Topic 1 Flashcards by Lucky Cat

What is the color of an object illuminated by white light explained by?

Subtractive color model

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Define:

Subtractive color model

If object absorbs a specific colour of light, that colour is “subtracted” from the light that is reflect/transmitted

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Subtractive Colour Model:

Absorbs yellow

Appears purple

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Subtractive Color Model:

Absorbs yellow-orange

Appears blue-violet

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Subtractive Color Model:

Absorbs orange

Appears blue

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Subtractive Color Model:

Absorbs red-orange

Appears blue-green

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Subtractive Color Model:

Absorbs red

Appears green

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Subtractive Color Model:

Absorbs red-violet

Appears yellow-green

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Subtractive Color Model:

Absorbs violet

Appears yellow

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Subtractive Color Model:

Absorbs blue-violet

Appears yellow-orange

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Subtractive Color Model:

Absorbs blue

Appears orange

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Subtractive Color Model:

Absorbs blue-green

Appears orange-red

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Subtractive Color Model:

Absorbs green

Appears red

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Subtractive Color Model:

Absorbs yellow-green

Appears red-violet

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For monochromatic light
1. What happens if the object absorbs the color?
2. What happens if the object reflects the color?

Light appears dimmer
Light appears bright

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Define:

HOMO

Highest Occupied Molecular Orbital

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Define:

LUMO

Lowest Unoccupied Molecular Orbital

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When a molecule absorbs light, an electron in the ——- orbital is promoted to another orbital

Higher-energy

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In organic compounds, what orbitals are the highest energy electrons in?

p orbitals

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More nodes equals…

Higher energy

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Define:

Conjugated system

“Put together”
* The pi bond system can alternate between multiple different bonds and are “put together/connected”

Singlet state

Derived from formula:
2 (sum of spin numbers) +1 = 1
* Essentially the state where the sum of spin numbers is 0

Ground state

What narrows the HOMO-LUMO gap?

Extended conjugation
Polar groups

What theory explains the narrowing of energy gaps caused by extended conjugation/polar groups?

Partcle-in-a-box theory

What explains the reason that light absorption happens in a band, not a single wavelength?

Vibrational energies of electrons

Electronic-excitation energies

200-400 kJ/mol

Vibrational energies

10-40 kJ/mol

Explain absorption band in terms of vibrational energies

In each electronic state, there are a series of vibrational levels * Multitude of vibrational levels are attained when electrons are excited, resulting in a band continuum

Non-radiative vibrational cooling

Also known as vibrational decay * Fast * Electron returns to v0 state of the S1 * Releases heat

From S1, v0 state, what are the 5 events that can happen?

1. Fluorescence (to S0) 2. Internal conversion (to S0) 3. Intersystem crossing (to triplet state) 4. Energy transfer to another molecule 5. Chemical reactions

Fluorescence vs. Internal conversion

Fluorescence is an emissive/radiative process Internal Conversion is a non-radiative process

Characteristics of fluorescence

* Fast (nanoseconds, microseconds) * Radiative process (emissive), releases energy as light * S1 to S0 transition

What happens in fluorescence?

Electron returns to S0/HOMO state BUT rarely in v0 * Vibrationally excited even though electronically grounded * Returns to v0 via vibrational cooling

What can fluorescence be measured with?

Fluorescence spectrometer (fluorimeter)

Characteristics of Fluorimeters

* Usually, only λ(max abs.) is used to excite sample * Emission beam is monitored perpendicular to excitation beam (maximize sensitivity, prevents any unabsorbed light from hitting detector)

Fluorescence emission spectrum

Plots fluorescence intensity as a function of emission wavelength * Absorption and emission sometimes plotted on same graph

Why does fluorescence emission usually occur at a higher wavelength than absorption?

In excited state, energy is lost as vibrational energy before fluorescence occurs * Results in lower energy thus longer wavelengths to be emitted

Characteristics of Internal Conversion

* S1, v0 to S0, v0 * Non-radiative, energy released as heat * Very slow or very fast (depends on molecule)

Why is internal conversion very fast in nucleobases?

Deactivating a high-energy intake to reduce the risk of DNA damage

Characteristics of Intersystem Crossing

* Excited electron undergoes a spin flip to have SAME SPIN as other electron * Results in a triplet state (T1)

Characteristics of electrons in T1 state

* Lower in energy than the singlet state * Also have vibrational levels (vibrational cooling to T1, v0 will occur)

What happens when T1 returns to S0?

Occurs by: 1. Phosphorescence (radiative, emissive) 2. Internal Conversion (non-emissive)

Phosphorescence

* Much slower than fluorescence * Microseconds to hours

What happens in BOTH phosphorescence and internal conversion of T1 to S0?

Intersystem crossing * Electron undergoes another spin flip back to the singlet (opposite spins)

Jablonski diagram

Summarizes photophysical processes * Non-radiative processes = Dashed lines * Radiative processes = Filled lines

# True or False: Decay Processes are competitive

True

# True or False: Most common proccess of decay can be determined from theory

False, must be determined experimentally and varies from molecule to molecule

What is important in conjugation?

Continuous, extended conjugation NOT Total number of double bonds

What is the role of polar/charged functional groups?

If present, less conjugation is required for a molecule to exhibit color

Give examples of compounds with conjugation + polar/charged functional groups

1. Indigo 2. Quinones 3. Triarylmethanes 4. Porphyrins 5. Diaryl Azo