Letter pubs.acs.org/OrgLett
Triplet Energy and π‑Conjugation Effects on Photoisomerization of Chiral N,C-Chelate Organoborons with PAH Substituents Soren K. Mellerup,† Lisa Haf̈ ele,‡ Andreas Lorbach,‡ Xiang Wang,† and Suning Wang*,†,§ †
Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada Department of Chemistry, Universität Konstanz, 78457 Konstanz, Germany § Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry, Beijing Institute of Technology, Beijing 1000081, P. R. China ‡
S Supporting Information *
ABSTRACT: Chiral, PAH substituted N,C-chelate boron compounds are systematically investigated to establish the effect of triplet energy and substitution position on their photoreactivity. They all undergo regioselective photoisomerization, forming new dark isomers with quantum efficiencies reflecting these various factors. New PAH fused 4bH-azaborepins are obtained via thermal isomerization of the dark isomers. These results further implicate a photoactive triplet state in the photoisomerization process and its utility in achieving rare PAH-fused azaborepin-like heterocycles.
C
Scheme 1. Photoreactivity and Thermal Reactivity of Chiral N,C-Chelate Organoboron Compounds
ontrolling molecular function with the use of external stimuli has become an increasingly popular strategy for the development of practically relevant chemical species.1 In particular, photoresponsive materials have garnered much attention owing to their simple, efficient, and readily reversible transformations between two or more perceptibly unique states.2 Among well-developed photochromic systems are those based on the cis−trans isomerization of alkene or azobenzene derivatives3 and the electrocyclic ring-opening/closing reactions of spiropyrans,4 fulgides,5 and diarylethenes,6 which have all been extensively studied to date. With respect to newer photochromic systems,7 we have recently focused our efforts toward improving the previously discovered photoreactivity of 2-phenylpyridine (ppy)-chelated organoboron compounds (B(ppy)(Ar)(Ar′); Ar = Ar′ = Mes), which isomerize upon exposure to UV light giving intensely colored “dark isomers” bearing a borirane ring (a; Scheme 1).8 Thus, far, the role of the chelate backbone has been firmly established with a number of different reactivities observed depending on the chosen chelating unit.9 Conversely, the importance of the aryl groups on boron has only recently been examined with no photoreactivity9a and regioselective photoisomerization10 observed for nonbulky (e.g., Ar = Ar′ = Ph) and chiral derivatives (e.g., Ar′ = Mes, Ar = Ph, (BMP; Scheme 1)), respectively. With respect to the latter, the bulky Mes group is required to initiate photoisomerization; however, the reaction was found to always occur at the less bulky phenyl or phenyl substituted group. Furthermore, dark isomers bearing hydrogen atoms on the borirane ring were found to undergo a second thermally driven process involving H atom transfer to pyridine, yielding unique azaborepin-like structures “b” (Scheme 1). Having established the photoreactivity of BMP and its derivatives, we postulated that similar reactivities could also be © 2017 American Chemical Society
observed in chiral boron compounds where the Ar groups are polycyclic aromatic hydrocarbons (PAH). This would allow us to study the impact of incorporating relatively low-lying π → π* states into the substituent that isomerizes. Furthermore, our earlier work indicated the possible involvement of a photoactive triplet state in the photoisomerization pathway of N,C-chelate boron compounds.11 The high regioselectivity of chiral N,Cchelate organoboron compounds permits us to introduce Ar groups with varying 3π−π* energy levels, enabling a better understanding of the photoactive triplet state and gaining new insights into the various factors/driving forces of the unique Received: June 7, 2017 Published: June 30, 2017 3851
DOI: 10.1021/acs.orglett.7b01724 Org. Lett. 2017, 19, 3851−3854
Letter
Organic Letters
PAH centered π−π* transitions. From the calculated transitions, the PAH π−π* character appears to increase with increasing π-conjugation of the substituent, following the order of naphthyl < pyrenyl < 9,10-diphenylanthryl. Compounds 1 − 7 display broad featureless emission spectra, originating mainly from CT transitions, while 8 displays a structured emission band with high ΦFL (0.75), attributable to the 9,10diphenylanthracene unit.13 Comparing the photophysical properties of 5 and 8 (λem = 477 vs. 418 nm and ΦFL = 0.28 vs 0.75, respectively), it is evident that attaching the anthryl moiety directly to the boron center dramatically alters the electronic structure of the PAH substituent. It is worth noting that unlike some of the parent PAH molecules,15 none of the compounds reported in this study display concentration dependent emission spectra thus ruling out excimer emission. Interestingly, all of the PAH-substituted compounds undergo photoisomerization in a similar manner as described previously for related B(ppy)(Ar)Mes molecules.8−10 Irradiation at 350 nm yields their respective dark isomers (denoted as a and a′ where appropriate) as indicated by UV−vis spectra (Figure 2
transformations involving B(ppy)(Ar)Mes. Based on these considerations, we synthesized eight new B(ppy)(Ar)Mes molecules with representative PAH units as shown in Scheme 1. All of the compounds investigated in this study were found to undergo regioselective isomerization at the PAH substituents with their respective photoisomerization efficiencies and product distributions reflecting the triplet energies, πconjugation, and site reactivity of the PAHs. Compounds 1−8 were prepared in low to moderate yields via a procedure similar to that used for BMP (Scheme 2),10 Scheme 2. Synthesis of B(ppy)(Mes)(PAH)
where the requisite prochiral B(OMe)(Mes)(Ar) reagents were prepared from B(OMe)2(Mes) and appropriate organomagnesium or organolithium derivatives of Ar (see Supporting Information, SI). All compounds were fully characterized by NMR and HRMS. The crystal structures of 3, 5, and 8 were determined by single-crystal X-ray diffraction analysis (see SI). As anticipated, the presence of one bulky mesityl group on the boron atom results in B−CAr/Mes bonds of 1.63−1.64 Å, which was previously shown to be critical for yielding photoactive systems.9a,10 As shown in the UV−vis and fluorescence spectra of 1−8 (Figure 1 and SI), all eight compounds absorb intensely in the
Figure 2. UV−vis spectra of the dark isomers of 1−8 in toluene at 10−4 M. Inset: Photographs showing their colors.
and SI). In all cases, new broad low energy absorption bands emerge between 510−600 nm following UV irradiation. It has been previously established8−10 that the color and absorption bands of the dark species arise due to HOMO → LUMO CT transitions, with the former being comprised of the σ-bonding orbitals on the borirane + π-orbitals of the dearomatized aryl ring and the latter residing on the ppy chelating backbone as π*-antibonding orbitals. Given that the chelating fragment remains constant in 1−8, the different colors observed directly reflect the nature of the PAH units attached to boron. Compared to BMP,10 naphthyl (1 and 2) and anthryl (5) substituents give hypsochromically shifted λabs for their dark isomers indicating a small stabilization of the HOMO due to the extended π-conjugation. The addition of more fused benzene rings (pyrenyl, 3 and 4) results in bathochromically shifted dark isomer λabs, comparable to that of the symmetric parent compound B(ppy)(Mes)2.8 Compounds 6a−8a have λabs which are nearly identical to that of BMPa, revealing that pendent PAHs linked through phenyl groups have little to no impact on the photophysical properties of their dark isomers. All of the pertinent photophysical and photochemical data for 1−8 and their respective dark isomers are listed in Table 1. Monitoring the photoreactions of 1−8 by 1H and 11B NMR confirms the formation of their dark isomers “a” with efficiencies and selectivity depending on the substitution patterns of the PAH at boron. In accordance with previous
Figure 1. UV−vis (solid lines) and normalized fluorescence (dashed lines) spectra of 1−8 in toluene at 10−5 M. Inset: Photographs showing the fluorescence colors of 1−8.
near-UV to visible region and fluoresce blue to green. While the derivatives incorporating naphthyl (e.g., 1, 2, and 6) all possess broad featureless absorption bands similar to previously reported N,C-chelate organoboron compounds,8−10 those bearing pyrenyl (3, 4, and 7) or 9,10-diphenylanthryl (5 and 8) exhibit absorption bands with well resolved vibrational features corresponding to intramolecular π−π* transitions centered on the PAH substituents.12,13 This is most pronounced for 3, as substitution at the 2-position of pyrene exerts the least electronic perturbation on the parent molecule.14 This assignment is consistent with TD-DFT calculation data (see SI), which indicate that the primary absorption bands of 1−8 originate from a combination of charge transfer (CT) transitions from π-PAH/Mes to π*-ppy, typical for this class of molecules,8−10 and the aforementioned 3852
DOI: 10.1021/acs.orglett.7b01724 Org. Lett. 2017, 19, 3851−3854
Letter
Organic Letters Table 1. Photoisomerization Quantum Efficiency of 1−8 and Absorption Data of 1a−8a compd 1a + 1a′ 2a 3a 4a 5a + 5a′ 6a 7a 8a
λabs (nm) (∈, M−1 cm−1)a 511 506 599 592 528 535 541 532
(8.16 (6.17 (8.32 (3.06 (2.77 (7.84 (8.48 (1.63
× × × × × × × ×
3
10 ) 103) 103) 103) 103) 103) 103) 103)
color
ΦPIb
salmon salmon blue black orange pink pink pink
0.59 0.77
