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<p>We aimed to determine whether the nuclear hyperfine effect of the meso protons of 9-deuterioanthracene (A<sub><i>d</i></sub>) influences the efficiency of triplet-triplet annihilation induced anthracene photodimerization by promoting interconversion of encounter spin states. While we answer this question in the affirmative: triplet sensitized A/A<sub><i>d</i></sub> photodimer formation rate ratios sensitized with fluorenone in C<sub>6</sub>D<sub>6</sub>, and with [Ru(dmb)<sub>3</sub>](PF<sub>6</sub>)<sub>2</sub> in CD<sub>3</sub>CN are 1.20 and 1.17, respectively, our reinvestigation of the photodimerization of A<sub><i>d</i></sub> revealed much larger head-to-head (D<sub>HH</sub>) to head-to-tail (D<sub>HT</sub>) dimer ratios than the expected 1:1 ratio. Furthermore, the D<sub>HH</sub>/D<sub>HT</sub> ratio is solvent and temperature dependent, ranging from 1.11 to 1.95 in benzene-<i>d</i><sub>6</sub> and from 1.45 to 3.41 in dichloromethane-<i>d</i><sub>2</sub> as <i>T</i> is raised from 4.6 to 39.4&#xa0;°C. Those are astonishingly large ratios considering that normal secondary kinetic isotope effects for formation of two CH/CD bonds with sp<sup>2</sup> to sp<sup>3</sup> hybridization change are inverse and do not exceed the 1:1.1 ratio relative to the formation of two CH/CH bonds. This deuterium isotope effect does not reflect the rate of bond formation. Rates of anthracene (A) depletion and of A<sub><i>d</i></sub> depletion, irradiated in degassed CD<sub>2</sub>Cl<sub>2</sub> or C<sub>6</sub>D<sub>6</sub> solutions, are the same. We attribute the D<sub>HH</sub>/D<sub>HT</sub> ratios to the concentration ratios of the precursor excimers. Analysis of the kinetics of photodimer formation suggests that the slopes of van't Hoff plots of the D<sub>HH</sub>/D<sub>HT</sub> ratios should be close to the enthalpy differences between HH and HT excimers. Values of Δ<i>H</i> = 4.1 ± 0.1 and 2.88 ± 0.06 favoring the HT dimer are obtained in CD<sub>2</sub>Cl<sub>2</sub> and C<sub>6</sub>D<sub>6</sub>, respectively. We conclude that the enthalpy difference favors the HT dimer, whereas the entropy difference favors the HH dimer. CD/CD affinity in the excimer is also manifested in the photodimerization of 9-bromo-10-deuterioanthracene (9BrA-10<sub><i>d</i></sub>). Whereas, 9-bromoanthracene gives only HT photodimer, 20% of 9BrA<sub>10<i>d</i></sub> photodimer in C<sub>6</sub>H<sub>6</sub> is HH.</p> Graphical abstract <p></p>

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The photodimerization of 9-deuterioanthracene: hyperfine 1H nuclear effect and an unprecedented secondary 2H isotope effect

  • 期刊论文

  • 发表日期：

    2026年3月12日

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  • Jack Saltiel,
  • Sulthana Fehroza Parambilpeediyakkal,
  • Scott Conrad,
  • David E. Townsend,
  • Edwin F. Hilinski

摘要

We aimed to determine whether the nuclear hyperfine effect of the meso protons of 9-deuterioanthracene (Ad) influences the efficiency of triplet-triplet annihilation induced anthracene photodimerization by promoting interconversion of encounter spin states. While we answer this question in the affirmative: triplet sensitized A/Ad photodimer formation rate ratios sensitized with fluorenone in C6D6, and with [Ru(dmb)3](PF6)2 in CD3CN are 1.20 and 1.17, respectively, our reinvestigation of the photodimerization of Ad revealed much larger head-to-head (DHH) to head-to-tail (DHT) dimer ratios than the expected 1:1 ratio. Furthermore, the DHH/DHT ratio is solvent and temperature dependent, ranging from 1.11 to 1.95 in benzene-d6 and from 1.45 to 3.41 in dichloromethane-d2 as T is raised from 4.6 to 39.4 °C. Those are astonishingly large ratios considering that normal secondary kinetic isotope effects for formation of two CH/CD bonds with sp2 to sp3 hybridization change are inverse and do not exceed the 1:1.1 ratio relative to the formation of two CH/CH bonds. This deuterium isotope effect does not reflect the rate of bond formation. Rates of anthracene (A) depletion and of Ad depletion, irradiated in degassed CD2Cl2 or C6D6 solutions, are the same. We attribute the DHH/DHT ratios to the concentration ratios of the precursor excimers. Analysis of the kinetics of photodimer formation suggests that the slopes of van't Hoff plots of the DHH/DHT ratios should be close to the enthalpy differences between HH and HT excimers. Values of ΔH = 4.1 ± 0.1 and 2.88 ± 0.06 favoring the HT dimer are obtained in CD2Cl2 and C6D6, respectively. We conclude that the enthalpy difference favors the HT dimer, whereas the entropy difference favors the HH dimer. CD/CD affinity in the excimer is also manifested in the photodimerization of 9-bromo-10-deuterioanthracene (9BrA-10d). Whereas, 9-bromoanthracene gives only HT photodimer, 20% of 9BrA10d photodimer in C6H6 is HH.

Graphical abstract