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Chapter 17b Fundamentals of spectroscopy 1 17.6 What Happens When a Molecule Absorbs Light? Molecule Promoted to a More Energetic Excited State Absorption of UV-vis light results in an electron promoted to a higher energy molecular orbital s  s* transition in vacuum UV n  s* saturated compoun...

Chapter 17b Fundamentals of spectroscopy 1 17.6 What Happens When a Molecule Absorbs Light? Molecule Promoted to a More Energetic Excited State Absorption of UV-vis light results in an electron promoted to a higher energy molecular orbital s  s* transition in vacuum UV n  s* saturated compounds with non-bonding electrons n  p*, p  p* requires unsaturated functional groups (eq. double bonds) most commonly used, energy good range for UV/Vis 1 17.6 Absorption of light- Formaldehyde 120 pm  Consider formaldehyde: In its ground state, S0, the molecule is planar, with a double bond between carbon and oxygen. From the electron dot description of formaldehyde, we expect two pairs of nonbonding electrons to be localized on the oxygen atom. The double bond consists of a sigma bond between carbon and oxygen and a pi bond made from the 2py (out-of-plane) atomic orbitals of carbon and oxygen. By excitation, Formaldehyde becomes of pyramidal structure in both the S1 and T1 excited states. Promotion of a nonbonding electron to an antibonding C―O orbital lengthens the C―O bond and changes the molecular geometry. O H 116.5o C H 110 pm Excitation H 31o 119o C O 132 pm H 109 pm Chapter 18 3 Electronic states of formaldehyde In the molecular orbital (MO) diagram for formaldehyde, one of the nonbonding AOs of oxygen is mixed with the three sigma bonding orbitals. These four orbitals, labeled σ1 through σ4, are each occupied by a pair of electrons with opposite spin (+ and -). At higher energy is an occupied pi bonding orbital (π), made of the py AOs of C and O. The highest energy occupied MO (HOMO) is nonbonding orbital (n), composed principally of the 2px AOs from O. The lowest energy unoccupied MO (LUMO) is the pi antibonding orbital (π*). Electrons in this orbital produce repulsion, rather than attraction, between the C and O atoms. 4 2 Electronic states of formaldehyde  n  p*(T) occur at 397 nm (green- The lowest energy electronic transition of formaldehyde promotes a nonbonding (n) electron to the antibonding pi orbital (π*). yellow)  n  p*(S) occur at 355 nm (colorless) Two possible transitions, depending on the spin quantum numbers in the excited state: The state in which the spins are antiparallel is called a singlet state (S1) and if the spins are parallel, we have a triplet state (T1). Excited electron changes direction of spin Excited electron keeps direction of spin, Pairing stays the same 5 Fundamentals of Spectrophotometry What Happens When a Molecule Absorbs Light? Infrared and Microwave Radiation  Not energetic enough to induce electronic transition  Change vibrational, translational and rotational motion of the molecule - The entire molecule and each atom can move along the x, y, z-axis When correct wavelength is absorbed,  Oscillations of the atom vibration is increased in amplitude  Molecule rotates or moves (translation) faster Vibrational States of Formaldehyde Energy: Electronic >> Vibrational > Rotational 3 What Happens When a Molecule Absorbs Light? Combined Electronic, Vibrational and Rotational Transitions Absorption of photon with sufficient energy to excite an electron will also cause vibrational and rotational transitions There are multiple vibrational and rotational energy levels associated with each electronic state   - Excited vibrational and rotational states are lower energy than electronic state Therefore, transition between electronic states can occur between different vibrational and rotational states  Relaxation Processes from Excited State   There are multiple possible relaxation pathways Vibrational, Rotational relaxation occurs through collision with solvent or other molecules -  Energy is converted to heat (radiationless transition) Electronic relaxation occurs through the release of a photon (light) What happens to absorbed energy Jablonski Diagram h1 Phosphorescence Absorbance Fluorescence Intersystem crossing (radiationless transition) Internal conversion (radiationless transition) h2 8 4 After an absorption process, the system is at the S1 level, several events can happen: Internal conversion (IC): The molecule could enter a highly excited vibrational level of S0 having the same energy as S1. From this excited state, the molecule can relax back to the ground vibrational state and transfer its energy to neighboring molecules through collisions. This radiationless process is labeled R1. If a molecule follows the path A–R1–IC–R2, the entire energy of the photon will have been converted into heat. A; Absorption h 1 Phosphorescence Intersystem crossing (radiationless transition) Fluorescence Absorbance (A) Internal conversion (radiationless transition) h 2 9 Intersystem Crossing (ISC): The molecule could cross from an S1 state into an excited vibrational level of T1. After the radiationless vibrational relaxation R3, the molecule finds itself at the lowest vibrational level of T1. From here, the molecule might undergo a second intersystem crossing to S0, followed by the radiationless relaxation R4. All processes mentioned so far simply convert light into heat. h 1 Phosphorescence Intersystem crossing (radiationless transition) Fluorescence Absorbance (A) Internal conversion (radiationless transition) h 2 10 5 Fluorescence (F): A molecule could also relax from S1 to S0 by emitting a photon. The radiational transition S1→S0 is called fluorescence. Typical lifetimes of fluorescence processes are 10−8 to 10−4 s (fast process). Phosphorescence (P): A molecule could also relax from T1 to S0 by emitting a photon. The radiational transition T1→S0 is called phosphorescence. Typical lifetimes of fluorescence processes are 10−4 to 102 s, because the transition involves a change in spin quantum numbers (2 unpaired electrons to 0 unpaired electrons), which is improbable! h 1 Phosphorescence Intersystem crossing (radiationless transition) Fluorescence Absorbance (A) Internal conversion (radiationless transition) h 2 11 Fundamentals of Spectrophotometry What Happens When a Molecule Absorbs Light? 5.) Fluorescence and Phosphorescence     Relative rates of relaxation depends on the molecule, the solvent, temperature, pressure, etc. Energy of Phosphorescence is less than the energy of fluorescence Phosphorescence occurs at a longer wavelengths than fluorescence Lifetime of Fluorescence (10-8 to 10-4 s) is very short compared to phosphorescence (10-4 to 102 s) Fluorescence and phosphorescence are relatively rare 6