Question

If a = -15 eV and b = -6 eV, which is the shortest conjugated carbon chain molecule capable of absorbing some visible light (and thereby becoming colored)?

Answer 1

Here we are talking about excitation of pi-electrons in a conjugated system in UV- region.

Now for the excitation of electrons, we need energy. And higher the jump, higher energy is required. So the relation between energy required (E) and the frequency of light (ν) is

• E = hν

where h = plan's constant

• Again we know v = 1/Λ

where Λ = wavelength of the light.

So we can write

• E = h/Λ

It tells that if we need to jump higher, we need more energy. Now a particular wave length corresponds to a particular energy. And generally lights with lower wavelength has higher energy.

• So for the smaller wave length, the energy and frequency should be higher.

Now this energy transfered ca be calculated by another formula-

• E = V * q

where V= potential difference

q = amount of charge transfered.

• This equation indicates that higher is the potential difference, higher will be the Energy required.
• So higher the energy, shorter is the wave length.

So combining these two concepts, we will have high potential difference for B. So it will have smaller wave length.

• Now we know UV spectra has lesser wave lengths than Visible region. So if we talk about this question, then the smaller wave length will be appeared in the UV region, where as the larger one for Visible region.

So particle A- will absorb light from visible region.