1. Literature seminar 2013.06.14 Yuna Kim

2. Liquid crystals
- functional soft materials combining both ordered structures and a dynamic nature.
- Liquid-crystalline (LC) phases; tuning the molecular shapes and intermolecular interactions.
- Phase transitions of liquid crystals through external stimuli.
light
mechanical shearing
accompanied by a change of the molecular self-assembled structures.
Electric fields
Electric Field-Induced Cubic Phase
Modulus
change
shear-induced nematic-isotropic transition
Kutsumizu et al
Nature 409, , 2001
- In the condensed states,
Multi-emission colors from a single luminophore and switch between their colors using external stimuli have been rarely achieved for organic and organometallic materials

3. Emission Color Determined by Thermal and Mechanical Processes
Previous study
Tricolored Mechanochromic Luminescence from a Single-Luminophore Liquid Crystal
Emission Color Determined by Thermal and Mechanical Processes
Excimer emi.
micelles
metastable LC
stable LC
Excimer emi.
Partial excimer emi.

4. Inducing smectic LC
bulky substituents; Usually are used to disturb intermolecular interactions and to suppress excimer emission (steric effects)
in the present design; tert-butyl substituents instead induce excimer formation.

5. Synthesis

8. partial-overlap excimers in addition to normal excimers.
Bands shown btn 350~500 nm
(S0-S1 transition) w/ vibronic structure; monomeric states
522 nm
624 nm
partial-overlap excimers in addition to normal excimers.
The partial-overlap excimers display emission bands at shorter
wavelengths than those observed for normal excimers.

9. Luminescent cores of 2 can approach each other up to about 3 Å and form excimers that avoid the steric hindrance of tert-butyl substituents; the relative orientation of the two molecules is partially limited by the tert-butyl substituents

10. 2.6 ns
2.9 ns
monomeric states
8.1 ns
33 ns
partial excimers is dominant
normal excimer

11. Partial overlap excimers of the emission cores.
cubic
Columnar
Mesomorphic

12. bathochromic shift in emission of 2
In the ground state; no intermolecular interaction among adjacent molecules of 1 and 2 in the SmA phases
after excitation; some of the luminescent moieties form excimers.
compound 2- the normal excimer is mainly formed because of the steric effects.
The monomeric emission is little observed because energy transfer may occur from most of the excited luminescent moieties to the excimer sites.
compound 1- without tert-butyl substituent can form various kinds of partial-overlap excimers as well as the normal excimer
the ratio of the normal excimer is low, resulting in the broader and blue-shifted emission compared to compound 2.

13. Conclusion
Luminescent smectic LC materials 1 and 2 based on a 9,10-bis(phenylethynyl)anthracene moiety have been prepared and they show smectic A phases. Significant differences in photophysical properties between 1 and 2 are observed in the smectic A phase.
Bulky tert-butyl substituents enhance the possibility to form well-overlapped excimers. This approach may provide a new tool to control the photoluminescent properties of p-conjugated molecular assemblies in addition to hydrogen bonding, charge transfer interactions and other interactions and may lead to the further functionalization of organic materials.