hatcher content part 11

what is the lifetime?

Imagine the measured quantity comprises a number of discrete
elements in a set [e.g. no. of molecules in the excited state]
The “lifetime” is the AVERAGE time that an element remains in the set
[i.e. average time an excited molecule takes to decay back to the
ground state]. It is often called the “mean lifetime”

what does single crystal XRD give you?

the 3D structure of a crystalline solid as an average over all the molecules in the crystal under study and the time taken to conduct the experiment

what is a crystal vs a single crystal?

crystal - solid consisting of a regular array of atoms, molecules or ions, repeating in 3 dimensions to give an ordered structure

single crystal - or monocrystalline, solid has a continuous and unbroken lattice throughout the crystal

How does XRD work?

on the diffractometer, a single crystal is irradiated with monochromatic x rays

the ordered atoms/molecules in the single crystal act like a diffraction grating and x rays are diffracted according to Bragg’s law

What is Bragg’s law?

n x gamma = 2 d sin(theta)

d = spacing between planes

theta = angle of diffracted beams

gamma = x ray wavelength

what is photocdrystallography?

for understanding how the 3D structure of a photoactive material contributes to their useful properties

Photocrystallography is the study of light-induced processes in
crystalline materials, using X-ray diffraction techniques

in-situ illumination of a crystal whilst the 3D crystal structure of its photoexcited state is collected

what is pump probe photocrystallography?

requires a pulsed laser pump and pulsed x ray probe

Lab X-ray sources cannot be easily pulsed, requires equipment not readily available

For fast time-resolution, the X-ray pulse is so short that diffraction from the crystal is
very weak: experiments are “diffraction limited” - these are usually performed at national facilities

what are synchrotrons?

circular particle accelerator that can produce very bright light
(particularly X-rays, though other types of radiation are also produced)

Electrons are accelerated by arrays of magnets, to almost c

When their direction in changed around the ring, the fast-moving electrons produce
very intense light

This light is directed down experimental “beamlines”, and used to conduct analytical
experiments

for the naturally pulsed synchrotron, what is the bunch separation and width?

separation = nanoseconds

width = picoseconds

what is an x ray free electron laser?

linear particle accelerator

electrons are accelerated by sequences of magnets

linac is long to allow enough acceleration of electron bunches

separation and bunches are on the femtosecond scale

1000 million and 1 billion times brighter than synchrotron

time resolution down to femtoseconds

what are the benefits of conducting pump-probe diffraction studies on photoactive molecules embedded in supramolecular structures?

Opportunity to control useful properties via interaction with host: e.g. lifetime
Dilution of photoactive species: less crystal strain = potential for higher % conversion

what is an excimer?

excited state dimer

dissociates immediately on relaxation back to the ground state

formed between two fluorophore molecules of the same type: one in excited state (1M*) and one in ground state (1M)

Typically this occurs by a π–orbital interaction

Due to dimer formation, molecules move closer together (as seen for xanthone by pump-probe photocrystallography)
Lifetime of excimer is short (typically nanoseconds)
The fluorescence of the excimer is red-shifted from those of the dissociated monomer
The lack of vibrational structure in the excimer results in a broad emission band