative to 1, yet a blue shift occurs in 2 and 3 because the substituents at the sterically sensitive ortho-positions inhibit excited-state geometric relaxation and result in less ICT character than 1. Coupled with theoretical calculations, the data support a planar ICT (PICT) excited state where the diphenylamino nitrogen in an sp2-like geometry is integral with the plane containing the fluorene and benzothiazole moieties. Ultrafast transient absorption experiments show that ICT occurs rapidly (
