In-Depth Notes on Luminescence Spectroscopy (PPT)

Overview of Luminescence Spectroscopy

  • Luminescence and fluorescence are based on multiplicity

    • Fluorescence (emission) doesn’t involve a spin flip and Ms=1 (singlet state) electrons still paired

    • Phosphorescence (emission) involves a change in spin state, leading to a triplet state (Ms=3) where the electrons are unpaired, allowing for longer-lived excited states and delayed emission of light.

    • Spin flip shouldn’t happen but can in heavy atoms due to interactions in the magnetic fields

  • A technique widely utilized in chemistry for the emission of photons.

  • Notably significant for biochemical applications due to its high sensitivity and low detection limits, making it effective for trace analysis.

    • nanomolar or picomolar detection limit

  • Electronic states influenced by environment so luminescence can give a lot of info.

Instrumentation

Basics

  • Laser-Based System: Sample is irradiated with photons, which are absorbed.

    • fluoresce is proportional to the intensity of the source

  • Steady State: Two principal types of instruments based on spectral analysis.

    • Source is continually exciting the sample and equal amounts of ground state and excited state atoms

    • Have an excitation wavelength and scan the emission wavelength

    • Example: Pyrene absorbs at 338nm

  • Frequency Domain: looking at the actual wave itself

    • wavelength hits sample and there is a delay in the absorption and emission

    • see the difference in the intensity and wavelength shift

  • Time Domain: pulsing the domain and seeing emission at some time t

    • usually in nanosecond

    • time resolved spectroscopy: pulsing the sample and slowly going to higher or shorter energies overtime

      • red edge effects: start exciting at the red edge of the band then you can look at molecules in different environments. Viscous solutions you can see the different electronic effects

      • Blue edge effects (higher energy state molecules)

  • Jablonski Diagram: Illustrates radiative (photon emission) and non-radiative processes (energy loss without photon emission; thermal energy).

    • S0 = ground state

    • S1 = first excited singlet state (Majority of molecules emit from the lowest state of S1)

      • Manifold: all the vibrational and electronic states that are a part of that level (S1 is a level)

    • S2 = second excited state (only one molecule the emits from S2)

    • S2 → S1; S1→S0: internal conversion; non-radiative energy loss (thermal)

    • All these processes taking place simultaneously

    • Intersystem crossing: the process where a molecule transitions between different spin states, typically from a singlet state to a triplet state, resulting in a longer lived excited state.

    • Delayed fluorescence: electron crossed over to the T1 state but there is enough energy to push it back and the spin can flip and cause emission

    • The rates constants determine the fluoresce or phosphorescence output

    • Stokes shift: The lose of energy of the excited photon before emission

      • wavenumber of max wavelength absorption - wavenumber of max wavelength the emission = delta v

    • Mirror image rule: emission spectrum is identical to the absorption spectrum in shape but occurs at a different energy, typically lower due to the Stokes shift.

    • Timescales of Emission:

      • Absorption: 10^{-15} seconds (femtoseconds).

      • Internal conversion/vibrational relaxation: 10^{-14} to 10^{-10} seconds.

      • Fluorescence: 10^{-9} to 10^{-7} seconds. (intersystem crossing)

      • Phosphorescence: 10^{-3} to 10^{-2} seconds (very slow due to forbidden transitions).

      • All affect whether or not you get emission

Cell Geometries

  • Different geometries facilitate the varying requirements for luminescence measurements.

  • 45 deg used for solid samples.

  • direct is not really used

Key Concepts

Rates of Luminescent Processes

  • Fluorescence: Transition from the first excited singlet state to the singlet ground state.

    • random phenomenon, all molecules have an equal probability to emit after being excited

    • Wavefunctions of the ground state to the excited have to be similar

  • Phosphorescence: Transition from the first excited triplet state to the singlet ground state; requires a spin flip, which is quantum mechanically not allowed.

  • Kasha’s Rule: Photon emission typically occurs from the lowest excited state rather than higher states, ensuring effective emission.

    • Emission should arise from E=0 in singlet S0 and E=0 in T1

    • Azulene is the only exception to Kasha’s rule

      Azulene - American Chemical Society

Delayed Fluorescence

Types

  • E-type (Eosin): Temperature-dependent; excitation leads to intersystem crossing; significant for understanding relaxation under variable thermal environments.

  • P-type (Pyrene): Intensity proportional to the square of excitation intensity; requires two photons and two molecules interacting to regenerate fluorescence.

  • Recombination Fluorescence: Involves photooxidation leading to an excited molecule without immediately returning to the ground state.

Excimers and Exciplexes

  • Excimer: Dimer formed in an excited state involving the same molecule, where the excitation energy is shared.

  • Exciplex: Involves two different molecules; one in an excited state and one in the ground state, typically functioning as electron donor and acceptor.

Quantum Mechanics in Luminescence

  • Frank-Condon Factor: Transition probability correlated with vibrational wave function overlap between two states; larger overlap leads to more intense fluorescence.

  • Quantum Yield: A measure of the efficiency of photon emission; high quantum yields correlate with effective luminescence.

External Influences

  • Heavy Atom Effects: Influence on spin-orbit coupling that enhances intersystem crossing.

  • Variability in Quantum Yield: Explored in both fluorescence and phosphorescence contexts.

Single Photon Counting Techniques

Purpose and Application

  • Utilizes Time-Correlated Single Photon Counting (TCSPC) for detecting short-lived excited states of molecules on the ns timescale.

  • Experimental Setup: Incorporates advanced electronics to facilitate the timing and correlation of excitation and emission signals.

Necessary Equipment

  • Components include Constant Fraction Timing Discriminators, Time-to-Amplitude Converters, Pulse Height Multichannel Analyzers, and delay lines to boost accuracy in measurements.

Quenching Phenomena

Types of Quenching

  • Dynamic and Static Quenching: Involve nonradiative energy transfer to neighboring molecules or interactions forming ground state complexes.

  • Stern-Volmer Equation: A tool to evaluate dynamic quenching effects and calculate quantum yield in the presence of quenchers.

Classifications of Quenchers

  • Inorganic (e.g., heavy atoms), Organic (various organic compounds), and Self-quenching where the quencher is the same as the detecting molecule.

Molecular Structure and Its Impact on Luminescence

  • Molecular Features: Compounds with rigid polyconjugated π-systems and aromatic moieties promote fluorescence.

  • Nonradiative Processes: Can be minimized in rigid matrices or at low temperatures, ensuring maximum emission efficiency.

Instrumentation Details

Fluorescence Microscope

  • Typically uses a Xe arc lamp generating broad output, necessary for high-intensity measurements.

Detectors

  • PMTs (Photomultiplier Tubes) and CCDs (Charge-Coupled Devices) are prime for luminescence.

  • PMTs are faster with pulsed applications, whereas CCDs excel in imaging applications due to higher quantum efficiency.

Summary

  • Luminescence spectroscopy is a valuable analytical technique involving intricate processes governed by quantum mechanics. Its versatility across scientific disciplines underscores the significance of thorough understanding and improved instrumentation outcomes.