Chapter 15 Molecular Luminescence Spectroscopy PDF

Chapter 15: Molecular Luminescence Spectroscopy What happens to the absorbed EM energy determines whether you have Absorbance o Molecule returns to ground/lower energy state via non-radiative transmission such as vibration, collision with other molecules etc....

Chapter 15: Molecular Luminescence Spectroscopy What happens to the absorbed EM energy determines whether you have Absorbance o Molecule returns to ground/lower energy state via non-radiative transmission such as vibration, collision with other molecules etc. ▪ These give off energy absorbed rather than the emission of light Fluorescence o Some energy is lost through various processes (e.g., non-radiative transitions) and then light is given off. →emission o Excitation of electrons very short living Phosphorescence o Molecule transitions from an excited triplet state to a lower energy singlet state and gives off light. Non-radiative transition intervenes. o Longer lifetime of electrons Chemiluminescence o Emission of radiation by an excited species formed during a chemical reaction ▪ Very characteristic…Ex. Oxidation product of an analyte or reagent rather than the analyte itself Very sensitive Detection limit often 1 or 2 orders of magnitude lower than in absorption Large in concentration range Excited states can be deactivated by collisions or other process Fluorescence, Phosphorescence, & Chemiluminescence 1. Fluorescence (10-5 to 10-8 s) occurs at a slower rate than absorbance (10-14 to 10-15 s) Fluorescence + phosphorescence has lower limit of detection - Sensitivity based on concentration parameter  2. Fluorescence – ground state to single state and back. Phosphorescence – ground state to triplet state and back. Forbidden transition; no direct excitation of triplet state because change in multiplicity - Lowest energy state selection rule (always paired) Low probability of direct transition from singlet When molecules get state to triplet state excited, they can either go to b or c (parallel spin, same direction) **The favored route to the ground state is the one that minimizes the lifetime of the excited state Deactivation Processes a. Vibrational relaxation: solvent collisions o VR is efficient and goes to lowest vibrational level of electronic state within 10-12s or less o Significantly shorter lifetime than electronically excited state o Fluorescence occurs from lowest vibrational level of electronic excited state but can go to higher vibrational state of ground level. o Wavelength emission > wavelength excitation (stokes shift) b. Internal conversion o Crossing of e- to lower electronic state (singlet-singlet or triplet-triplet) o S2-S1 (almost equal E) and S1 to S0 would also happen o Efficient, therefore many compounds do not fluoresce (aliphatic) o Especially probable if vibrational levels of two electronic states overlap, can lead to predissociation  o Predissociation: relaxation to vibrational state of lower electronic state with enough energy to break a bond o Dissociation: direct excitation (absorption) to vibrational state with enough energy to break a bond à no internal conversion occurs! c. External conversion o Deactivation via collision with solvent (collisional quenching) o Decrease collision → increase fluorescence or phosphorescence o Decrease temp. and/or increase viscosity o Decrease concentration of quenching (Q) agent d. Intersystem crossing o Spin of electron is reversed o Change in multiplicity in molecule occurs (single to triplet) o Enhanced vibrational levels overlap o More common if molecule contains heavy atoms (I, Br) e. Phosphorescence o Deactivation from “triplet” electronic state to the ground state producing a photon Fluorescence & structure o Lower energy 𝜋→ 𝜋 ∗ transitions o Usually aromatic compounds o Quantum yield increases with number of rings and degree of condensation o Fluorescence especially favored for rigid structures o Fluorescence increases for chelating agent bound to metal Lack of rigidity → enhanced internal conversion rate and increase in likelihood for radiationless deactivation - Less likely to fluoresce Temperature, Solvent, & pH Effects o Decrease temp. → increase fluorescence (less collisions-less external conversion) o Increase viscosity → increase fluorescence (less collisions) o Fluorescence is pH dependent for compounds with acidic/basic substituents Effect of Dissolved O2: o Increase O2 → decrease fluorescence o Oxidize fluorescence species o Paramagnetic property increases intersystem crossing (spin flipping) Effect of Concentration Due to secondary absorption (wavelength of emission overlapping absorption band) Self-absorption by own analyte species Double beam Chemiluminescence o Chemical reaction yields an electronically excited species that emits light as it returns to ground state o Relatively new, few examples Homework (pg. 297-298) 15-1, 15-2, 15-3, and 15-6 15.1 Explain the difference between fluorescence emission spectrum and a fluorescence excitation spectrum. Which more closely resembles an absorption spectrum? In florescence emission spectrum excitation wavelength is held constant, and the emission intensity is measured as a function of the emission wavelength. In fluorescence excitation spectrum the emission is measured at one wavelength while excitation wavelengths are scanned. The excitation spectrum closely resembles an absorption spectrum since the emission intensity is usually proportional to the absorbance of the molecule. 15.2 Define the following terms a. Fluorescence: Process in which a molecule, excited by the absorption of radiation, emits photon while undergoing a transition from an excited singlet electronic state to a lower state of the same spin multiplicity (e.g., singlet → singlet transition) b. Phosphorescence: Process in which a molecule, excited by the absorption of radiation, emits a photon while undergoing a transition from excited triplet state to a lower state oof a different spin multiplicity (e.g., a triplet → singlet transition) c. Resonance fluorescence: observed when an excited species emits radiation of the same frequency as used to cause the excitation. d. Singlet state: one in which the spins of the electrons of an atom or molecule are all paired so there is not net spin angular momentum e. Triplet state: one which the spins of the electrons of an atom or molecule are unpaired so that their angular moments add to give a net non-zero moment. f. Vibrational relaxation: the process by which a molecule loses its excess vibrational energy without emitting radiation. g. Internal conversion: intermolecular process in which a molecule crosses to a lower electronic state with emitting radiation.  h. External conversion: radiationless process in which a molecule loses electronic energy while transferring that energy to the solvent or another solute i. Intersystem crossing: the process in which a molecule in one spin state changes to another spin state with nearly the same total energy (e.g., singlet → triplet) j. Predissociation: occurs when a molecule changes from a higher electronic state to an upper vibrational level of a lower electronic state in which the vibrational energy is great enough to rupture the bond k. Dissociation: occurs when radiation promotes a molecule directly to a state with sufficient vibrational energy for a bond to break l. Quantum yield: fraction of excited molecules undergoing the process of interest. Ex. The quantum yield of fluorescence is the fraction of molecules which have absorbed radiation that fluoresce. m. Chemiluminescence: process by which radiation is produced as a result of a chemical reaction 15.3 Why is spectrofluorometry potentially more sensitive than spectrophotometry? For spectrofluorometry, the analytical signal F is proportional to the source intensity Po and the transducer sensitivity. In spectrophotometry, the absorbance A is proportional to the ratio of Po to P. Increasing Po or the transducer sensitivity to Po produces a corresponding increase in P or the sensitivity to P. Thus, the ratio does not change. As a result, the sensitivity of fluorescence can be increased by increasing Po or transducer sensitivity, but the that of absorbance does not change. 15.6 Discuss the major reasons why molecular phosphorescence has not been as widely used as molecular fluorescence spectrometry. The triplet state has a long lifetime and is very susceptible to collisional deactivation. Thus, most phosphorescence measurements are made at low temperature in a rigid matrix or in solutions containing micelles or cyclodextrin molecules. Also, electronic methods must be used to discriminate phosphorescence from fluorescence. Not as many molecules give good phosphorescence signals as fluorescence signals. As a result, the experimental requirements to measure phosphorescence are more difficult than those to measure fluorescence and the applications are not as large.