We present quantum chemical electronic structure calculations to investigate the nature of the low-lying excited states of [n]cycloparaphenylenes ([n]CPPs) and the role of static and dynamic geometrical distortions in the bright states. The lowest-energy bright states involve single-electron excitations from S0 ground state to S2 and S3 states, which are at the Franck-Condon geometry the two components of a twofold degenerate 1E state. They couple to a twofold degenerate e vibration which induces Jahn-Teller (JT) deformation of the CPP geometry from circular to oval shape. Non-radiative decay from the S2/S3 states to the ground S0 and first excited, dark S1 states is suppressed due to symmetry rules. The emission spectral features in CPPs with large number of phenylene units n can therefore largely be attributed to the E ⊗ e JT system associated with S2 and S3. However, absorption and emission energies computed at the respective S0 and S2/S3 minimum energy geometries are found to be nearly identical, independent of the molecular size n in the CPP molecules. In contrast, molecular dynamics simulations performed on the excited state potential surfaces are able to explain the experimentally observed fluorescence blueshift of the strongest emission peaks with increasing molecular size. This unusual feature turns out to be a consequence of large vibrational amplitudes in small [n]CPPs, causing greater Stokes shifts, while large [n]CPPs are more rigid and therefore feature smaller Stokes shifts (“dynamic blueshift”). For the same reasons, symmetry rules are violated to a greater extent in small [n]CPPs, and it is expected that in their case a “static blueshift” due to emission from S1 contributes in the fluorescence spectra.

1. 1Nagoya University
http://qc.chem.nagoya-u.ac.jp
1
2Universität Regensburg
PACCON 2013
Bangsaen Beach, Chon Buri, Thailand
January 24, 2013
Origin of theSize-Dependent Fluorescence
Blueshift in [n]Cycloparaphenylene
Stephan Irle,1Cristopher Camacho,1 Thomas Niehaus,2Kenichiro Itami1
2Electron Dynamics in Complex Systems Group,
Universität Regensburg
1Quantum Chemistry of Complex Systems,
Nagoya University

2. 2
[n]Cycloparaphenylenes (Collaboration with Itami Group)
We have a dream:
Omachi, Matsuura, Segawa, Itami, Angew. Chem. Int. Ed. 49, 10202 (2011)
Prof. Itami

3. Ground state PES
3Segawa, Omachi, Itami, Org. Lett. 12, 2262 (2010) (Supporting Material)
Linear relationship between strain energy and size n
q
Similar to carbon nanotubes!
cf. Kudinet al., Phys. Rev. B61,
235406 (2001)

4. Absorption and Emission Spectra
4Iwamoto, Watanabe, Sakomoto, Suzuki, Yamago, JACS 133, 8354 (2011)
Blueshift in emission
with increasing n!

5. Absorption and Emission Spectra
5
“Nomal” redshift in open-polyparaphenylenes
TD-CAM-B3LYP/SV(P)

6. Biphenyl p-MOs
6
Constant amplitude, a+2b
Constant amplitudes, a-2b
2b
a
HOMO = -
LUMO = +
p-bond!

7. CPP frontier MO’s
• MO level diagram n=12, D6d symmetry:
• HOMO-LUMO excitation is symmetry-forbidden!
• e MOs behave like x and y functions
• LUMO+1HOMO and LUMOHOMO-1 excited states are both of
E1 symmetry, they mix!
• 4 states:
e1
a2
a1
e1
LUMO+1
LUMO
HOMO
HOMO-1
x y HOMO-2
LUMO+2
nodal plane
(LUMO+1xHOMO)+(LUMOHOMO-1x)
(LUMO+1yHOMO)+(LUMOHOMO-1y)
(LUMO+1xHOMO)-(LUMOHOMO-1x)
(LUMO+1yHOMO)-(LUMOHOMO-1y)
1E1
2E1 7
e
Bright!
Dark!

8. [n]CPP frontier MO’s
8
CAM-B3LYP/SV(P) at ground state optimized structures (all alternating conformations)
No Blue-shift inbright
transitions!
HOMO-LUMO gap increasing with increasing n!

9. 9
[n]cPP: [2n]cPP cut in half, fixed geometry
[n]CPP frontier MO’s
[n]OPP: [2n]cPP cut in half but linear, optzd.
HOMO-LUMO gap in [3]OPP
Stronger p-antibonding
Stronger p-bonding
in [n]CPP
[∞] OPP HOMO
CAM-B3LYP/SV(P) @ S0opt’d geometries
[∞] OPP LUMO

10. 10
Energy [eV]
1A1
21E1
11E1
1A’1A’
Qq
2 1Bx
-Qq
1 1Bx
3 1By
4 1By
2 1By
1 1By
3 1Bx
4 1Bx
Qq = x2-y2
-Qq= -(x2-y2)
Jahn Teller Energy Diagram (schematic)

11. 11
Jahn Teller DistortionCoordinates Qe&Qq
nodal plane
-Qq = -x2+y2Qe = xy
[10]CPP

12. Absorption and Emission Spectra
12
1E1
Emission: at least 2 intensive peaks!

13. Absorption and Emission Spectra
13
1E1
Absorption: 1 peak Emission: at least 2 peaks!

14. S0
S0
’
S1
S1
’
S0, S0’: ground state
S1, S1’: lowest excited singlet state energy/hartree
coordinate
Energy diagramMethod
 TURBOMOLE, GAMESS, DALTON
 B3LYP/SV(P) level
 sequence of calculations
1. optimization in the ground state
2. TD-DFT calculation at S0 structure
3. optimization in the excited state
TD-DFT calculation at S1’ structure
1
2
absorption
3
fluorescence
TD-DFT Methods
14
Geometry optimization in excited state

15. Ground state geometries
15Segawa, Omachi, Itami, Org. Lett. 12, 2262 (2010)
2 local conformers:
Many conformational isomers …
[12]CPP [12]CPP [12]CPP
[kcal/mol]
B3LYP/6-31G(d)

16. Ground state PES
16Segawa, Omachi, Itami, Org. Lett. 12, 2262 (2010)
Ground state transition states for phenyl group rotation around f
[12]CPP
B3LYP/6-31G(d)
f
f

17. [n]CPP UV/Vis spectra
17
Absorption: red
Emission: red

18. 18
[n]CPP UV/Vis spectra
Absorption: artificially constant
Emission: still red

19. 19
[n]CPP UV/Vis spectra
Absorption: ~constant
Emission: ~constant

20. 20
[n]CPP UV/Vis spectra
Absorption: ~constant
Emission: ~constant

21. 21
[n]CPP UV/Vis spectra
1E1
Absorption: ~constant
Emission: ~constant

22. Alternative to DFT: Approximate DFT
Density-Functional Tight-Binding: Method using atomic parameters
from DFT (PBE, GGA-type), diatomic repulsive potentials from B3LYP
•Seifert, Eschrig (1980-86): minimum
STO-LCAO; 2-center approximation
•Porezag, Frauenheim, et al. (1995):
efficient parameterization scheme: NCC-
DFTB
•Elstneret al. (1998): charge self-consistency: SCC-DFTB
•Köhleret al. (2001): spin-polarized DFTB: SDFTB
Marcus Elstner
ChristofKöhler
Helmut
Eschrig
Gotthard
Seifert
Thomas
Frauenheim
22
MD in excited state
Thomas
Niehaus
Linear response:
TD-DFTB

23. 23
CAM-B3LYPTD-DFTB/MD
Electron Dynamics in Complex Systems Group,
Universität Regensburg
Linear response
TD-DFTB:
Thomas Niehaus
Method
 TD-DFTB w/mio-1-1 parameters
 8 states considered, dynamics performed for S0, S1, S2/S3
 MD:
1. starting from optimized geometries
2. NVT 0.5 ps equilibration at 298 K
3. NVE for 4.7 ps, production runs
4. CAM-B3LYP/SV(P) single point excited state calculations
(up to 32 sample points)

24. 24
Simulated [n]CPP UV/Vis spectra
CAM-B3LYP/SV(P)TD-
DFTB-MD snapshots
Energy
Energy
Yes blueshift!
Yes blueshift!
Yes blueshift!

25. 25
PESs during excited state dynamics
f-fdihedral angle
S2
S1
S0
S2-S0
S1-S0
S2
S1
S0
S2-S0
S1-S0
State energies during MD
CAM-B3LYP/SV(P)TD-DFTB-MD
Transition energies during MD

26. 26
PESs during excited state dynamics
State energies during MD Transition energies during MD
CAM-B3LYP/SV(P)TD-DFTB-MD

27. 27
Why Emission from S2?
Energy difference between S1 and S2 very small, around
1 eV or smaller. Higher population of S2 than in usual
organic molecules.
Vibronic coupling matrix elements between S2 (u-type
symmetry) and S0/S1 (g-type symmetry) since low energy
molecular vibrations of circles behave as x2, y2 (g-type)
radiationless decay from S2 “blocked”
Consequence:
-Emission from S2 easily possible in case of large n,
while small CPP with ndistort also in x,y and emission
becomes possible from S1.
-Red shift for small n> red shift for large n
 appearance of “blue shift” with increasing size n
Published in: C. Camacho, Th. Niehaus, K. Itami, SI, Chem. Sci. 4, 187 (2013)
ggu

28. Absorption and Emission Spectra Explained
28
Absorption: 1 peak
Dynamic blue-
shift, emission from
S2/S3
Static blue-shift,
emission from S1

29. Prof. Dr. Stephan Irle
sirle@chem.nagoya-u.ac.jp
Assist. Prof. Dr. Daisuke Yokogawa
d.yokogawa@chem.nagoya-u.ac.jp
WPI-Institute of Transformative Bio-Molecules &
Department of Chemistry, Nagoya University
of Complex Systems
November 5, 2012
Back row: Yoshifumi Nishimura (D3), KosukeUsui (M2), Jun Kato (B4), Tim Kowalczyk (JSPS,
PhD,), Cristopher Camacho-Leandro (PhD), Yoshio Nishimoto (JSPS, D1)
Front row: Takayo Noguchi (secty), Naoto Baba (B4), Matt Addicoat (JSPS, PhD), SI, Arifin (G30,
D1), Daisuke Yokogawa (Assist. Prof.)

30. OUR GOAL is to develop “transformative bio-molecules”, innovative functional
molecules that make a marked change in the form and nature of biological science and
technology.
Institute of Transformative Bio-
Molecules (Nagoya University)
EXPECTED OUTCOME
Our ten-year campaign will culminate in a wealth
of synthetic bio-molecules that will be key to
solving urgent problems at the interface of
chemistry and biology. The innovation in
food/biomass production, optical technologies,
and generation of new bio-energy can be
imagined as our dream.
THE IDENTITYof the Center is its capability to
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OUR UNIQUE APPROACH is to apply our
cutting-edge synthesis (molecule-activation
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advanced systems biology in plants and
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32. Several Postdoc positions open at the WPI!
2 positions in my group, specialty: biofluorescence, binding free
energies (advertized on CCL.net)
Institute of Transformative Bio-
Molecules (Nagoya University)

33. 33
Promotion
http://admissions.g30.nagoya-u.ac.jp
Undergraduate and Graduate Courses in Physics,
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Admissions for 2013 are currently ongoing.
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34. Thank you for your attention!
34