CSIR-NET Photochemistry — The Essential Rules
Photochemistry looks like a memory chapter and is not. Nearly every CSIR-NET question comes from one of three places: a named law, a competition between rate constants, or a quantum-yield calculation. This article states the laws precisely, lays out the Jablonski scheme as a table you can reconstruct from memory, and then does the calculations in full arithmetic.
The two laws that frame everything
Stark–Einstein law (law of photochemical equivalence, second law): in the primary photochemical act, one molecule is activated by one absorbed photon.
Read the second law carefully. It restricts the primary step only. Secondary thermal steps are unlimited, which is why an overall quantum yield can be enormous — and why the law is not violated when it is.
Photon energy — and why one photon is a lot
h = 6.626 × 10−34 J s, c = 2.998 × 108 m s−1, NA = 6.022 × 1023 mol−1. One einstein = one mole of photons.
Energy of one einstein at λ = 300 nm.
hc = (6.626 × 10−34)(2.998 × 108) = 1.9865 × 10−25 J m
Ephoton = 1.9865 × 10−25 ÷ (300 × 10−9) = 1.9865 × 10−25 ÷ 3.00 × 10−7
= 6.622 × 10−19 J
Eeinstein = 6.622 × 10−19 × 6.022 × 1023 = 3.988 × 105 J mol−1
= 399 kJ mol−1
That single number explains most of photochemistry. 399 kJ per mole is comparable to a C–C or C–H bond energy, so near-UV light can break bonds that room-temperature collisions never will.
The Jablonski scheme, written out
A diagram is only a bookkeeping device for these processes. Learn the table instead — the timescales are what decide which process wins.
| Process | States | Radiative? | Spin change? | Typical timescale |
|---|---|---|---|---|
| Absorption | S0 → S1, S2 | yes | no | 10−15 s |
| Vibrational relaxation | within one electronic state | no | no | 10−12–10−10 s |
| Internal conversion (IC) | S2 → S1, S1 → S0 | no | no | 10−12–10−8 s |
| Fluorescence | S1 → S0 | yes | no | 10−9–10−7 s |
| Intersystem crossing (ISC) | S1 → T1 | no | yes | 10−10–10−8 s |
| Phosphorescence | T1 → S0 | yes | yes | 10−6–102 s |
Three consequences follow directly from that table and are examined constantly:
- Kasha's rule. Internal conversion from S2 to S1 is far faster than emission, so emission is observed only from the lowest excited state of a given multiplicity — S1 for fluorescence, T1 for phosphorescence.
- Vavilov's rule. Because of Kasha's rule, the fluorescence quantum yield is essentially independent of the excitation wavelength.
- Stokes shift. Emission is always at longer wavelength than absorption, because vibrational relaxation wastes energy before the photon is emitted.
The Franck–Condon principle explains the band shapes: electronic transitions are so fast that the nuclei do not move during them, so a transition is drawn as a vertical line and its intensity depends on the overlap of the two vibrational wavefunctions.
Selection rules
| Rule | Statement | How it is broken |
|---|---|---|
| Spin | ΔS = 0; singlet → triplet is forbidden | Spin–orbit coupling; the heavy-atom effect (internal or external) makes ISC and phosphorescence competitive |
| Laporte (parity) | In a centrosymmetric molecule only g ↔ u transitions are allowed, so d–d transitions are forbidden | Vibronic coupling, or loss of the centre of symmetry (tetrahedral complexes are far more intensely coloured for this reason) |
| Symmetry / overlap | n → π* in a carbonyl is symmetry-forbidden | Vibronic mixing. It still appears, but weakly: ε roughly 10–100, against 103–105 for an allowed π → π* |
The molar absorption coefficient ε is therefore diagnostic. A weak, long-wavelength band in a ketone is n → π*; the strong short-wavelength band is π → π*. Solvent polarity separates them further — n → π* shifts blue in polar solvents, π → π* shifts red.
Quantum yield — the definition and the arithmetic
= (moles of product formed) ÷ (einsteins absorbed)
Worked example. A solution absorbs 1.00 J of radiation at 300 nm and produces 2.00 × 10−6 mol of product. Find Φ.
From above, one einstein at 300 nm carries 3.988 × 105 J.
Einsteins absorbed = 1.00 ÷ (3.988 × 105) = 2.508 × 10−6 einstein
Φ = (2.00 × 10−6) ÷ (2.508 × 10−6) = 0.798 ≈ 0.80
Cross-check by the photon route. Photons absorbed = 1.00 ÷ (6.622 × 10−19) = 1.510 × 1018. Molecules formed = 2.00 × 10−6 × 6.022 × 1023 = 1.204 × 1018. Φ = 1.204 ÷ 1.510 = 0.798. The two routes agree.
Two standard cases sit at the extremes. Photolysis of HI has Φ(−HI) = 2: one photon splits one HI into H• and I•, and the hydrogen atom immediately consumes a second HI (H• + HI → H2 + I•). The hydrogen–chlorine chain reaction has a quantum yield of the order of 104–106, because a single photon starts a radical chain that runs many thousands of cycles before termination. Neither breaks the Stark–Einstein law, which governs only the primary act.
Lifetimes, efficiency and quenching
τ0 = 1 / (kf + Σknr) · natural radiative lifetime τr = 1/kf
Stern–Volmer: Φ0/Φ = 1 + KSV[Q], with KSV = kqτ0
Worked example. A dye has kf = 5.0 × 107 s−1 and total non-radiative decay 5.0 × 107 s−1.
τ0 = 1 ÷ (5.0 × 107 + 5.0 × 107) = 1 ÷ (1.0 × 108)
= 1.0 × 10−8 s = 10 ns
Φf = (5.0 × 107) ÷ (1.0 × 108) = 0.50
Cross-check: Φf = kfτ0 = 5.0 × 107 × 1.0 × 10−8 = 0.50. And by the lifetime ratio, τr = 1/kf = 2.0 × 10−8 s = 20 ns, so Φf = τ0/τr = 10/20 = 0.50. All three routes agree.
Now add a quencher. At [Q] = 0.020 M the emission falls to one third, so
Φ0/Φ = 3.0.
KSV = (3.0 − 1) ÷ 0.020 = 100 M−1
kq = KSV/τ0 = 100 ÷ (1.0 × 10−8) =
1.0 × 1010 M−1 s−1
That value sits at the diffusion-controlled limit for a small molecule in water, so this quenching is as fast as encounters allow — a conclusion you can only reach by finishing the arithmetic.
Photochemical reactions worth knowing by name
- Norrish type I — α-cleavage of an excited ketone into an acyl and an alkyl radical, which may then lose CO.
- Norrish type II — intramolecular abstraction of a γ-hydrogen through a six-membered transition state, giving a 1,4-biradical that either cleaves to an enol plus an alkene, or closes to a cyclobutanol (Yang cyclisation). No γ-hydrogen means no type II.
- Paternò–Büchi — an excited carbonyl adds across an alkene to give an oxetane.
- Photoreduction of benzophenone in propan-2-ol, giving benzpinacol via hydrogen abstraction by the n,π* triplet.
Orbital symmetry control is examined through the Woodward–Hoffmann rules, and the photochemical row is simply the thermal row reversed:
| Reaction type | Electron count | Thermal | Photochemical |
|---|---|---|---|
| Electrocyclic | 4n | conrotatory | disrotatory |
| Electrocyclic | 4n + 2 | disrotatory | conrotatory |
| Cycloaddition (suprafacial–suprafacial) | 4n + 2, e.g. 4 + 2 | allowed | forbidden |
| Cycloaddition (suprafacial–suprafacial) | 4n, e.g. 2 + 2 | forbidden | allowed |
Mistakes that cost marks
- Claiming Φ > 1 breaks the Stark–Einstein law. It does not — the law applies to the primary act only.
- Confusing τ0 with τr. The measured lifetime includes every decay channel; the natural radiative lifetime is 1/kf alone. Stern–Volmer uses the measured one.
- Using energy per photon where einsteins are needed. Decide at the start whether you are counting molecules or moles and stay consistent.
- Expecting phosphorescence in fluid solution at room temperature. T1 lives long enough to be quenched by oxygen and by collisions, which is why phosphorescence is usually observed in a rigid glass at low temperature.
- Forgetting the first law. Light that passes straight through does nothing. If the compound does not absorb at that wavelength, no photochemistry occurs however intense the lamp.
- Ignoring the fraction absorbed. With A = 0.30 only about half the incident light is absorbed (1 − 10−0.30 = 0.50); quantum yield is defined per photon absorbed, never per photon supplied.
Where this appears in the exam
| Exam | Typical demand |
|---|---|
| CSIR-NET Chemical Sciences | Quantum yield arithmetic, Stern–Volmer analysis, Jablonski processes and their ordering, Norrish reactions |
| GATE Chemistry | Photon energy, laws of photochemistry, fluorescence versus phosphorescence, selection rules |
| IIT-JAM / CUET-PG | hc/λ calculations, Beer–Lambert, absorption versus emission |
| MSc coursework | Actinometry (potassium ferrioxalate is the standard chemical actinometer) and lifetime measurement |
Get the powers of ten right. Photochemistry arithmetic is small numbers multiplied by large ones — 10−34 against 1023 — and that is exactly where marks are lost. The ABC Chemistry Calculator Suite keeps the scientific constants, unit converter and calculation tools in one page while you work.
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