Fundamentals of photochemistry 1

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: [email protected] (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/

Topics • Introduction to photophysics • Established organic chromophores • Triplet-triplet upconversion • Two-photon absorption and nonlinear optics

• Marcus theory and photoinduced electron transfer • Photosystem II and artificial photosynthesis • Dye-sensitized solar cells

• Inorganic bonding, photophysics

• Photocatalytic CO2 reduction

• Ru(bpy)3

• Organometallic light emitting diodes

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

“Principles of Molecular Photochemistry: An Introduction” - Nicholas J. Turro, V. Ramamurthy, J. C. Scaiano. (University Science Books, 2010). (aka baby Turro)

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Students interested in the applications of organic photochemistry are 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 Science Books, 2010). (aka big Turro)

(see course syllabus for further recommended reading)

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The following text is available as an e-book from the library and is a useful reference text to at an introductory level.

“Principles and applications of photochemistry” - Brian Wardle (Wiley)

Motivation •

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

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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?

Molecular photophysics & photochemistry •

Molecular photophysics and photochemistry is a very broad and interdisciplinary topic embracing the fields of chemical physics, molecular spectroscopy, and supramolecular chemistry.

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

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Upon laying a solid foundation of general photophysical principles we will progress to apply this knowledge specifically to inorganic systems – again from a theoretical and application perspective.

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We will touch on the topic of molecular inorganic photochemistry towards the end of this lecture course. Molecular organic photochemistry will not be covered in any depth due to time constraints.

Photophysics vs. Photochemistry ? •

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

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Molecular photophysics & photochemistry is often divided into two (sometimes four) sub-topics
Organic photophysics & photochemistry
Inorganic photophysics & photochemistry

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

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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.

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The “molecular” part of molecular photophysics and photochemistry emphasizes 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 and describing the possible, plausible, and probable pathways of photochemical reactions from the absorption of a photon by a reactant R to form its excited state *R ultimately resulting in a product P produced by 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) is resonant with an electronic transition in R responsible for producing the excited state *R. • The excited state *R is considered an independent chemical entity relative to R. each have a unique structural geometry & electronic structure.

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There are 3 fundamentally distinct primary photochemical pathways that *R may follow 1) *R → I → P via a distinct “reactive intermediate” (I) which typically has the characteristics of 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 and electronically excited product (*P).

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Primary photochemical pathway 1 1) *R → I → P via a distinct “reactive intermediate” (I) which typically has the characteristics of 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 of primary photochemical pathways?

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For any reaction to be possible molecules (including their vibrational and spin substructures) 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

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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.

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Only the set of structures and pathways that obeys the conservation laws is considered 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 induce absorption of a photon to produce *R, and how does this interaction of a photon with the electrons of R relate to the theoretical and experimental quantities, such as extinction coefficients, radiative lifetimes, and radiative efficiencies? 2) What are the possible and plausible structures, energetics, and dynamics available to *R and I that occur along the reaction pathway from *R → P ? 3) What are the possible and plausible sets of primary photochemical processes corresponding 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 are postulated 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 pathways for the photophysics and photochemistry of *R ? 7) What are the absolute rates (rate constants) at which each elementary step occurs along the reaction pathway from *R → P ? 8) What sorts of structures, energetics, and dynamics correspond to *R and I in typical photoreactions?

Scheme 1.2

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

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At this basic level of theory electron-electron repulsions and electron spin configurations are not considered.

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When the energies of the non-bonding orbitals in I are significantly different both electrons are spin-paired in the lower lying orbital. Such electronic configurations correspond to species called zwitterions.

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Questions: 

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



What is the electronic configuration of *R ?



What are the plausible primary photophysical and photochemical processes typicaly 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 initial analysis 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 of displaying 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 rapid deactivation 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 xcoordinate has no physical meaning (it is not a reaction coordinate or potential energy surface).

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What is the basis of isomerism in the state energy diagram?
Isomerism results from differences in the electronic configurations (electronic isomers) or in the spin configurations (spin isomers) between different states. Different electronic or spin isomers may also be stereoisomers of each other.

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

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

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All possible photophysical transitions from S1 and T1 must be considered in an overall *R → R photochemical process. If photophysical processes are very fast relative to photochemical processes, the latter may be plausible but will be improbable because of the former plausible photophysical processes which are kinetically favored.

Possible radiative absorption and emission processes 1. Spin allowed singlet-to-singlet photon absorption characterized by an extinction coefficient ε(S0 → S1) S0 + hν → S1 2. Spin forbidden singlet-to-triplet photon absorption characterized by an extinction coefficient ε(S0 → T1) S0 + hν → T1 3. Spin allowed singlet-to-singlet photon emission, aka fluorescence emission characterized by a rate constant kFl S1 → S0 + hν‘ 4. Spin forbidden singlet-to-triplet photon emission, aka phosphorescence emission characterized by a rate constant kPh T1 → S0 + hν‘’

Plausible non-radiative photophysical processes 5. Spin allowed radiationless electronic transitions between states of the same multiplicity, aka internal conversion characterized by a rate constant kIC S1 → S 0 + ∆ 6. Spin forbidden radiationless electronic transitions between states of the differing multiplicity, aka intersystem crossing characterized by a rate constant kST S 1 → T1 + ∆ 7. Spin forbidden radiationless electronic transitions between T1 and S0 also know as intersystem crossing and characterized by a rate constant kTS T1 → S0 + ∆

Scheme 1.4

Jablonski diagram

S T

hv

S fl T ph

S

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 1.77 1.99 2.14 2.34 2.64 2.95 >4.13