A cautionary tale of basic azo photoswitching in dichloromethane finally explained

The study addresses long-standing, irreproducible and counterintuitive results reported for azobenzene and azopyridine photoswitch thermal isomerization kinetics in chlorinated solvents such as dichloromethane (DCM), particularly under UV irradiation. Azobenzenes, including azopyridines, are widely used molecular photoswitches whose applications depend critically on thermal isomerization rates. Prior work has debated the mechanism of thermal isomerization (inversion, rotation, or multistate rotation involving S1 and T1) and has revealed strong environmental and substituent dependence. For azopyridines, inconsistent kinetics and reliability have been reported, and protonation is suspected to play a role, with contradictory literature about whether protonation suppresses photoisomerization or accelerates thermal relaxation. The authors hypothesize that UV irradiation in DCM generates protons via solvent photodecomposition, leading to adventitious protonation of azopyridines and altered kinetics and mechanisms. The purpose is to definitively identify the cause of anomalous behavior in DCM and elucidate the mechanistic consequences of protonation on thermal isomerization.

• Azobenzenes are T-type photochromes with thermal cis-to-trans relaxation spanning wide timescales; spectral classification follows Rau, with π→π* (UV, intense) and n→π* (visible, weak), and typically hours-long thermal isomerization.
• The thermal isomerization mechanism has been debated (inversion vs rotation or hybrid). Recent studies support a multistate rotation mechanism involving S1 and T1, explaining the experimentally observed negative activation entropies (the entropy puzzle).
• Application needs vary: some require rapid thermal isomerization (μs or faster), others require thermal stability.
• Azopyridines extend functionality via a basic pyridine ring, enabling environmental responsiveness; however, fundamental thermal kinetics are poorly understood and often inconsistent across solvents and conditions.
• Reports show contradictory effects of protonation: quantum chemical work suggested protonation lowers the barrier so much that bulk photoisomerization becomes unfavorable (shutting down formation of cis), whereas experimental studies observed that photoisomerization can persist at low pH with drastically accelerated thermal isomerization (hours to milliseconds). Other works reported abrupt regime changes in isomerization kinetics for azopyridine-based sensors.
• Prior observations in chlorinated solvents (e.g., chloroform) noted UV-induced color/spectral changes attributable to acid generation and/or electron transfer processes; DCM photolysis can produce HCl and other products under UV/VUV, with mechanisms influenced by water and oxygen. Pyridine derivatives can form adducts with DCM under ambient conditions, potentially facilitating photodecomposition.

Experimental:

• Compound: 4-phenylazopyridine (AzPy) synthesized via coupling of 4-aminopyridine with nitrosobenzene (modified literature procedure). Purified by silica gel chromatography. Characterized by 1H NMR (400 MHz, CDCl3) and ATR-FTIR.
• Solvents and reagents: spectroscopic grade DCM (main solvent), additional solvents (toluene, ethanol, etc.), methanolic HCl (0.01–0.5 M), proton sponge (1,8-bis(dimethylamino)naphthalene, PS) as proton scavenger.
• UV–vis spectroscopy: Varian Cary Bio 100 spectrophotometer, 10 mm quartz cuvettes, 21 °C. Steady-state spectra collected between irradiations. LEDs: 365 nm (104 mW, 10 nm FWHM) and 450 nm (56 mW, 10 nm FWHM). Custom pump–probe setup (schematic in SI Fig. S3).
• Photoirradiation protocols:
  • Spectral characterization: 60 μM AzPy in DCM irradiated at 365 nm (15 s intervals) or 450 nm (15 s intervals), collecting spectra after each exposure. Control experiments in other solvents (e.g., toluene).
  • Acid titration: incremental additions (total 50 μL) of 0.01 M methanolic HCl to 2.5 mL of 60 μM AzPy in DCM; spectra collected between additions.
  • Proton scavenging: 60 μM PS in DCM irradiated with 365 nm in 30 s intervals to monitor PS protonation signature; 450 nm control (no effect).
  • Kinetics measurements: dual-pump scheme. For each cycle, irradiate AzPy solution at 365 nm to photogenerate protons (via DCM photodecomposition), then pump at 450 nm (4 min) to drive trans→cis photoisomerization. Monitor recovery (cis→trans) at the π→π* peak (312–313 nm) to obtain thermal relaxation kinetics. Repeat cycles to assess cumulative effect of prior 365 nm exposure. After final cycle, add PS (1:1 molar to AzPy) and remeasure kinetics.
• Kinetic analysis: Assume first-order relaxation. Effective rate constant k_eff reflects parallel relaxation of neutral (AzPy) and protonated (AzPyH+) species. Extract k_eff by monoexponential fitting of absorbance A(t) at the π→π* band: A(t) = (A0 − A∞) e^(−k t) + A∞. Address irreproducibility due to trace water by replicates and controls; normalize curves to A0.

Computational:

• Software: ORCA 5.0.3 with RIJCOSX and def2/J auxiliaries.
• Protonation site energetics: M06-2X/def2-QZVPP (gas phase) to evaluate first protonation of cis/trans geometries; confirm minima via vibrational analysis.
• Thermal isomerization pathways: Spin-flip TDDFT (Tamm–Dancoff) with BH&HLYP-D3(BJ)/def2-QZVPP for S0 and T1 surfaces. Evaluate rotation (CNNC dihedral) and inversion (∠CNN) pathways. Increment angles (10°, then 5° near TS). For S0 single-surface pathways, take highest-energy optimized structure as TS; energies reported relative to ground-state cis (calculated at identical level). For enforcing rotation in AzPyH+ S0, constrain internal CNN angles (average of cis/trans) to avoid inversion.
• Solvent effects: Linear-response polarizable continuum model (PCM) for DCM to refine AzPyH+ inversion barrier.
• Electronic structure analysis: Kohn–Sham wavefunctions for MO visualization, population analysis, bond lengths/orders. Devised isodesmic hydrogenation reaction to estimate inductive weakening of the azo bond in AzPyH+; computed reaction enthalpies (298.15 K). Visualization via Chemcraft.

• UV-induced spectral changes in DCM at 365 nm indicate protonation:
  • For 60 μM AzPy in DCM, 365 nm irradiation generated new peaks at 340.0 nm (π→π*) and 473.0 nm (n→π*), with isosbestic points at 236.0, 258.0, and 323.5 nm, and a decrease of the original π→π* band at 312.5 nm (ε ≈ 24,000 M−1 cm−1). Changes saturated after ~6 min. 450 nm irradiation produced only standard trans↔cis photoisomerization without bathochromic/hyperchromic shifts. Similar irradiation in toluene showed only isomerization behavior.
  • Titration with 0.01 M methanolic HCl reproduced identical bathochromic/hyperchromic shifts and visible color change (yellow→pink at higher concentration), confirming protonation of the pyridine nitrogen.
  • Proton sponge (PS) in DCM exhibited characteristic spectral changes upon 365 nm irradiation, evidencing UV-mediated acid generation in DCM; 450 nm caused no changes.
• Source of protons: UV photodecomposition of DCM (facilitated possibly by adduct formation with pyridines) generates HCl/acidic species even in rigorously dried solvent, explaining historical irreproducibility in chlorinated solvents under UV.
• Kinetic consequences:
  • Thermal isomerization rates of AzPy in DCM are highly sensitive to trace water and adventitious protons, leading to intra-sample discrepancies up to an order of magnitude.
  • In a cyclic 365 nm (proton generation) / 450 nm (photoisomerization) scheme, the effective thermal relaxation rate approximately doubled with each additional 365 nm exposure (initial stages observed). Addition of PS (1:1) abolished the acceleration and restored baseline kinetics.
• Mechanistic insights from theory:
  • Neutral AzPy: Hybrid inversion–rotation mechanism. S0/T1 surfaces cross at CNNC ≈ 77.5° and 102.5° with barriers ~120 and 115 kJ mol−1 (MECPs); inversion barrier ~112 kJ mol−1. Positive activation entropy from single-surface inversion (KS-DFT) contradicts experiment, supporting multistate rotation as operative for neutral AzPy.
  • Protonated AzPyH+: S0 and T1 do not cross (multistate rotation abolished). Thermal isomerization proceeds primarily via inversion at the azo nitrogen near the pyridinium. Barriers drop markedly: ~25 kJ mol−1 (gas phase); ~62 kJ mol−1 with DCM PCM. Simple Arrhenius estimates indicate ~10^6-fold acceleration upon protonation (assuming ~120 kJ mol−1 for neutral AzPy).
  • Origin of acceleration: Not resonance stabilization but an inductive effect that withdraws electron density from the azo bond, weakening it and facilitating rotation/inversion. Isodesmic hydrogenation enthalpies (ΔH298): 37.9 kJ mol−1 (trans) and 46.4 kJ mol−1 (cis) support partial azo bond weakening in AzPyH+ and account for a substantial fraction of the barrier reduction.
• Practical implication: UV in chlorinated solvents can unintentionally protonate azopyridines, drastically accelerating thermal isomerization; using a proton scavenger reverses the effect.

The study clarifies the long-standing observation that azopyridine (and acid-sensitive azobenzene) thermal isomerization kinetics in chlorinated solvents under UV irradiation are erratic and often irreproducible. The experiments demonstrate that 365 nm irradiation in DCM generates acid capable of protonating the pyridine nitrogen of AzPy, as evidenced by bathochromic/hyperchromic spectral shifts replicated by acid titration and mirrored by proton sponge protonation. This adventitious protonation, even at trace levels, profoundly accelerates thermal cis→trans isomerization and explains inconsistent kinetics historically observed in DCM. Kinetic measurements using a dual-pump protocol show cumulative acceleration with additional UV (365 nm) exposure and full reversal upon addition of a proton scavenger, directly linking protons to the rate enhancement. Computational analysis bridges the observations to mechanism: for neutral AzPy, a multistate rotation pathway involving S0/T1 crossings is consistent with the entropy puzzle and high barriers (~112–120 kJ mol−1). Protonation eliminates S0/T1 crossings, favoring a single-surface inversion pathway with a much lower barrier (25–62 kJ mol−1 depending on solvation), predicting order-of-magnitude increases in rate consistent with experimental acceleration. The origin of the lowered barrier is attributed to inductive weakening of the azo bond upon pyridinium formation rather than resonance stabilization of a linear transition state. Collectively, these results resolve the “chlorinated solvent caution” by identifying UV-generated protons as the root cause and show that protonation can tune azopyridine thermal kinetics over many orders of magnitude. The findings carry broader significance for designing and characterizing photoswitches containing basic sites or acid-sensitive groups and for avoiding artifacts in kinetic studies conducted in chlorinated solvents under UV.

This work definitively identifies UV-induced protonation in dichloromethane as the cause of anomalous azopyridine photoswitching behavior. Long-wave UV (365 nm) photodecomposes DCM to generate acid, which protonates the pyridine nitrogen of AzPy, producing characteristic spectral redshifts and dramatically accelerating thermal isomerization. Protonation eliminates the multistate S0/T1 rotational mechanism operative in the neutral species and lowers the barrier for thermal isomerization via an inductive weakening of the azo bond, not via resonance stabilization. Experimentally, repeated 365 nm exposure progressively increases the relaxation rate, while a proton scavenger restores baseline kinetics. Practically, studies of azobenzenes with acid-sensitive functionalities in chlorinated solvents should avoid UV conditions that generate acid or include scavengers/controls to prevent adventitious protonation. Conceptually, controlled protonation provides a reversible handle to tune azopyridine thermal isomerization over several orders of magnitude. Future work will expand time-resolution to capture the full acceleration dynamics, refine quantum chemical treatments of inversion pathways, and explore generality across related azo systems and solvents.

• Instrumental time resolution limited the observation to initial stages of UV-induced acceleration; full kinetic evolution requires improved temporal capability (noted as forthcoming).
• Computational limitations: KS-DFT is unreliable near rotational transition states due to multiconfigurational character; spin-flip TDDFT can exhibit spin contamination near crossings; solvent effects were approximated via PCM.
• The exact concentrations and identities of acidic photoproducts from DCM under 365 nm were not quantified; mechanisms likely depend on water/oxygen and potential pyridine–DCM adduct formation.
• Irreproducibility due to trace water was observed; while addressed experimentally, a systematic quantification of water/proton content during measurements was not provided.
• Constraints were applied to enforce rotational pathways in AzPyH+ on S0, which may bias exact barrier values though trends remain clear.