Azobenzene Photoswitch: cis–trans Isomerization

Illumination & Temperature

UV intensity (365 nm, π→π*) 40%

Visible intensity (450 nm, n→π*) 0%

Temperature 298 K

Population (N = 288 molecules)

trans 100%cis 0%

trans (extended) cis (bent)

cis fraction

0.0%

trans fraction

100.0%

Thermal cis→trans half-life at this T

–

How it works

Azobenzene is the archetypal molecular photoswitch: an N=N azo bridge links two phenyl rings. Absorbing a UV photon (π→π* transition, ~340–365 nm) promotes the stable, planar trans (E) form to an excited state that relaxes into the bent cis (Z) form, in which the rings fold to the same side of the N=N bond. Absorbing visible light (n→π*, ~440–450 nm) or simply waiting drives the reverse, cis → trans reaction, because trans is thermodynamically more stable by ~50 kJ/mol.

Each molecule in the ensemble below switches stochastically with the rates:

d[cis]/dt = k(t→c)·[trans] − (k(c→t) + k_therm)·[cis]

Photoisomerization: k = Φ · σ · I
  Φ = quantum yield, σ = absorption cross-section, I = photon flux

Thermal relaxation (Arrhenius):
  k_therm(T) = A · exp(−Ea / R·T)

UV slider — sets the 365 nm photon flux driving trans→cis (real Φ ≈ 0.1–0.3 for this channel).
Visible slider — sets the 450 nm flux that photochemically resets cis→trans (real Φ ≈ 0.4–0.6).
Temperature slider — drives thermal cis→trans decay through the Arrhenius law; the half-life readout (t½ = ln2 / k_therm) updates live.

The rate constants here are scaled to switch on a human-watchable timescale so the dynamics stay visible; real azobenzene has a much higher thermal barrier (Ea ≈ 90–100 kJ/mol), giving a cis half-life of hours to days at room temperature — which is exactly why it is used in photoswitchable drugs, dye-sensitized materials and molecular data-storage research: the cis state is metastable, not fleeting.

Tracked molecule: trans

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Azobenzene Photoswitch: cis–trans Photoisomerization