Electromagnetic Spectrum and Its Absorption by Molecules

X-rays UV/Vis IR Radio

Near Mid Far

HighEnergy

LowEnergy

200

800

2500

2500

0Wavelength (nm)

Ee Ev Er EnsCore Bonding/

Valence

Mol

ecul

arE

nerg

y ty

pes

Light is quantizedE= hc/λ

Energy levels in molecules are quantized

Elight = ΔEEmoleclues (between energy levels) → absorbance or emission

4000

400

Wavenumbers, cm-1

Review of IR Spectroscopy (a kind of vibrational spectroscopy)Rules for Infrared Spectroscopy

Vibrational Spectroscopy

● Not vibrating light● Vibrating molecules

http://www.chemeddl.org/resources/models360/models.php?pubchem=313

https://www.chemtube3d.com/spectrovibhcl1-ce-final/#models

http://sites.science.oregonstate.edu/~gablek/CH335/Chapter10/IR_vibrations.htm

Using light to probe of vibrational modes of molecules (molecular ions)

Spectroscopy is probing atoms, molecules, ions with light

Scattering of Light

Jablonski Diagram

Ground electronic state

Electronic excited state

v=1v=2v=3

NIRAbsorption

IR (Mid)Absorption

RayleighScattering

RamanScattering

ResonanceRaman

Scattering

hν0 hν0

hν0 hν0 – hν1

hν0 hν0 – hν1

Virtualstate

Ramanhν0 = energy of excitation lighthν1 = energy of scattered light

Rayleighhν0 = energy of excitation lighthν0 = energy of scattered light

Stokes vs. antiStokes

Ground electronic state

Electronic excited state

v=1v=2v=3

NIRAbsorption

IR (Mid)Absorption

RayleighScattering

StokesRaman

Scattering

AntistokesRaman

Scattering

hν0 hν0

hν0

hν0 – hν1

Virtualstate

hν0 – hν1

hν0

Laser line

Measured data to Raman spectra

Wavelength /nm

Convert to Raman Spectrum(convention)

Wavenumber /cm-1

Example Raman Spectrum

Laser 785 nm

Selection Rules

+

+

+++

+

+

++

----------

e-e-

e-e-

δ+e-

e-e-

e-

δ+

e-e-

e-e-

e-e-

e- e-

δ+

Rule of Savannah: More electrons in the valence orbitals/involved in bonding, the more polarizable

Polarizability (α): The ability to form instantaneous dipoles ): The ability to form instantaneous dipoles

e-e-

e-e-

e-

e- e-

e-

Induced dipole

Selection Rule:Changing α): The ability to form instantaneous dipoles during vibration is key to strong signal.

Selection RulesRule of Mutual Exclusion

https://science.widener.edu/svb/ftir/ir_co2.html

Raman spectrum of CO2

IR spectrum of CO2

Center of inversion

Vibrational Modes3N-5 = 4

Intensity of Raman Scattered lightConcentration Dependence

Notice how Raman Intensity is not a Ratio of light through sample over light though blank (used in absorbance spectroscopy)● Intensity of laser correlates with intensity

of Raman signal (if your laser intensity is fluctuating then your Raman scattered light is fluctuating)● This would be the same problem with

Fluorescence● Intensity of Raman scattered light is laser

intensity dependent● Solution is often an internal standard

● Add a known amount of a chemical that does not chemically or optically interfere with the analyte to all standards

Relative Intensities of Raman Scattering

Type Iscattered/emitted --------

IincidentFluorescence 1 – 0.01Rayleigh 10-4

Stokes Raman 10-7

AntiStokes Raman 10-9

Resonance Raman 10-2 - 10-4

SE Raman > 1

Given a specific molecular system, what can you control?

Io and λ

Scattering Cross-Section

Process Cross-section of σ (cm2)

absorption UV 10-18

absorption IR 10-21

emission Fluorescence 10-19

scattering Rayleigh 10-26

scattering Raman 10-29

scattering Resonance Raman

10-24

scattering SERS 10-16

scattering SERRS 10-15

Conceptual construct:Area presented by the molecule for the process

Useful for comparing processes

Bigger the cross-section more likely the process

Take home message: Raman is a very, very, very unlikely process to occur.

Electronic excited state

v=1v=2v=3

RayleighScattering

hν0 hν0

hν0

hν0 – hν1

Virtualstate

hν0 – hν1

hν0

Vibrational relaxation

Fluorescence emissionStokesRaman

Scattering

AntistokesRaman

Scattering

● Rayleigh scattered light is same narrow bandwidth as laser.

● All molecules that Raman scatter very narrow band shifted from laser λ

● Fluorescence emission bandwidth is very broad

● Each molecule can have a slightly different wavelength

From McCreery 2000 p 4

Fluorescence interference almost always out “shines” Raman scattered light

samplewavelength

selectorand Rayleigh light

rejection

detector

LAS

ER Why this geometry?

Laser transmssion (if using a cuvette/transparent sample holder)

Laser cleanup filterNotch or edge filterRayleigh rejection filter

Sample handling

Raman can measure gases, liquids and solids. Close and far.

Fiber coupled Probe system

Raman Microscope

cocaine

Ways to enhance the Raman Signal1. make Raman signal taller2. make the fluorescence signal lower

Time-gated Raman or Lifetime filtering

based on the different time-dependence of fluorescence and Raman scattering signals upon a short-pulse optical excitation

Pulsed laser

process lifetime

fluorescence 100 ps - ns

Raman “instantaneous” fs - ps

Time difference allows for separating (filtering) the two types of light.

Uses a non-linear optical material that switches that rotates the light that passes through.

Filters allows only one polarization of light to pass

Transmission properties change with very high intensity light https://doi.org/10.1039/C9CP02430A

Time-gated Raman

DOI:10.1364/OE.21.031632

● Fluorescence intensity changes (x and y) when you change the laser excitation wavelength.

● Raman does not.

https://agora.cs.wcu.edu/~huffman/ia.html#lumin

Change excitation Wavelength

Shift excitation Raman difference spectroscopy (SERDS)

Uses a tunable laser (Ti:sapphire, most common) to take 2 Raman spectra with 2 different excitation wavelengths (νlaser = 785 nm and νlaser = 784 nm)Principle: fluorescence does not change that much with such a small excitation shift, but Raman bands shift by 1 nm (very predictable )

Bleaching● What is Bleaching?

● Chemical - ● Photo -

AntiStokes Raman

v=1v=2v=3

RayleighScattering

StokesRaman

Scattering

hν0 hν0

hν0

hν0 – hν1

Virtualstate

hν0 – hν1

hν0

AntiStokesRaman

Scattering

Fluorescences is only Stokes shifted

Resonance Raman

Electronic excited state

v=1v=2v=3

RayleighScattering

RamanScattering

ResonanceRaman

Scattering

hν0 hν0

hν0 hν0 – hν1

hν0 hν0 – hν1

Virtualstate

Enhancement of Raman Signal ● Enhancement factor (EF)● EF = 102 – 106● Additional Selection Rule

● Only vibrations coupled to electronic absorption are enhanced

Hamaguchi, 2013, Jena

Remember the requirements for fluorescence? π electrons

Substance λmax /nm

benzene 255naphthalene 286

anthracene 375

UV Raman

Improves● Intensity of Raman scattering● “suppresses” fluorescence

Problems:● Can not see the laser to align it.● Danger to human● Danger to sample

Nanotechnology: What is this?

Literal definition:● Technology – stuff that is useful to people● Nano – one the namometer scale (10-9 meters)

Working definition:Controlled creation, measurement, and understanding of matter in the range ~ 1 to 100 nm

Nanoscale Somewhat subjectiveor situational

What is the behavior?Why does it behave that way?

Instrumentation● Did you make it?● What are its

properties?

Think BIGsynthesis,● Bottom up● Top down

SynthesisMeasurement

Theory

Informationflow

Controlled Creation: assembly of molecules into nanostructures

Formation of Fe(CO)2 oneatom at a time

35 argon atoms spelling out “IBM”

Dip-pen nanolithography (DPN):

● “writing” of electronic circuits

● as well as patterning of biomaterials for cell biology research,

● nanoencryption

https://www.nanonics.co.il/solutions/nanolithography#videos_nano

Moore’s Lawthe density of transistors on an integrated chip (IC) doubling every 12 months (later amended to every 2 years).

Fullerenes and Carbon Nanotubes

C60

An array of aligned carbon nanotubes (scale bar 0.001 mm)

Schematic diagram showing a CNT-based top-gate FET with an ideal 5- to 10-nm CNT pitch. S, source; D, drain.

DOI: 10.1126/science.aba5980

Properties of Carbon Nanotubes● CNTs have high thermal conductivity● CNTs have high electrical conductivity● CNTs aspect ratio● CNTs are very elastic ~18% elongation to failure● CNTs have very high tensile strength● CNTs are highly flexible — can be bent considerably without damage● CNTs have a low thermal expansion coefficient● CNTs are good electron field emitters

https://en.wikipedia.org/wiki/Carbon_nanotube

Type of nanotube controls its property:Conductivesemiconductor

Surface-Rolling MoleculesAbstract: Design, syntheses, and testing of new, fullerene-wheeled single molecular nanomachines, namely,nanocars and nanotrucks, are presented. These nanovehicles are composed of three basic componentsthat include spherical fullerene wheels, freely rotating alkynyl axles, and a molecular chassis. The use ofspherical wheels based on C 60 and freely rotating axles based on alkynes permits directed nanoscalerolling of the molecular structure on gold surfaces. The rolling motion observed by STM resembles thesame motion performed by macroscopic entities in which rolling occurs perpendicular to the axles. A newsynthesis methodology, in situ ethynylation of fullerenes, was developed for the realization of the fullerene-wheeled molecular machines. Four generations of the fullerene-wheeled structures were developed, andthe latest fourth generation nanocar, 3b, along with three-wheeled triangular compounds, 4a and 4b, provideddefinitive evidence for fullerene-based wheel-like rolling motion, not stick-slip or sliding translation. Thestudies here underscore the ability to control directionality of motion in molecular-sized nanostructuresthrough precise molecular design and synthesis.

Examples of Nano machines

MCM-41 arrays

MCM-41: a solid of SiO2, a mesoporous material

“synthesis of MCM-41, surfactants (typically cetyltrimethylammonium bromide (CTAB)) are added to the synthesis solution. The surfactant initially forms rod-shaped micelles that eventually align together in hexagonal arrays. Silica species are added to cover the rods, and then calcination condenses the silanol groups, which bridges the silicon atoms with oxygen atoms. Ultimately, this organic template oxidizes and disappears, leaving behind fully-formed MCM-41.”

ordered and large pores with a diameter in the range 1.5-10 nm

Some Applications

● base material for platinum complexes used in luminescent oxygen sensing materials

● uptake of CO2 for gas purification

● surface functionalized MCM-41 a catalyst in the synthesis of bisphenol A.

Mesoporous materials

Nanoscale? Way below the defraction limit of light.

https://www.edmundoptics.com/knowledge-center/application-notes/imaging/diffraction-limit/

Good video describing defraction limitation of imaging (Just first few minutes)

Quantum Dots

Nanotechnology: What is this, just another word for chemistry? Sort of

localized surface plasmon resonance (LSPR)

Why Are Metals Shiny?

Plasmon: oscillation of electrons

To explain why metals (and graphite) are shiny, we invoke a combination of reflection, refraction, and the energy levels of MOs. When a photon of light is absorbed and reemitted, the electron moves from one orbital to another. Let us consider a piece of metal at room temperature. When a photon arrives at the metal’s surface it encounters the almost continuous band of MOs. Most photons, regardless of their wavelength, can be absorbed because there is an energy gap between orbitals corresponding to the energy of the photon. This process promotes electrons up to a higher energy level. As the electrons drop back down to a lower energy level, the photons are re-emitted, resulting in the characteristic metallic luster. Metals actually emit light, although this does not mean metals glow in the dark (like a light bulb or the Sun). Instead, metals absorb and re-emit photons, even at room temperature.

Why Are Metals Shiny?

https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_CLUE_(Cooper_and_Klymkowsky)/3%3A_Elements_Bonding_and_Physical_Properties/3.4%3A_Metals/Why_Are_Metals_Shiny%3F

Fig. 1 Study of absorption and scattering plasmonic optical properties of colloidal Ag NPs using UV-vis absorption spectroscopy. (A) Photos of colloids show: (a) light yellow; (b) yellow; (c) light orange; (d) orange-red; (e) red; (f) dark red; (g) purple; (h) purple violet; (i) violet; (j) blue; (k) light blue; and (l) green colors. (B) Normalized absorbance of UV-vis absorption spectra of the colloids of Ag NPs in (A) shows the peak wavelength (λmax) with FWHM at: (a) 393 nm (64 nm); (b) 405 nm (69 nm) with a weak shoulder peak at 526 nm; (c) 427 nm (110 nm) with a weak shoulder peak at 364 nm; (d) 461 nm (192 nm) with a weak shoulder peak at 382 nm; (e) 502 nm (160 nm) with two weak shoulder peaks at 422 and 342 nm; (f) 518 nm (146 nm) with two weak shoulder peaks at 430 and 340 nm; (g) 536 nm (158 nm) with two shoulder peaks at 418 and 340 nm; (h) 552 nm (166 nm) with two shoulder peaks at 414 and 340 nm; (i) 572 nm (172 nm) with two shoulder peaks at 416 and 338 nm; (j) 606 nm (212 nm) with a shoulder peak at 334 nm; (k) 646 nm (215 nm) with two shoulder peaks at 450 and 336 nm; and (l) 738 nm (130 nm) with two shoulder peaks at 420 nm (78 nm) and 330 nm.

https://doi.org/10.1039/C0JM01990A

Highly tunable color change that is a function of NP size

javascript:popupOBO('CMO:0000292','c0jm01990a')javascript:popupOBO('CMO:0000842','c0jm01990a')

Other peoples videos on Nano materials and Plasmonics

Surface plasmons https://www.youtube.com/watch?v=eVpxn5Cw6YM

Nanoplasmonics 1 https://www.youtube.com/watch?v=8iyShOidtYg8iyShOidtYgAbout SERS Nanoplasmonics 2 https://www.youtube.com/watch?v=iUyPssG9f_MNanoplasmonics 3 https://www.youtube.com/watch?v=s7iYjwYQmjM

Fiberprobe W/ sers https://www.youtube.com/watch?v=81rfymWn8jI

https://www.youtube.com/watch?v=eVpxn5Cw6YMhttps://www.youtube.com/watch?v=8iyShOidtYg8iyShOidtYghttps://www.youtube.com/watch?v=iUyPssG9f_Mhttps://www.youtube.com/watch?v=81rfymWn8jI

SERS: Electromagnetic Mechanism

hνL

hνL

AgNP

Plasmon forms in resonance with electric field of light

Optical cross-section is larger than NP

Example: SERS

Rhodamine 6G

633 nm laser (HeNe) 3mW

Hot spots: coupled resonances at the ‘gaps’ between metallic nano-objects.

Hot spot

The trick it to reproducibly ● create arrays of nanostructures● Get the analytes inside of them

SERS: Overcoming fluorescence with surface quenching

v=1v=2v=3

hν0

Vibrational relaxation

Fluorescence emission

Metal surfaceAdds new relaxation mechanism (no radiative)

SERS selection rules

Application of SERS to molecular orientation on surfaces

NanoMaterials: SERS-active substrates

Enhancement Distance Dependance

Stiles P.L. et all, Annual Review of Analytical Chemistry, 1, 2008, p.601-26

Surface Enhanced Resonance Raman Spectroscopy (SERRS)

RRS provides additionalEF = 10 2 -10 6

Resonance

Resonance

No resonance

Due to matching of excitation to absorption of a specific part of the molecule, theRaman spectrum associated with this part of the molecule is selectively enhanced

Creation of Metal Nanostructures: AgNP AgNP = silver nanoparticlesSynthesis● Reduction of Ag salt (AgCl, Ag2SO4, AgNO3) or Ag2O● Reducing agents (H2 gas, glucose, NaBH3)

Thermal Evaporation/Deposition

Electrochemical Roughening

Delivering molecules to metal structures

laser serslaser sers

laser sers

laser sers Measure in the solvent

RamanSelection Rules

Wavelength RangeVaries depending on laser (UV-Vis-NIR)

Advantages● Water is not a problem● Special situations very sensitive● Remote is possible● Visible excitation (cheaper, more robust

optics in theory)● Relatively easy to measure bands below

400 cm-1

● Signal enhancement is possible● With SERRS single molecule detection!

DisadvantagesVery low signal, low sensitivityFluorescence interferenceGas is “difficult”

IR Selection Rules

Wavelength Range2500 – 25000 nm

Advantages● More sensitive● Remote operation possible● More robust for quantitative analysis

(absorbance), FT

Disadvantages● Too sensitive to water● Major change to spectrometer to get

below 400 cm-1

ν =

ReferencesMcCreery 2000, DOI:10.1002/0471721646

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