Structural Control of Nonadiabatic Photochemical Bond Formation: Photocyclization in Structurally Modified ortho-Terphenyls

Abstract

Understanding how molecular structure impacts the shapes of potential energy surfaces and prospects for nonadiabatic photochemical dynamics is critical for predicting and controlling the chemistry of molecular excited states. Ultrafast transient absorption spectroscopy was used to interrogate photoinduced, nonadiabatic 6π cyclization of a collection of ortho-terphenyls (OTP) modified with alkyl substituents of different sizes and electron-donating/withdrawing character positioned on its central and pendant phenyl rings. OTP alkylated at the 4,4″ and 4',5' positions of the pendant and central rings, respectively, exhibiting biphasic excited-state relaxation; this is qualitatively similar to relaxation of OTP itself, including a fast decrease in excited-state absorption (τ1 = 1-4 ps) followed by formation of metastable cyclized photoproducts (τ2 = 3-47 ps) that share common characteristic spectroscopic features for all substitutions despite variations in chemical nature of the substituents. By contrast, anomalous excited-state dynamics are observed for 3',6'dimethyl-OTP, in which the methyl substituents crowd the pendant rings sterically; time-resolved spectral dynamics and low photochemical reactivity with iodine reveal that methylation proximal to the pendant rings impedes nonadiabatic cyclization. Results from transient measurements and quantum-chemical calculations are used to decipher the nature of excited state relaxation mechanisms in these systems and how they are perturbed by mechanical, electronic, and steric interactions induced by substituents.

Two-photon absorption spectroscopy of stilbene and phenanthrene: Excited-state analysis and comparison with ethylene and toluene

“…The choice of stilbene is motivated by its interesting photochemistry, which includes cis/trans isomerization and electrocyclization pathways that represent the key reaction channels in many photochromic molecular switches. 13,[27][28][29][30][31][32][33][34] The 2PA spectroscopy of stilbene has been a subject of theoretical studies for several decades, 15,35,36 however, only a handful of studies have employed correlated many-body electronic structure methods. [37][38][39] To treat stilbene and larger conjugated systems, many studies employed semi-empirical approaches and timedependent density functional theory (TDDFT).…”

Ultrafast Pump–Repump–Probe Photochemical Hole Burning as a Probe of Excited-State Reaction Pathway Branching

“…Excitation of the lowest energy transition of OTP (250−300 nm) results in the immediate appearance of two transient absorption bands peaking at 375 and 600 nm attributed to S 1 OTP. 33,34 Decay of OTP* is bimodal over the course of picoseconds, 31,32 with DHT formed as a result of excited-state deactivation. DHT exhibits two characteristic absorption features centered at 340 and 580 nm and is short-lived (45 ns) due to thermally activated ring reopening over a low energetic barrier (0.25 eV).…”

Multiphoton Control of 6π Photocyclization via State-Dependent Reactant–Product Correlations

“…OTP is structurally similar to diarylethenes, including cis -stilbene and many well-known photochromic dyes . Previously we investigated the photochemical dynamics that follow one-photon excitation of OTP through its lowest-energy optical transition; − the absorption spectrum of OTP is plotted in Figure b. OTP in low-lying excited electronic states (OTP*) relaxes within a few picoseconds to yield trans -4 a ,4 b -dihydrotriphenylene ( trans -DHT, bottom left of Figure a); spectra of OTP* and trans -DHT are plotted in Figure b.…”

“…Our understanding of OTP photochemistry derives from analysis of ultrafast pump–probe transient absorption (TA) data. − We have previously observed that S 2 OTP* (which is populated on a time scale <100 fs) decays biexponentially with lifetimes of 700 fs and 3.0 ps in tetrahydrofuran (THF) solution. ,, However, we could not determine from TA which of these time scales corresponds with cyclization because the OTP* absorption spectrum completely spans that of S 0 trans -DHT, such that the kinetics of the photoproduct appearance cannot be determined unambiguously through global modeling of TA spectral dynamics . To resolve this issue, pump–repump–probe spectroscopy (PRP) was applied as a pathway-selective probe of dynamics .…”

“…In order to isolate the 1 + 1′ product absorption it is advantageous to investigate photoresponses of an analogous reactant structure that is structurally inhibited from cyclization, thereby minimizing spectral contributions from a repump-induced trans -DHT bleach in SR-PRP signals. We have previously characterized steric and electronic impacts on the rate and efficiency of cyclization in a series of structurally modified OTPs and ortho -arenes. ,− Here we examine dynamics of 3,3″,5,5″-tetra methyl- ortho -terphenyl (TMOTP), drawn in the inset of Figure a; details of its synthesis are presented in the Supporting Information. We selected this structure with the expectation that steric interactions between adjacent methylated rings would inhibit electrocyclization from the lowest excited state but would not perturb the reactant structure so significantly as to change its electronic structure and spectroscopy within the Franck–Condon region of the ground and excited states.…”