第八讲分子材料

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第八讲 molecular materials
•Introduction to molecular materials •Electronic states in conjugated molecules •Optical spectra of molecules •Aromatic hydrocarbons •Conjugated polymers •Organic optoelectronics
① absorption:
a


E2

(n2

1 2
)2



E1

1 2
1

0 n22

0

E2

E1

1 2
(2

1)
•vibrational-electronic process : electron to jump
to the excited electronic state, and vibrational
课程内容
绪论 经典传播 带间吸收，激子吸收 发光 半导体量子阱、自由电子、分子材料 发光中心
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Features of molecular materials
------Optical properties of molecular materials have been intensely studied since 1990s, due to its potential application in electroluminescent and photovoltaic devices.
•Absorption: promoting an electron to the excited state without altering equilibrium nuclei separation r1 of the ground state.
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Electronic states in conjugated molecules
•HOMO level : highest occupied molecular orbital •LUMO level: lowest unoccupied molecular orbital •They are must be orbitals in conjugated molecule, because bond electrons are very tightly bound. •Lowest energy transition: * •HOMO level, S=0; excited states, S=0 or 1
The Franck-Condon principle
•The electronic transitions take place so rapidly that the nuclei do not move significantly during the transition. ---The Franck-Condon principle (nuclei are much heavier than the electrons)
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Introduction to molecular materials
Focus on conjugated organic compounds with delocalized orbitals
1. aromatic hydrocarbons ( 芳烃化合物）：carbon-hydrogen compounds containing benzene rings
Reason: photons carry no spin.
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第八讲 molecular materials
•Introduction to molecular materials •Electronic states in conjugated molecules •Optical spectra of molecules •Aromatic hydrocarbons •Conjugated polymers •Organic optoelectronics
•Delocalized electrons quantum confinement effects a reduction in the
energy into visible spectral region electrons transition!!!
•the lowest electronic transition : ethylene at 6.9 eV, benzene at 4.6 eV.
------The optical physics of organic materials is different to that of the inorganic crystals in two aspects:
The electronic and vibrational states tend to be localized near individual molecules-----the properties of the solids are quite similar to those of the constituent molecules. The optical transitions are vibronic in character: the transition simultaneously changes the electronic and vibrational state of the molecule.
quanta created. absorption spectrum will
consist of a series of discrete lines
② ④ non-radiative relaxation
③ emission:
e


E2

1 2
2


E1

(1 2

n1
•visible and ultraviolet spectra: <1m. transitions between the electronic states of the molecule.
•In this chapter, we will be concerned with electronic and vibrational transition.
2. conjugated polymers （共轭聚合物）：long chain molecules made by bonding many individual molecular units ('monomers') together.
Molecular solids are held by van der Waals type interactions of neutral organic compounds, therefore:
•The molecule can also change its vibrational state during the electronic transition-----vibrational-electronic transtion.
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Electronic-vibrational transitions
•The localized nature of the electronic and vibrational states means
that the optical properties of the solid are rather similar to those of
the individual molecules.
)1


0

n11
•emission spectrum consists of a series of vibrational-electronic lines.
absorption occurs at a higher energy than the emission,
The difference in energy between the maximum absorption and the maximum emission is called the Stokes Shift. 10
•singlet-singlet transition are spin-allowed, dominate the absorption and emission
process; triplet-singlet transitions are spin-forbidden and have very low probabilities.
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Three spectral regions
•far-infrared spectra: wavelength > 100 m. transitions between the rotational states of the molecule.
•infrared spectra: 1-100 m. transitions between the vibrational states of the molecule.
1. Electronic states remain tightly bound to the constituent molecules. -----localized electronic states.
2. Phonons of molecular solids are localized modes with discrete frequencies.