CHIROPTICAL SPECTROSCOPIES FOR INVESTIGATING SOFT MATTER AND ORGANIC- BASED CONDUCTING MATERIALS Giovanna Longhi Dipartimento di Medicina Molecolare e Traslazionale Università degli Studi di Brescia PhD Course of Materials Engineering Politecnico di Milano
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CHIROPTICAL SPECTROSCOPIES FOR
INVESTIGATING SOFT MATTER AND ORGANIC-
BASED CONDUCTING MATERIALS
Giovanna Longhi
Dipartimento di Medicina Molecolare e Traslazionale
Università degli Studi di Brescia
PhD Course of Materials Engineering Politecnico di Milano
Chiral interaction molecule-light: ‘chiral’ light In order to detect molecular chirality, some sort of chiral probe must be used: Right- and left-circularly polarized light beams are mirror-image chiral systems and so can act as chiral probes:
right
left
z
z counter-
clockwise
clockwise
Chiral molecules respond slightly differently to right- and left-circularly polarized light: a difference in absorption corresponds to Circular Dichroism a difference in refractive index leads to Optical Rotation
How to generate a circularly polarized wave:
superposition of two waves polarized in two perpendicular planes, with same amplitude and wavelength and
with a phase difference of 90 degrees between them.
Linearly polarized light :
Linearly polarized light can be seen as the sum of two circularly polarized
components of same magnitude and opposite phase
ORD: optical rotatory dispersion
Wavelength Dependence of Optical Rotation Circular Birefringence Refractive indexes for left and right circularly
polarized light are different.
we measure rotation angle: (degrees)
CD:
Circular dichroism Ellipticity Left and right components components
are differently absorbed.
we measure ellipticity angle:
or absorbance difference: A
for small angles: A °/32.98
Optically active system: how does it changes trasmitted ligth?
Incident linear polarized wave
Incident plane polarized light: polarization of the light out of an optically active medium.
Both effect are observed in general:
Circular Birefringence:
The two circular components have
different velocity, nLnR
Major axis is no more parallel to the initial
polarization direction
Rotation of the polarization plane
Circular Dichroism
The two circular components are
absorbed differently, ALAR
Elliptic polarization
[α]l = α · 100 / (d · c)
[] l = 32980 (AL-AR)/ (d · c)
α = (nL-nR)180 d /l = (kL-kR) 180 d /l
Normal optical rotation (OR):
For an optically active compound, in a
spectroscopic region with no absorbtion
bands, the Optical Rotation Dispersion
(ORD) curves are monotonic continuous
functions (positive or negative )
nm
OR
Circular Dichroism: in corrispondence of
an absorbtion band there is a sligtly
different absorbtion between left and right
circularly polarized ligth In correspondence with a Circular Dichroism band
Optical Rotaion Dispertion becomes anomalous
varying wavelength……..CD spectra and ORD dispersion
(+) dimethyl-L-tartrate
(-) dimethyl-D-tartrate
Circular Polarization Spectroscopy of Chiral Molecules
At molecular level:
optical activity asymmetry of electronic distribution The absorbed light induces an elicoidal shift of charges, this explain the preferred
absorbtion of one circularly polarized component.
Absorption band
Electric dipole transition moment
l = 265 nm
= 6400
l = 380 nm
= 7600
l = 252 nm
= 204000
CH
l = 234 nm
= 15000
D = 0
D = 92 10-40max
l
l nm
(unità cgs)
è orientato all'interno di un cromoforo:
Una banda d'assorbimento appare nello spettro VIS-UV di una molecola organica quando la transizione elettronica ad essa associata possiede
un momento di dipolo elettrico
l = 265 nm
= 6400
l = 380 nm
= 7600
l = 252 nm
= 204000
CH
l = 234 nm
= 15000
D = 0
D = 92 10-40max
l
l nm
(unità cgs)
è orientato all'interno di un cromoforo:
Una banda d'assorbimento appare nello spettro VIS-UV di una molecola organica quando la transizione elettronica ad essa associata possiede
un momento di dipolo elettrico
Optically active transitions
A 0m transition to be optically active needs both a transition electric dipole
moment m and a transition magnetic dipole moment m.
By analogy with the dipole strength associated with normal absorption, we can define a
rotational strength that indicates the intensity, or probability, of a CD transition.
R0m = 0m ∙ m0m
max
max40- 23x10
l
l
R
•Optical activity requires both a finite μ and a finite m. The product of these two vectors corresponds to a
helical displacement of charge.
R0m = ∙ m ≠ 0 if and only if both and m ≠ 0 and non ortogonal
For molecules with inversion symmetry i there are states g e u :
0m ≠ 0 per g u
m0m ≠ 0 per g g o u u Rom = 0
For molecules with a symmetry plane s there are states + e - :
0m ≠ 0 per + -
mom ≠ 0 per + +, - -
Rom = 0
Selection Rule for CD
-Charge displacements (μ) that accompany absorption events are linear.
electric dipole allowed transitions
- The circular component (if present) generates a magnetic dipole (m).
Transitions that generate a magnetic dipole are magnetically allowed.
- Optical activity requires both a finite μ and a finite m. The dot product of these
two vectors is a pseud-scala and is associated to a helical displacement of charge.
ECD: UV-VIS range 800-180 nm electronic transitions
VCD: IR range 800-4000 cm-1 vibrational transitions:
“fingerprint” region
NIR-VCD: NIR range 850-1750 nm
-vibrational overtone transitions VCD)
-electronic transitions
(metal-ions and highly conjugated materials). ECD
ORD: optical rotation dispersion (far from absorbtion bands)
CHIROPTICAL TECHNIQUES COMPRISE -OPTICAL ROTATION (OR and ORD),
-CIRCULAR DICHROISM (CD)
(IN THE IR-NIR: VCD AND IN THE UV-Vis-NIR: ECD)
-CIRCULARLY POLARIZED LUMINSCENCE (CPL)
-RAMAN OPTICAL ACTIVITY (ROA)
CIRCULAR DICHROISM
R L
Raman optical activity
ROA Circularly Polarized Luminescence
CPL
Circularly Polarized Luminescence CPL
CPL apparatus:
PEM ¼ photoelastic modulator;
PM photomultiplier tube).
LED sources, LASER, optical fiber indicated by arrows, in
the vicinity of the sample position.
optional depolarizer
optional polarizer
Raman optical activity (ROA)
ROA measures a tiny difference in the intensity of Raman scattering from
chiral molecules in right (R)- and left (L)-circularly polarized incident light:
ROA has the same relationship to Raman as ECD does to