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Decoding the Curious Tale of Atypical Intersystem Crossing Dynamics in Regioisomeric Acetylanthracenes Abbey M. Philip, Mahesh Gudem, Ebin Sebastian, and Mahesh Hariharan J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b00766 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 5, 2019
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The Journal of Physical Chemistry
Decoding the Curious Tale of Atypical Intersystem Crossing Dynamics in Regioisomeric Acetylanthracenes Abbey M. Philip‡, Mahesh Gudem‡, Ebin Sebastian and Mahesh Hariharan* School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram (IISER-TVM) Maruthamala P. O., Vithura Thiruvananthapuram, Kerala, 695551 (India) E-mail:
[email protected] ABSTRACT: Mapping the primary photochemical dynamics and transient intermediates in functional chromophores is vital for crafting archetypal light-harvesting materials. Though the excited state dynamics in 9-acetylanthracene is well explored, the origin of near-quantitative triplet population and the atypical intersystem crossing (ISC) rate as compared to the regioisomeric analogs (1/2-acetylanthracene) have rarely been scrutinized. We present a comprehensive account of the photoinduced dynamics in three regioisomeric monoacetylanthracenes using ultrafast transient absorption and quantum chemical calculations. The conjoint experimental and computational investigations suggest that (i) greater stabilization of the 1nπ* relative to 1ππ* state; (ii) dissimilar 1ππ*→1nπ* crossover barriers and (iii) the strong spin-orbit coupling (ν𝑆𝑆𝑆𝑆 ) of 1nπ* state with the receiver 3ππ* state command the divergent triplet population in 1-/2-/9-acetylanthracenes. A tacit understanding of the subtle structural alteration facilitated contrasting ISC dynamics in carbonylated arenes can act as a stepping stone for the evolution of potent photofunctional materials.
1.
INTRODUCTION
Unravelling the photodriven relaxation dynamics1-5 of disparate electronic states is vital for the evolution of next generation molecular materials.6-7 Amid the numerous photoinduced transitions,8 scrutiny of intersystem crossing (ISC) dynamics9-11 at singlet-triplet (S-T) interfaces demands attention due to its significance in solar light harvesting.12-14 Consequently, attaining mechanistic insights into the spin-orbit coupling (SOC)15 mediated triplet population becomes perennially important to perceive ISC in innately triplet deficient aromatics.16-17 El-Sayed rationalized the significant role of symmetry and the electronic configuration of the interacting molecular orbitals (1,3ππ* and 3,1 nπ*) in defining the strengths of SOC.18-19 Accordingly, a strong (weak) SOC and a fast (slow) ISC is expected among the interacting 1nπ* (1ππ*) and 3ππ* manifolds.18-19 ISC dynamics in nitro-,20-22 thio-,23 carbonyl-,24-25 heavy metals26-27/atoms17 appended aromatic chromophores is well documented earlier. Among the carbonylarenes,24, 28-30 the photodynamics in carbonylanthracenes is profoundly explored to probe the (de)population dynamics of the singlet and triplet states.31-33 Concomitantly, solvent polarity34-36/temperature37-38/pressure39 dependent emission, time-resolved absorption,31-32, 40 and photodimerization41 experiments in solution/supersonic jets have been carried out with carbonylanthracenes. Fascinatingly, the photodynamics in 9-carbonylanthracenes is unique owing to the unique torsional dynamics,38, 42 ultrafast ISC and the prevalent triplet population.31-33, 37, 39-40, 43 Though ISC dynamics in carbonylanthracenes is well-explored, the influence of the substituent position on the triplet population have hitherto been overlooked.44-45 In this article, we report the origin of the atypical photoinduced relaxation dynamics and the access to the triplet excited state in regioisomeric 1-/2-/9-acetylanthracenes (1A, 1B and
1C, respectively, Scheme 1). Investigation of the ultrafast dynamics via transient absorption measurements reveal that the derivative 1C undergoes an ultrafast ISC (τ𝐼𝐼𝐼𝐼𝐼𝐼 ≈ 290 fs) with a near-unity triplet yield, whereas 1A and 1B display relatively low triplet yields and a few orders of magnitude slow ISC. Interpretation of the observed photodynamics via quantum chemical calculation suggests that the (i) contrasting stabilization of the 1nπ* state and crossover barriers for 1ππ*→1nπ* transition and (ii) the strong SOC (ν𝑆𝑆𝑆𝑆 = 19.89 cm-1) among a)
O
θ = 27.50°
b)
1A O
θ = 6.40°
1B c)
O
θ = 88.05°
1C
Scheme 1. Chemical and X-ray molecular structures (along with torsions along the carbonyl group) for a) 1A; b) 1B; and c) 1C.
the stabilized 1nπ* and 3ππ* manifolds afford the remarkably unique triplet yields and ISC dynamics in 1A, 1B and 1C.
2. METHODS Photophysical Measurements. Absorption spectra were recorded on Shimadzu UV-3600 UV-VIS-NIR, while the fluorescence and excitation spectra were recorded on a Horiba Jobin Yvon Fluorolog 3 spectrometers, respectively. All spectroscopic experiments were performed by using standard quartz
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cuvettes of path length 1 cm for solution in dried and distilled solvents. The fluorescence quantum yields in solution state were determined using optically matched solutions. Quinine sulfate dissolved in 0.1 M H 2 SO 4 (λ𝑒𝑒𝑒𝑒 = 350 nm, reported quantum yield (ϕ𝑓𝑓 ) = 0.577)46 was used as the standard for 1-3 derivatives. Picosecond time-resolved fluorescence lifetime measurements were carried out in Horiba Jobin Yvon-IBH time-correlated single photon counting system. Solutions were excited at 377 nm using a pulsed diode laser (NanoLED-375L;
