Inorganic Chemistry: Structure and Reactivity (CH 612) at UMass Boston

Prof. Jonathan Rochford

CH 612, Fall 2013

Office: S-01-130

Telephone: 617-287-6133

E-Mail: jonathan.rochford@umb.edu

(Office Hours: TBD)

Lectures will take place in the Chemistry Department Conference Room, S-01-089

Tuesdays and Thursdays 5.30 – 7.00 pm

http://alpha.chem.umb.edu/chemistry/ch612/

• Introduction to photophysics

• Established organic chromophores

• Triplet-triplet upconversion

• Two-photon absorption and

nonlinear optics

• Inorganic bonding, photophysics

• Ru(bpy)3

Topics

• Marcus theory and photoinduced

electron transfer

• Photosystem II and artificial

photosynthesis

• Dye-sensitized solar cells

• Photocatalytic CO2 reduction

• Organometallic light emitting

diodes

• The first stage of this lecture course covering theoretical aspectsof molecular organic photophysics is covered in Chapters 1-7 ofthe following textbook.

“Principles of Molecular Photochemistry: An Introduction”

- Nicholas J. Turro, V. Ramamurthy, J. C. Scaiano. (University ScienceBooks, 2010).

(aka baby Turro)

• Students interested in the applications of organic photochemistryare highly recommended to check out the following textbook(Chapters 1-7 are taken directly from the above text):

“Modern Molecular Photochemistry of Organic Molecules”

- Nicholas J. Turro, V. Ramamurthy, J. C. Scaiano. (University ScienceBooks, 2010).

(aka big Turro)

(see course syllabus for further recommended reading)

• The following text is available as an e-book from thelibrary and is a useful reference text to at anintroductory level.

“Principles and applications of photochemistry”

- Brian Wardle (Wiley)

• The motivation for understanding molecular photochemistry comes froman intellectual goal and an essential need to understand and developmodern photonic technologies. Both aspects will be addressed this fall.

• Questions often asked of photochemical systems:

� What are the fundamental entities that exist along a photophysical or

photochemical pathway?

� What are the structural, energetic, and dynamic properties of these

entities?

� What are legitimate theoretical concepts and experimental tools that

are required to understand and to measure the properties of these

entities?

Motivation

• Molecular photophysics and photochemistry is a very broad andinterdisciplinary topic embracing the fields of chemical physics, molecular

spectroscopy, and supramolecular chemistry.

• This course will focus initially on the photophysics of organic moleculesfrom both a theoretical and application perspective.

• Upon laying a solid foundation of general photophysical principles we willprogress to apply this knowledge specifically to inorganic systems – againfrom a theoretical and application perspective.

• We will touch on the topic of molecular inorganic photochemistry towardsthe end of this lecture course. Molecular organic photochemistry will notbe covered in any depth due to time constraints.

Molecular photophysics & photochemistry

Photophysics vs. Photochemistry ?

• Molecular photophysics & photochemistry is a science concerned with thestructures and dynamic processes that result from the interaction of light withmolecules.

• Molecular photophysics & photochemistry is often divided into two (sometimesfour) sub-topics

� Organic photophysics & photochemistry

� Inorganic photophysics & photochemistry

• The same general principles can be applied in each case however transition metalatoms introduce additional complexity into excited state processes due to theirincomplete d-subshells and structure dependent splitting of these subshells.

• What is the difference between photophysics and photochemistry?

� Molecular photophysics concerns the interaction of light with molecules

resulting in net physical change.

� Molecular photochemistry concerns the interaction of light with molecules

resulting in net chemical change.

The field of molecular photochemistry is concerned with the interaction of

light (represented by photons or oscillating electromagnetic waves) and

matter (represented by the electrons and nuclei of molecules) that lead to the

formation of an electronically excited state *R which is eventually converted

to a product P (photochemistry) or relaxes back to its initial state R

(photophysics) through a variety of pathways.

• The “molecular” part of molecular photophysics and photochemistryemphasizes the use of

� molecular structure and its implied dynamics

transitions between states, excited state kinetics

� molecular substructure

electronic configuration, nuclear configuration, spin configuration

as both crucial and unifying intellectual units for organizing anddescribing the possible, plausible, and probable pathways ofphotochemical reactions from the absorption of a photon by a reactant R

to form its excited state *R ultimately resulting in a product P producedby one of three primary photochemical pathways.

A global paradigm for understanding

molecular photochemistry

• In simplest terms molecular organic photochemistry involves the overall process

where R is an organic molecule that absorbs a photon (hν) whose frequency (ν) isresonant with an electronic transition in R responsible for producing the excitedstate *R.

• The excited state *R is considered an independent chemical entity relative to R.each have a unique structural geometry & electronic structure.

• There are 3 fundamentally distinct primary photochemical pathways that *R mayfollow

1) *R → I → P

via a distinct “reactive intermediate” (I) which typically has the characteristicsof a radical pair (RP), biradical (BR) or a zwitterion (Z).

2) *R → F → P

via a “funnel intermediate” without passing through a reactive intermediate.F can be described as a “conical surface intersection” or as a “minimum”produced by surface-avoided intersections.

3) *R → *I → P or *R → *P → P

via formation of an electronically excited intermediate (*I) or andelectronically excited product (*P).

• Primary photochemical pathway 1

1) *R → I → P

via a distinct “reactive intermediate” (I) which typically has the characteristicsof a radical pair (RP), biradical (BR) or a zwitterion (Z).

Possible photochemical processes

• How do you characterize a reaction pathway *R → P according to the paradigm ofprimary photochemical pathways?

• For any reaction to be possible molecules (including their vibrational and spinsubstructures) must obey all four of the conservation laws of chemical reactions:

1) The conservation of energy

2) The conservation of momentum (linear and angular)

3) The conservation of mass (number of atoms + kind of atoms…nuclear?)

4) The conservation of charge

• These conservation laws place restrictions on the number of possible structures(*R, I, F, *I, *P, P) and possible pathways that a photochemical reaction can follow.

• Only the set of structures and pathways that obeys the conservation laws isconsidered possible and all others are ruled out without exception.

Questions YOU should be asking yourself

1) How do we visualize a photon interacting with the electrons of R to induceabsorption of a photon to produce *R, and how does this interaction of a photonwith the electrons of R relate to the theoretical and experimental quantities, suchas extinction coefficients, radiative lifetimes, and radiative efficiencies?

2) What are the possible and plausible structures, energetics, and dynamics availableto *R and I that occur along the reaction pathway from *R → P ?

3) What are the possible and plausible sets of primary photochemical processescorresponding to the *R → I process?

4) What are the legitimate theoretical approaches, experimental design strategies,experimental techniques, and computational strategies for experimentally“observing” or validating the occurrence of the species *R and I that arepostulated to occur along the reaction pathway from *R → P ?

5) What is the most probable pathway from *R → I ?

Questions YOU should be asking yourself

6) How is the most probably pathway determined by the competing kinetic pathwaysfor the photophysics and photochemistry of *R ?

7) What are the absolute rates (rate constants) at which each elementary step occursalong the reaction pathway from *R → P ?

8) What sorts of structures, energetics, and dynamics correspond to *R and I intypical photoreactions?

• Scheme 1.2 is an elaboration of Scheme 1.1 including the HOMO and LUMOfrontier orbitals of the key structures R, *R, I, and P.

Scheme 1.2

• At this basic level of theory electron-electron repulsions and electron spinconfigurations are not considered.

• When the energies of the non-bonding orbitals in I are significantly different bothelectrons are spin-paired in the lower lying orbital. Such electronic configurationscorrespond to species called zwitterions.

• Questions:

� What are the electronic characteristics of the HOMO and LUMO energy levelsinvolved in the R + hν → *R process ?

� What is the electronic configuration of *R ?

� What are the plausible primary photophysical and photochemical processestypicaly of *R based on its biradical type electron configuration ?

� What are the electronic natures of the non-bonding orbitals of I ?

� What are the plausible secondary thermal reactions of I that lead to P ?

State energy diagrams: electronic & spin isomers

• There are three critical molecular states that need to be considered upon initialanalysis of a photochemical reaction

R(S0), *R(S1) and *R(T1)…..how does S0 define an electronic state relative to MOs?

• State energy diagrams, aka Jablonski diagrams, provide a concise means ofdisplaying relative energies, electronic configurations and keeping track of the S0, S1

and T1 states.

• Higher energy Sn and Tn states where n > 1 may also be included if desired. However,excitation of these higher-energy excited states generally results in rapiddeactivation to S1 and T1 faster than any other measurable process (Kasha’s rule).

• The y-coordinate represents the potential energy (PE) of the system, whereas the x-coordinate has no physical meaning (it is not a reaction coordinate or potentialenergy surface).

• What is the basis of isomerism in the state energy diagram?

� Isomerism results from differences in the electronic configurations (electronicisomers) or in the spin configurations (spin isomers) between different states.Different electronic or spin isomers may also be stereoisomers of each other.

• Transitions between any two electronic states (apart from S0→Sn) occur fromhigher to lower potential energy with a corresponding dissipation of energy(conservation of energy) in the form of heat (radiationless aka thermal decay) or inthe form of a photon (radiative aka emissive decay).

• The plausibility and probability of a transition between any two states requiresknowledge of specific molecular structures and reaction conditions.

• All possible photophysical transitions from S1 and T1 must be considered in anoverall *R → R photochemical process. If photophysical processes are very fastrelative to photochemical processes, the latter may be plausible but will beimprobable because of the former plausible photophysical processes which arekinetically favored.

Possible radiative absorption and emission processes

1. Spin allowed singlet-to-singlet photon absorption characterized by an extinctioncoefficient ε(S0 → S1)

S0 + hν → S1

2. Spin forbidden singlet-to-triplet photon absorption characterized by an extinctioncoefficient ε(S0 → T1)

S0 + hν → T1

3. Spin allowed singlet-to-singlet photon emission, aka fluorescence emissioncharacterized by a rate constant kFl

S1 → S0 + hν‘

4. Spin forbidden singlet-to-triplet photon emission, aka phosphorescence emissioncharacterized by a rate constant kPh

T1 → S0 + hν‘’

5. Spin allowed radiationless electronic transitions between states of the samemultiplicity, aka internal conversion characterized by a rate constant kIC

S1 → S0 + ∆

6. Spin forbidden radiationless electronic transitions between states of the differingmultiplicity, aka intersystem crossing characterized by a rate constant kST

S1 → T1 + ∆

7. Spin forbidden radiationless electronic transitions between T1 and S0 also know asintersystem crossing and characterized by a rate constant kTS

T1 → S0 + ∆

Plausible non-radiative photophysical processes

Scheme 1.4

Jablonski diagram

S

S

S

T

T

hv

fl

ph

Basic concepts

� wave-particle duality of photons/electrons

� quantization and atomic and molecular structures

� Schrödinger wave equation Hψ = Eψ

� principle (n), angular momentum (l), magnetic (ml) and spin (ms)

� ψ2 = the probability of finding the electron at a particular location in space

� Pauli exclusion principle - no two e−s share the same quantum numbers

� Total spin = Σms multiplicity m = 2s + 1

Basic concepts

� Planck’s law; E = hν = hc/λ where h is Planck’s constant (6.626 × 10−34 Js)

� wavelength uses units of nm (10-9 m) or Å (10-10 m)

� wavenumber (ν) uses units of cm-1 (= 107/nm)

� 1 einstein = 1 mol photons = N(hc/ λ) J

� 1 eV = 1.602 × 10−19 J

� Frequency uses units of Hz (= s − 1).

eV

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