Thermally-Activated, Delayed Fluorescence in O,B,O- and N,B,O

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Thermally-Activated, Delayed Fluorescence in O,B,Oand N,B,O-Strapped Boron Dipyrromethene Derivatives Patrycja Stachelek, Abdulrahman A. Alsimaree, Rua B. Alnoman, Anthony Harriman, and Julian G. Knight J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b11131 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on March 2, 2017

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The Journal of Physical Chemistry

Thermally-Activated, Delayed Fluorescence in O,B,O- and N,B,O-Strapped Boron Dipyrromethene Derivatives Patrycja Stachelek,a Abdulrahman A. Alsimaree,b Rua B. Alnoman,†,b Anthony Harriman,a,* and Julian G. Knightb (a) Molecular Photonics Laboratory, School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, United Kingdom and (b) School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, United Kingdom.

ABSTRACT: A small series of boron dipyrromethene (BODIPY) dyes has been synthesized whereby the boron atom is constrained in a 5-membered ring formed from either orthodihydroxypyridine or ortho-aminophenol. In the latter case, the amino group has been converted into the corresponding amide derivative so as to curtail the possibility for light-induced charge transfer from strap to BODIPY. These compounds are weakly emissive in fluid solution but cleavage of the strap, by treatment with a photo-acid generator, restores strong fluorescence. Surprisingly, the same compounds remain weakly fluorescent in a rigid glass at 80K where lightinduced charge transfer is most unlikely. In fluid solution, the fluorescence quantum yield increases with increasing temperature due to a thermally-activated step, but does not correlate with the thermodynamics for intramolecular charge transfer. It is proposed that the strap causes rupture of the potential energy surface for the excited state, creating traps that provide new routes by which the wave-packet can return to the ground state. Access to the trap from the excited state is reversible, leading to the delayed emission. Analysis of the temperature-dependent emission intensities allows estimation of the kinetic parameters associated with entering and leaving the trap.

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INTRODUCTION The main attribute of the popular boron dipyrromethene (BODIPY) family of heterocyclic dyes relates to the strong fluorescence in both solution and solid states.1 Such dyes, often having fluorescence quantum yields approaching unity, have found numerous practical applications as emissive probes, sensors and labels.2-6 A key feature of the generic BODIPY nucleus is that spinorbit coupling is extremely weak7 and, as a direct consequence, intersystem crossing to the triplet manifold is ineffective. At the same time, internal conversion to the ground state does not compete significantly with the corresponding radiative process. As such, the design of BODIPYbased fluorescent probes demands that a radiationless decay route is introduced that responds selectively to the targeted species or environmental change. For example, omitting alkyl substituents from the upper rim of the dipyrrin unit opens a new nonradiative channel8 for certain BODIPY derivatives possessing an aryl group at the pseudo-meso-position.9,10 This latter feature has led to the design of fluorescent rotors able to monitor changes in temperature, pressure and local rheology.11,12 Although many different BODIPY-based rotors have been reported,13,14 there is but poor understanding of the underlying mechanics that relate the emission yield and/or lifetime to frictional forces imposed by the surrounding medium. Indeed, our perception of the mechanism has been challenged by the recent report15 that aryl substitution at the lower rim of the dipyrrin core modulates the rotary behavior. A related matter concerns the photophysical properties of BODIPY derivatives equipped with redox-active subunits strapped across the boron center. This situation is exemplified by certain BODIPY-based chromophores where the conventional B-F bonds are replaced with a 5membered O,B,O ring.16 Here, the compounds tend to be non-emissive until cleavage of one or both of the strapped B-O bonds. Removal of the cyclic appendage (Chart 1) not only eliminates


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the possibility for intramolecular charge transfer but also alleviates any undue internal steric strain imposed by the strap. The net result16 is to switch on fluorescence but the switching process is essentially irreversible. Here, we set out to extend the series of known redoxswitchable BODIPY derivatives by introducing new analogues bearing an N,B,O strap and compare these asymmetrical derivatives with a symmetrical O,B,O strapped compound. An advantage of the N,B,O-strapped dyes is that the redox properties can be varied over a wide range by substitution at the N atom. This site also provides a simple means by which to anchor the dye to a surface or biological entity. With the strap in place, these new compounds are nonfluorescent even when light-induced charge transfer between dye and strap is highly unfavorable on thermodynamic grounds. This situation raises the question as to what is behind the emission quenching! In answering this point, we encounter a new type of thermally-activated, delayed fluorescence.

Chart 1. Cartoon representation of constrained (left-hand side) and unconstrained (right-hand side) BODIPY-based dyes where the redox-active strap eliminates fluorescence (hνF) by virtue of intramolecular charge transfer (CT).


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RESULTS and DISCUSSION Synthesis and compound characterization Replacement of the fluorine atoms in BODIPY dyes by oxygen substituents has been realized by a number of methods.17-20 Although the spiroheterocyclic derivative AMN could be formed in 57% yield simply by treatment of the known21 BODIPY BOD with an excess of 2-aminophenol under basic conditions (5 eq. Cs2CO3, 5 eq. aminophenol, 1,4-dioxane, 95 oC, 7 h), the yield was improved to 64% by employing TMSOTf-induced defluorination of the BODIPY; a method which requires fewer equivalents of the aminophenol and milder conditions.20 This protocol (Scheme 1) was applied subsequently to the synthesis of the related spirocyclic system PYR (57%) using 2,3-dihydroxypyridine. Moreover, DMAP-catalysed acetylation of the nitrogen atom in the aminophenol adduct AMN gave the amide AMD in 63% yield when one molar equivalent of acetyl chloride was employed. The use of three equivalents of the acylating agent led to isolation of the β-acetoxyenamide ADD in 59% yield, presumably via base-induced aldol condensation to form a β-ketoamide followed by acetylation of the derived enolate.22 The structure of each compound was confirmed by single crystal X-ray structure determination (see Supporting Information). Further details are supplied as part of the Supporting Information. Compounds were subjected to additional purification by TLC prior to making the spectroscopic measurements.


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Scheme 1. Synthetic route to the spiroheterocyclic BODIPY derivatives PYR, AMN, AMD and ADD.

Photophysics of the ortho-dihydroxypyridine derivative

It has been reported16 that certain O,B,O-strapped BODIPY-based derivatives formed from

ortho-dihydroxybenzene are non-fluorescent in solution due to intramolecular charge-transfer from the strap to the excited-state of the dye (Chart 1). Fluorescence is recovered when the strap is severed by acid treatment. A further example of this generic platform of dyes is found with the O,B,O-pyridine derivative PYR (Scheme 1). Thus, in 2-methyltetrahydrofuran (MTHF) solution at ambient temperature, PYR exhibits absorption (λMAX = 535 nm) and emission (λFLU = 540 nm) spectral properties comparable to those of the conventional BF2-based BODIPY derivative, BOD,1,23 but with a modest red shift (Figure 1). Even so, there is good agreement between absorption and excitation spectra, indicating that emission does indeed arise from the target compound and not from a trace impurity. The Stokes’ shift (∆SS) is unusually small (Table 1) but in keeping with the other strapped BODIPY dyes to be described later.24 More importantly, the fluorescence quantum yield (ΦF = 0.015 ± 0.002) and excited-singlet state lifetime (τS = 180 ± 20 ps) are reduced drastically compared to those of BOD recorded under identical conditions (Table 1). The time-resolved emission profile indicates the presence of a minor component with a much


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longer lifetime as will be described later. The radiative rate constant derived from the experimental data (kRAD = ΦF/τS = 8 x 107 s-1) falls in perfect agreement with that calculated25 from the Strickler-Berg expression (kSB = 8 x 107 s-1). Cyclic voltammetry carried out in CH2Cl2 solution with tetra-N-butylammonium tetrafluoroborate (0.2 M) as background electrolyte showed the half-wave potential for oneelectron reduction (ERED) of PYR to be -1.34 V vs Ag/Ag+. This is in the range normally found with BODIPY-based dyes.26,27 The cyclic voltammogram displays successive one-electron oxidation processes but the first wave, which occurs at a peak potential of 0.83 V vs Ag/Ag+, is electrochemically irreversible. The second oxidative wave occurs with a half-wave potential (EOX) of 1.10 V vs Ag/Ag+. On the basis of molecular orbital calculations (see Supporting Information), and by reference to other BODIPY derivatives,26,27 the first oxidative step is assigned to the removal of a single electron from the ortho-dihydroxypyridine-derived strap. On this basis, the second oxidative step can be assigned to the one-electron oxidation of the BODIPY nucleus.


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Figure 1. Absorption and fluorescence spectra recorded for PYR in MTHF solution at ambient temperature. The inset shows the corresponding spectra after treatment with the photo-acid. The photographic images show solutions of PYR under near-UV illumination in the absence and presence of photo-acid generator. Making use of the above assignment, the difference (∆ECV = 2.44 eV) between the half-wave potentials for the BODIPY nucleus lies close to the optical bandgap (∆EOP = 2.31 eV), as determined from the crossover point of normalized absorption and emission spectra. In fact, the measured EOX value is expected to be raised to higher potentials because of electrostatic repulsion caused28 by prior oxidation of the strap. A crude calculation for this Coulombic effect, based simply on the approximate center-to-center distance, suggests that the EOX is overstated by ca. 0.2 eV. This brings ∆ECV into good agreement with ∆EOP. More importantly, this information (see Supporting Information) can be used to imply that light-induced charge transfer from the strap to the excited-singlet state of the BODIPY dye is thermodynamically favorable.29,30 Since


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the electrochemistry is irreversible, the driving force (∆G ≈ -0.21 eV) can only be estimated from the peak potential and should be used cautiously. Even so, a convenient explanation for the extremely low fluorescence quantum yield observed for PYR in solution is that light-induced, intramolecular charge transfer competes with radiative deactivation of the excited-singlet state.3134

Table 1. Compilation of photophysical and electrochemical properties recorded for the strapped BODIPY derivatives in solution at room temperature. Property

BOD

PYR

AMN

AMD

ADD

λMAX / nm

495

535

533

533

533

εMAX / M-1 cm-1

93,470

28,160

36,500

28,320

27,655

f (a)

0.50

0.13

0.18

0.12

0.12

λFLU / nm

508

540

540

543

549

ΦF

0.94

0.015

0.003

0.001

0.004

τS / ns

5.6

∆SS / cm-1

510

175

245

345

550

ERED / V vs Ag/Ag+ (b)

-1.33

-1.34

-1.58

-1.55

-1.46

EOX / V vs Ag/Ag+ (c)

NA

0.83(irr)

0.23(irr)

1.29(irr)

1.17(irr)

EOX / V vs Ag/Ag+ (d)

1.11

1.10

0.90

0.80

0.77

∆EHL / eV

2.41

2.19

2.11

2.21

2.20

0.18; 2.1

Thermally-Activated, Delayed Fluorescence in O,B,O- and N,B,O

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