Senge's Photochem

Question 1

What does hv do to a double bond?

Answer 1

Cis/Trans isomerisation - rotation requires breakage of double bond as alkene is not freely rotatable

Question 2

Colour vs Pigmentation

Answer 2

Colour (think of dyes) - chemical structure is that colour, absorption of light and emission of light as photons
Pigmentation - reflection/diffraction of light in certain way so that it appears a colour

Question 3

What type of structures are generally able to absorb and emit light?

Answer 3

Rigid Structures - cannot disperse energy through vibrational or rotational means, have to disperse as photons.
May contain carbonyl groups
Normally large structures
Conjugated systems

Question 4

Solvatochromism

Answer 4

Instead of mixing two colours to get colour you want from dyes, can sometimes absorb in different polar solvents where the dipole energy stabilises ground state thus increasing energy of HOMO LUMO gap and therefore the colour of the dye!

Question 5

What state does oxygen exist at in room temp?

Answer 5

Triplet state as the triplet state is lower in energy than the ground singlet state. To undergo a chemical reaction, triplet oxygen has to undergo the spin forbidden transition to a singlet state.

Question 6

Fluorescence vs phosphorescence

Answer 6

Fluorescence - emission of electromagnetic radiation within about 10^-8 seconds after absorption
phosphorescence - slow emission of electromagnetic radiation as electrons drop to triplet state and then to ground singlet. It is slower because these are spin forbidden transitions.

Question 7

How does photorepair work in our bodies?

Answer 7

As light hits DNA, photomutation may occur and “wrong” base pairs added creating this ‘bulge’ in the dimer. Occurs millions times a day. CPD photolyase binds to damaged dimer, and repairs via photolyase -absorption of light >300 nm (violet/blue end of spectrum) and refills with complementary base pairs and unbinds.

Question 8

How do humans protect their skin from sun damage?

Answer 8

Absorbers of UV radiation,
Reflectors of UV radiation (sunscreen)
Anti-oxidants (help repair mechanism)

Question 9

Stilbene Photoisomerisation

Answer 9

E vs Z stilbene - Opsin (protein) contains retinal in eye, in dark cis double bond, in light is trans, Change in shape changes shape of protein which changes shape of next protein etc., until it hits tunnel protein.

Question 10

Tunnel protein

Answer 10

Na+/K+ pump in neurons, something that pumps protons of cations to create an electrochemical gradient which causes neurons to fire.

Question 11

How can energy absorbed by light be dissipated?

Answer 11

1. Light to light - fluorescence, phosphorescence
2. Light to chemistry - Chemical change e.g. singlet or triplet state.
3. Light to heat - Physical - Internal conversion or inter system crossing.

Question 12

Photosensitisation

Answer 12

Doing photo chemistry with a compound that doesn’t absorb light, e.g. use a compound that is able to absorb light and let it affect the compound we want to change - e.g. with triplet oxygen, absorb light turn to singlet oxygen, substrate is then oxidised by higher energy oxygen and get an oxidised substrate.

Question 13

Dyes structure vs fluorescent dyes

Answer 13

dyes - have extended conjugated pi system, molecular structure determines colour. Absorption of light energy and conversion to vibrational energy (heat) - i think absorb energy opposite on colour wheel to what we see
Fluorescent dyes - more rigid structures and therefore conversion to vibrational energy not possible therefore release energy in form of fluorescence.

Question 14

Hypsochromic shift

Answer 14

Blue shift - higher energy, lower wavelength shift

Question 15

Bathochromic shift

Answer 15

Red shift - lower energy, higher wavelength

Question 16

Hyperchromic vs Hypochromic shift

Answer 16

Hyperchromic shift - positive, increased absorption, hypochromic = decreased

Question 17

Types of photosensitisation

Answer 17

Type 1 - Activate sensitizer with light, make triplet sensitiser. Sensitiser reacts with substrate to form radical substrate and radical sensitizer, then reaction with triplet oxygen to form oxidised products.
Type 2 - Activate sensitizer with light, make triplet sensitiser, react with triplet oxygen first to make singlet oxygen and that oxidises substrate.

Question 18

Which functional groups go through pi to pi* transitions (singlet or triplet) upon exposure to hv

Answer 18

Aromatics and polyenes

Question 19

Which functional groups go through n to pi* transitions (singlet or triplet) upon exposure to hv

Answer 19

Mainly carbonyls

Question 20

What is LLLT

Answer 20

Low Level Light Therapy

Question 21

Describe Neonatal jaundice blue light treatment and why is it necessary

Answer 21

When bruised - blood vessels burst releasing red blood cells into tissue, cells slowly degrade and haemoglobin released, proteins on their own aren’t very stable and start to degrade.
Porphryin and light - form singlet oxygen which is TOXIC, therefore the ring breaks open to form bilverdin and then further reduction gives bilirubin. Bilirubin is initially formed as Z alkene but under blue light (460-90 nm) it goes to E bilirubin which is passed safely from body.
UNSURE: i think that that isomerisation allows more solubility in water to be passed from body. Diseases associated with this is indication of liver failure. Difference between biliverdin and bilirubin is like one double bond in the difference

Question 22

PUVA

Answer 22

Psoralen and UV-A Treatment - organic molecule, light and oxygen - uv activates chromophore which produces oxygen which takes part in photochem reaction which kills bacteria causing disease

Question 23

What is Porphyrias?

Answer 23

In body, we synthesize uroporphyrin 3 which turns into iron(II) protoporphryin heme (which is not a photosensitizer). Feedback inhibition from uroporphyrin 3 tells enzyme ALA to stop making uroporphyrin, however sometimes there is wrong ‘AP’ configuration (uroporphyrinogen 1) which because of its different configuration, doesn’t tell ALA to stop making molecule. This uroporphyrin is sensitive to light and insoluble, reacts iwht light to form singlet oxygen which is TOXIC (but only on exposure to light). Causes red urine among other side effects.

Question 24

Photodynamic cancer therapy

Answer 24

Inject with photosensitiser, may make it targetted to accumulate in cancer cells. Hit with light which forms singlet oxygen and cell death. Want intense absorption in red or nIR region (greater tissue penetration) Limitations : low penetration in skin.