第八章 分子发光光谱法molecular_luminescence

STRUCTURAL RIGIDITY -- Chelation
OH 8-hydro
No fluorescence
Strong fluorescence
• For a highly fluorescent molecule such as fluorescein, 1
• Chemical species that do not fluoresce appreciably have efficiencies that approach 0
Quantum Efficiency ()
屈
Variables That Affect Fluorescence & Phosphorescence
• Quantum Yield • Chemical Structure • Environment
Quantum Efficiency ()
• i.e. the ratio of the number of molecules that luminesce to the total number of excited molecules.
1．Jablonski 能级图
Professor Aleksander Jabłoński (born in 1898, died in 1980) was a Polish physicist.
Fluorescence is schematically illustrated with the classical Jablonski diagram, first proposed by Jablonski in 1935 to describe absorption and emission of light.
外转换使荧光或磷光减弱或“猝灭”。 系间跨越：不同多重态,有重叠的转动能级间的非辐射跃迁。
改变电子自旋，禁阻跃迁，通过自旋—轨道耦合进行。
（2）辐射能量传递过程
荧光发射：电子由第一激发单重态的最低振动能级→基态（
多为 S1→ S0跃迁），发射波长为 ‘2的荧光； 10-7～10 -9 s
。
发射荧光的能量比分子吸收的能量小，
单重态的寿命一般为10-8秒，最长可达10-6秒，但有时却 可以观察到单重态寿命长达10-3秒，这种长寿命的荧光被称 为延迟荧光。其寿命与该物质的分子磷光相当。
延迟荧光在激发光源熄灭后，可拖后一段时间，但和磷 光又有本质区别，同一物质的磷光总比发射荧光长。
延迟荧光
延迟荧光
二、荧光量子效率
Emission and Excitation Spectra
FLUORESCENCE AND STRUCTURE (CONTD.)
Fusion of heterocycles to benzene rings increases the molar absorptivity for n- * absorptions and shortens the lifetime of the n- * singlet preventing its conversion to triplet. This increases fluorescence quantum efficiency.
传递途径
辐射跃迁
无辐射跃迁
荧光 延迟荧光 磷光
系间跨越 内转移 外转移 振动弛预
Jablonski能级图
内转换 S2
S1
能 量
发
射
吸
荧
收
光
振动弛豫
内转换
系间跨越
外转换
T1
T2
发
射
磷
光
振动弛豫
S0 λ1
S单重态
λ3
λ2
λ
2
Jablonski diagram
T三重态

电子激发态的多重性：M=2S+1 （S为总自旋量子数）； M=1（S=0）单重态用S表示；M=3（S=1）三重态用T表示； 电子自旋状态：平行自旋比成对自旋稳定(洪特规则)； 所以三重态能级（平行自旋）比相应单重态（成对自旋）能 级低。
phenanthrene, 菲
Emission Spectrum: Plot of the emission intensity at 90o to the incident radiation as a function of emission wavelength for a fixed excitation wavelength.
HEAVY ATOM EFFECT
HEAVY ATOM EFFECT
Heavy atom effect : The quantum yield decreases with the atomic number of the halogen.
The probability for intersystem crossing increases with increasing atomic number of the halogen which reduces fluorescence.
第八章 分子发光光谱法
(Molecular Luminescence Spectroscopy)
分子发光光谱法包括：
光致发光
分子荧光（Molecular Fluorescence）
(Photoluminescence)
化学发光（Chemilumin分es子ce磷nc光e）（Molecular Phosphorescence）
Excitation Spectrum: Plot of the emission intensity at a fixed emission wavelength at 90o to the incident radiation as a function of excitation wavelength. Wavelength of excitation maximum is often where the emission intensity is a maximum.
• Highly conjugated double-bond structures
FLUORESCENCE AND STRUCTURE (CONTD.)
Simple heterocycles do not exhibit fluorescence. The n- *singlet quickly converts to the n- * triplet and no fluorescence is observed.
生物发光（Bioluminescence） 电致发光（Electroluminescence）
§8-1 分子荧光与磷光的基本原理
Principle of molecular fluorescence and phosphorescence
一、分子荧光与磷光的产生
物质分子在一定条件下吸收辐射能而被激发到较高电子能态后， 在返回基态的过程中将以不同的方式释放能量。 荧光和磷光是以发射辐射的形式释放能量，属于光致发光。
‘ 2
>
2
>
1
；
磷光发射：电子由第一激发三重态的最低振动能级→基态（
T1 → S0跃迁）； 电子由S0进入T1的可能过程：（ S0 → T1禁阻跃迁）
S0 →激发→振动弛豫→内转移→系间跨越→振动弛豫→ T1 发光速度很慢： 10-4～100 s 。
光照停止后，可持续一段时间。
延迟荧光（delayed fluorescence)也被称为缓发荧光，它来源 于从第一激发三重态（T1）重新生成的S1态的辐射跃迁。
• It is determined by the relative rate constants for the processes by which the lowest excited singlet state is deactivated.
kf
k f ki kec kic k pd kd
Chemical Structure
• Fluorescence from singlet states of - * have more intensity than those from n- * transitions as the molar absorptivities for - * absorptions are much higher than those for n- * absorptions.
STRUCTURAL RIGIDITY
quantum
yields
1.0
0.2
Fluorescence is favored in molecules with structural rigidity.
The increased rigidity of fluorene stabilizes the - * singlet state leading to higher quantum yield.
FLUORESCENCE AND STRUCTURE (CONTD)
Substitutions on benzene ring affect fluorescence maxima and fluorescence intensity as shown in the table.
Electron withdrawing groups decrease intensity
FLUORESCENCE AND STRUCTURE (CONTD)
Substitutions on benzene ring affect fluorescence maxima and fluorescence intensity as shown in the table.