ABSTRACT
Triplet arylnitrenes may provide direct access to aryl azo‐dimers, which have broad commercial applicability. Herein, the photolysis of p‐azidostilbene (1) in argon‐saturated methanol ...yielded stilbene azo‐dimer (2) through the dimerization of triplet p‐nitrenostilbene (31N). The formation of 31N was verified by electron paramagnetic resonance spectroscopy and absorption spectroscopy (λmax ~ 375 nm) in cryogenic 2‐methyltetrahydrofuran matrices. At ambient temperature, laser flash photolysis of 1 in methanol formed 31N (λmax ~ 370 nm, 2.85 × 107 s−1). On shorter timescales, a transient absorption (λmax ~ 390 nm) that decayed with a similar rate constant (3.11 × 107 s−1) was assigned to a triplet excited state (T) of 1. Density functional theory calculations yielded three configurations for T of 1, with the unpaired electrons on the azido (TA) or stilbene moiety (TTw, twisted and TFl, flat). The transient was assigned to TTw based on its calculated spectrum. CASPT2 calculations gave a singlet–triplet energy gap of 16.6 kcal mol−1 for 1 N; thus, intersystem crossing of 11N to 31N is unlikely at ambient temperature, supporting the formation of 31N from T of 1. Thus, sustainable synthetic methods for aryl azo‐dimers can be developed using the visible‐light irradiation of aryl azides to form triplet arylnitrenes.
Irradiation of p‐azidostilbene (1) in argon‐saturated methanol yielded stilbene azo‐dimer through the dimerization of triplet p‐nitrenostilbene (31N). The formation of 31N was verified by electron paramagnetic resonance spectroscopy and absorption spectroscopy in cryogenic matrices. At ambient temperature, laser flash photolysis of 1 in methanol formed the triplet excited state of 1 that decayed to form 31N. CASPT2 calculations gave a singlet–triplet energy gap of 16.6 kcal/mol for 1N; thus, intersystem crossing of 11N to 31N is unlikely at ambient temperature, supporting the formation of 31N from triplet precursor of 1.
Excited-state intramolecular hydrogen transfer on the triplet surface of salicylideneaniline derivatives has received much less attention than the corresponding ultrafast process on the singlet ...surface. To enhance the understanding of this triplet reactivity, the photochemical properties of a series of salicylidene-α-hydroxy acid salts with different substituents on the phenol moiety (1–3) were characterized. UV/vis absorption and phosphorescence measurements in ethanol revealed that 1–3 exist as both enol and keto tautomers, with the enol form being predominant. Irradiation of 1 at 310 nm in ethanol glass (77 K) yielded an absorption band with a λmax at ∼405 nm, which was assigned to the trans-keto tautomer (trans- 1K). In contrast, laser flash photolysis of 1–3 in methanol or acetonitrile resulted in a transient absorption with λmax at 440–460 nm. This transient, which decayed on the microsecond timescale and was significantly shorter lived in methanol than in acetonitrile, was assigned to the triplet excited state (T1) of the cis-keto tautomer (cis-1K–3K) and residual absorption of trans-1K–3K by comparison with TD-DFT calculations. The assignment of the T1 of cis-1K was further supported by quenching studies with anthracene and 2,5-dimethyl-2,4-hexadiene. Laser flash photolysis of 1 in the temperature range of 173–293 K gave an activation barrier of 6.7 kcal/mol for the decay of the T1 of cis-1K. In contrast, the calculated activation barrier for cis-1K to undergo a 1,5-H atom shift to reform 1 was smaller, indicating that intersystem crossing of the T1 of cis-1K is the rate-determining step in the regeneration of 1.
The potential of pyrimidines to serve as ditopic halogen‐bond acceptors is explored. The halogen‐bonded cocrystals formed from solutions of either 5,5′‐bipyrimidine (C8H6N4) or ...1,2‐bis(pyrimidin‐5‐yl)ethyne (C10H6N4) and 2 molar equivalents of 1,3‐diiodotetrafluorobenzene (C6F4I2) have a 1:1 composition. Each pyrimidine moiety acts as a single halogen‐bond acceptor and the bipyrimidines act as ditopic halogen‐bond acceptors. In contrast, the activated pyrimidines 2‐ and 5‐{4‐(dimethylamino)phenylethynyl}pyrimidine (C14H13N3) are ditopic halogen‐bond acceptors, and 1:1 halogen‐bonded cocrystals are formed from 1:1 mixtures of each of the activated pyrimidines and either 1,2‐ or 1,3‐diiodotetrafluorobenzene. A 1:1 cocrystal was also formed between 2‐{4‐(dimethylamino)phenylethynyl}pyrimidine and 1,4‐diiodotetrafluorobenzene, while a 2:1 cocrystal was formed between 5‐{4‐(dimethylamino)phenylethynyl}pyrimidine and 1,4‐diiodotetrafluorobenzene.
The role of activated pyrimidines and bipyrimidines as ditopic halogen‐bond acceptors is demonstrated.
