Conformation of the Ester Group Governs the Photophysics of Highly Polarized Benzo[g]coumarins

Photosensitizers that display “unusual” emission from upper electronically excited states offer possibilities for initiating higher-energy processes than what the governing Kasha’s rule postulates. Achieving conditions for dual fluorescence from multiple states of the same species requires molecular design and conditions that favorably tune the excited-state dynamics. Herein, we switch the position of the electron-donating NMe2 group around the core of benzo[g]coumarins (BgCoum) and tune the electronic coupling and the charge-transfer character of the fluorescent excited states. For solvents with intermediate polarity, three of the four regioisomers exhibit fluorescence from two different excited states with bands that are well separated in the visible and the near-infrared spectral regions. Computational analysis, employing ab initio methods, reveals that the orientation of an ester on the pyrone ring produces two conformers responsible for the observed dual fluorescence. Studies with solid solvating media, which restricts the conformational degrees of freedom, concur with the computational findings. These results demonstrate how “seemingly inconsequential” auxiliary substituents, such as the esters on the pyrone coumarin rings, can have profound effects leading to “anti-Kasha” photophysical behavior important for molecular photonics, materials engineering, and solar-energy science.


■ INTRODUCTION
For decades, Kasha's rule, along with its subsequent Vavilov's rule, have served as fundamental guidelines in molecular photonics. 1 They are corollary of the inherently fast internal conversion (IC) between electronically excited states making the deactivation of the lowest excited states with the same multiplicity the rate-limiting step. 2 This photophysical behavior prevents harvesting the full scale of optical excitation energy and accessing reaction pathways originating from upper excited states. 3−5 Therefore, systems that do not follow Kasha's rule and show, for example, two fluorescence bands from excited state with different energies, i.e., dual fluorescence, present important paradigms for areas such as solar-energy science, optical imaging, and biosensing. 6−9 Azulene is the "poster child" of chromophores that violate Kasha's rule. 10 The rate of radiative deactivation of its S 2 state is comparable to that of S 2 → S 1 IC, resulting in strong S 2 → S 0 fluorescence. 11,12 Similarly, other chromophores, such as ketocyanine dyes and sub-porphyrins can exhibit fluorescence from their S 2 and S 3 states. 13,14 Modulating the rates of excited-state charge transfer (CT), intersystem crossing (ISC) and proton-transfer (PT)-mediated tautomerization can also produce two emissive states with different energies that undergo radiative decay with equal efficacy. Chromophores with emissive (CT) and locally excited (LE) states display dual fluorescence when the rates of CT are comparable to the rates of deactivation of the LE state. 15,16 In solids, ISC from upper states can provide a means for populating states with different multiplicity and attaining dual emission encompassing fluorescence and phosphorescence bands. 17,18 Excited-state PT producing multiple tautomers can similarly lead to dual fluorescence from the populations with different energy gaps between their emissive and ground states. 19 Tautomerization of free-base corroles allows picosecond access to higher energies than that of their lowest excited states. Overall, excited-state isomerization that populates two local minima of emissive structures with different energies above the Franck−Condon (FC) ground states provides a plausible means for attaining dualfluorescence behavior. 20 In the context of excited-state CT and conformational dynamics that may lead to dual fluorescence, we turn our attention to aminocoumarins. Developed as strongly fluorescent and photostable laser dyes, they have become popular photoprobes for biomedical imaging. 21,22 The amine and the electron-deficient pyrone ring form a donor−acceptor pair efficiently mediating ICT in the excited state and producing broad CT fluorescence with a huge Stokes' shift. 23−25 Although studied for more than a century, coumarins and πexpanded coumarins are well-known to produce emission from their low-lying CT states with no traces of dual fluorescence from upper states.
Coumarin was first isolated in 1820 from the beans of Dipteryx odorata, which gave its name. 26 The first π-expanded coumarin was synthesized by von Pechmann in 1887. 27 Nevertheless, this field has only catapulted to prominence over the last decade. In this short time period, multiple classes of synthesized and characterized π-expanded coumarins, including conjoined and helical coumarins, have emerged. 28−38 The fluorescence of 7-aminocoumarins is one of the most intensively studied and puzzling topics in the field of photophysics of organic chromophores. 39 Important early contributions by Jones 40−42 revealed that substituents on and in the proximity of the amino-nitrogen atom strongly affect their emission characteristics. These reports were followed by breakthrough analysis by numerous authors 43−46 with critical contributions coming from Rettig 47 and Cole. 48 Recently two structural motifs that retain reasonably high fluorescence quantum yields of 7-aminocoumarins in very polar solvents emerged. 49−51 At the same time the work by Cole, Maciejewski, and others revealed that shifting the position of the amino group to position 6 has a profound effect on the photophysics, typically enlarging the Stokes' shift and decreasing the fluorescence quantum yield. 25,52−55 The first example of benzocoumarin possessing NMe 2 group was published by Cho and co-workers in 2007. 56 This benzo[h]coumarin ( Figure 1) displayed remarkably strong fluorescence in water and moderate bathochromic shift of emission. In the breakthrough discovery, Ahn was the first to recognize that linear expansion of coumarin into benzo[g]coumarin while maintaining the presence of the dialkylamino group ( Figure 1) leads to superb photophysical properties (i.e., large fluorescence quantum yields, reasonable Stokes' shift, and so forth) accompanied by a redshift of the emission band to ca. 600 nm. 57 Indeed, the methyl ester of 8-dimethylaminobenzo-[g]coumarin has very promising properties from the point of view of bioimaging, featuring strong emission even in polar solvents. 58 The subsequent systematic study reveals that further modification toward benzo[f ]coumarin series has a detrimental effect on emission intensity. 59 The 8dialkylaminobenzo[g]coumarin motive has been subsequently explored by Ahn and co-workers in numerous contributions addressing, e.g., autofluorescence in deep tissue imaging 58,60 and discrepancy of fluorescence properties in solutions and in cells. 61 The discovery of 8-dimethylamino-benzo[g]coumarin led to numerous papers exploring this dye as next generation fluorescent probes. 61−64 Considering this wealth of knowledge, the question at hand is, can altering the position of the amine in π-expanded coumarins tune its electronic coupling with the aromatic core in order to attain dual fluorescence from different electronic states?
Herein, we report a discovery of dual fluorescence from benzo[g]coumarin (BgCoum) derivatives ( Figure 1). While methyl 8-dimethylaminobenzo[g]coumarin-3-carboxylate (8-BgCoum) shows a single broad fluorescence band exhibiting pronounced solvatochromism and a large Stokes' shift consistent with radiative decay of a CT state, 65 moving the amine substituent to positions 6, 7, and 9 induces the emergence of short-wavelength emission for solvents with intermediate polarity. As important as the amine position is, to our surprise, it is the rotation of an ester substituent on the pyrone ring that defines the emergence of the dual fluorescence, as ab initio computational studies reveal. This unexpected paradigm of photoinduced dynamics makes πexpanded coumarins promising candidates for exploring initiation of intermolecular processes from upper excited states.

Molecular Design and Synthesis
Introducing a dimethylamino group (NMe 2 ) at positions 6, 7, 8, and 9 of a benzo[g]coumarin scaffold allows examining the effects of the position of strong electron-donating substituents on the photophysics of this class of π-expanded dyes. Such alterations of the position of NMe 2 varies its electronic coupling with the coumarin core and, especially, with the electron-deficient pyrone ring modified with an ethoxycarbonyl group (CO 2 Et). As an electron withdrawing group with Hammett constants of 0.37 and 0.45, CO 2 Et enhances the propensity of the pyrone ring to act as an electron acceptor. 66 Nevertheless, while the Swain−Lupton (SL) field constant of CO 2 Et is 0.34, consistent with its electron-withdrawing property, the SW resonance constant of ethoxycarbonyl is only 0.11, 67 suggesting relatively weak mesomeric effects and perturbation of the coumarin electronic transitions involving frontier orbitals with a π-character. In addition to the 8-BgCoum derivative that we have already reported, 65 we synthesize the new 6, 7, and 9 regioisomeric benzo[g]coumarins in four steps using a method analogous to that described in the literature. 58 Monosubstitution of 2,6-dihydroxynaphthalene with dimethylamine under Bucherer reaction conditions yields compound 2a (Scheme 1). For coumarins 6-BgCoum and 9-BgCoum, we use the commercially available 5-amino-2-hydroxynaphthalene and 8-amino-2-hydroxynaphthalene as substrates (Scheme 1). Employing chloromethyl methyl ether (MOM-Cl) allows protecting the hydroxyl group to afford compounds 2b and 2c. Using the same method to protect the hydroxyl group of 2a, it is converted into the methoxymethyl ether 3a. Methylation of the amino group using methyl p-toluenesulfonate leads to compounds 3b and 3c with an overall efficiency of 50%. The use of other methylation reagents gives significantly lower yields.
Ortho-formylation relative to the protected hydroxyl group of the dimethylaminonaphthalene derivatives leads to the next key intermediates 4a−c. Specifically, introducing methoxymethyl to the hydroxyl group allowed for the Snieckus-type lithiation of 3 (Scheme 2). Treating these lithioorganic derivatives with N,N-dimethylformamide (DMF) as a formylation agent, and afterward with an aqueous solution of HCl, yields the aldehydes 4a−c directly, without the need to separately deprotect the hydroxyl group. The Knoevenagel condensation of diethyl malonate with 4 results in the formation of compounds 6-BgCoum, 7-BgCoum, and 9-BgCoum in yields of 59, 31, and 41%, respectively (Scheme 2).

Photophysical Behavior: Unusual Dual Fluorescence
All four BgCoum NMe 2 derivatives show a broad fluorescence band that manifests solvatochromic behavior ( Figure 2). In addition to the bathochromic shifts of the emission, an increase in solvent polarity induces a decrease in the fluorescence quantum yields (Φ f ) of the BgCoum compounds ( Table 1). The origin of the observed broad fluorescence band, therefore, is consistent with radiative deactivation of an excited state with a pronounced CT character, i.e., S 1 (CT) → S 0 . That is, an increase in solvent polarity stabilizes the emissive CT states of the aminocoumarins, bringing them closer to the conical intersection with the S 0 state and drastically increasing the non-radiative-decay rate constants, k nr ( Table 1).
The absorption spectrum of 8-BgCoum shows a largeamplitude band around 420−450 nm, consistent with a S 0 → S 1 (CT) transition, along with a weak ultraviolet (UV) signal originating from excitation to upper singlet state (Figure 2c). The solvatochromic response of the S 0 → S 1 (CT) absorption, however, is not as pronounced as that of the S 1 (CT) → S 0 emission, and the Stokes' shift (Δν) increases with an increase in solvent polarity (Table 1). Enhancing the charge separation (CS), therefore, appears to follow the initial photoexcitation to the Franck−Condon (FC) CT state.
The UV transitions dominate the absorption spectra of 6-BgCoum, 7-BgCoum, and 9-BgCoum, and they do not show as strong CT bands in the 400 nm region as 8-BgCoum ( Figure 2). The overlap between the natural transition orbitals (NTOs) for the S 0 → S 1 (CT) excitation is, thus, smaller in 6-BgCoum than in 8-BgCoum, and it appears especially minute in 7-BgCoum and 9-BgCoum.
Most importantly, ultraviolet (UV) excitation of 6-BgCoum, 7-BgCoum, and 9-BgCoum in solvent with intermediate  Figure 2). In the emission spectra of the weakly fluorescent 7-BgCoum and 9-BgCoum, the amplitude of this newly emerged short-wavelength band is only about two-to-four times smaller than that of the broad CT band. Conversely, the 400 nm emission of 6-BgCoum in THF is barely visible on the background of the strong CT fluorescence. For 8-BgCoum, which generally exhibits the largest Φ f among these four dyes, we do not observe dual fluorescence. It is noteworthy to emphasize that in polar solvents (MeOH, DMF, and DMSO), emission of 7-BgCoum and 9-BgCoum is below the detection limit. Fluorescence intensity of 6-BgCoum in DMF and DMSO is low but on the same level as that in ACN. On the other hand, 8-BgCoum in accordance to previous studies 57, 58 is strongly emissive across the solvents polarity scale (Table  S1). The photophysical properties of these novel benzo[g]coumarins ought to be compared with known benzo[f ]coumarin and benzo[h]coumarin possessing NMe 2 group ( Figure 1). 56,59 In analogy to methyl 9-(dimethylamino)benzo[f ]coumarin-2-carboxylate, 6-BgCoum has negligible emission in DMSO and in DMF, whereas its λ abs max = 360/ 427 nm (Tables 1, and S1). In the case of 7-BgCoum and 9-BgCoum, fluorescence in these solvents is below any detection limit. Also, methyl 8-(dimethylamino)-benzo[h]coumarin-3carboxylate has similar absorption and emission data. The dual emission has not, however, been observed for these previously reported, π-expanded coumarins. Moreover, their emission in low-polarity solvents is less bathochromically shifted in comparison to that of 6-BgCoum, 7-BgCoum, and 9-BgCoum.
Overall, these π-expanded aminocoumarin fall into two subgroups based on their photophysical behavior: (1) type i dyes, which include 6-and 8-BgCoum, that show strong CT emission and high-amplitude CT-absorption bands with 6-BgCoum n-hexane 50

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pubs.acs.org/jacsau Article relatively week or completely undetected double fluorescence; and (2) type ii dyes, which include the weakly fluorescent 7and 9-BgCoum, that show dual fluorescence with substantial contributions from the short-wavelength bands and CT absorption with quite small extinction coefficients ( Figure 2). The photostability of the four benzo[g]coumarins was studied in toluene using a xenon light source ( Figure S5). All investigated dyes displayed superb stability as demonstrated by comparing with coumarin 153. In particular, 9-BgCoum displayed the highest photostability. Even prolonged exposure to a light source did not cause any noticeable changes in the absorption spectrum of 9-BgCoum.

Electrochemical Properties and Redox Energy Gaps
Cyclic-voltammetry (CV) analysis provides estimates of the reduction potentials of the dyes and their oxidized forms, i.e., about 100 mV more negative than those of the other three dyes. Similarly, the type ii 7-BgCoum appears to be the best electron donor with the smallest Table S3). At position 8, thus, the electron-donating amine has the strongest effect on the lowest unoccupied molecular orbital (LUMO), and at position 7�on the highest occupied molecular orbitals (HOMO).
While it should be used with caution, Koopmans' theorem correlates with the LUMO and HOMO energy levels, respectively, of the dyes. 69 The redox, or electrochemical, energy gaps, , of the four π-expanded aminocoumarins range between 2.2 and 2.5 eV, corresponding to about 500 to 560 nm, which is between the CT absorption and emission bands (see Figure S9 in the Supporting Information). That is, ε EC of the four aminocoumarins is similar to their zero-to-zero energy, ε 00 , corresponding to the optical HOMO−LUMO gaps. Therefore, the S 0 → S 1 (CT) and S 1 (CT) → S 0 radiative transitions involve predominantly the HOMOs and the LUMOs of these dyes. Since ε 00 is not much smaller than ε EC , the electron−hole electrostatic interactions in the emissive CT do not provide significant stabilization, which concurs with a large extent of CS.

Computational Analysis: How Important is the Ester Group?
To elucidate the origin of the dual fluorescence and the nature of the excited states responsible for the short-wavelength emission bands, we resort to ab initio calculations of the four aminocoumarins, implemented at the MP2/ADC(2)/cc-pVDZ level of theory using the Turbomole 7.3 software package. 70−75 It is advantageous for the computational studies to truncate the alkyl chain of the ester substituent to a methyl. In addition to the gas-phase studies, employing the conductor-like screening model (COSMO) allows us to introduce DCM as a solvating medium. 76 Ground-state optimizations in the gas phase leads to the emergence of two distinct conformers of the BgCoum structures with different orientations of the esters attached to their pyrone rings in position 3 ( Figure 1): (1) syn, with the C�O carbonyl bonds of the ester and the lactone pointing in the same direction, and (2) anti, with the ester and lactone C�O bonds oriented in the opposite directions (Figure 3). Electronic conjugation between the ester and the aromatic ring appears to warrant a preference for planar conformations.
The syn conformers are overall more stable than the anti ones, precluding the latter from always providing detectable contributions to the ground-state optical absorption spectra. In the anti structures, the ester methyl crowds the carbonyl oxygen of the lactone. Since the methyl hydrogens are not acidic enough, this spatial proximity should not lead to hydrogen bonding but rather to steric hindrance that destabilizes the anti geometries. In the gas phase, the ground-state energy levels of the anti conformers are higher than those of the syn ones (Figure 4a,c,e,g). Introducing DCM as a solvating medium decreases this energy difference ( Figure  4), which still leads to negligibly small equilibrium population of anti-6-BgCoum and anti-7-BgCoum in DCM, amounting to only about 0.003% at room temperature (Figure 4b,d). Conversely, the S 0 populations of the syn and anti conformers of 8-BgCoum in DCM are practically equal (Figure 4f), and that of anti-9-BgCoum in DCM amounts to 12% (Figure 4h). These trends suggest that for 6-BgCoum and 7-BgCoum, showing dual fluorescence, the excited-state dynamics appears to originate from photoexcitation of only their anti conformers. 77 Nevertheless, 7-BgCoum in DCM displays a shortwavelength fluorescence band (Figure 2b) that ought to originate from its syn conformer. A possible transition from the S 2 state of the syn conformer to the S 1 state of the anti one can account for the observation. Such estimates of conformer populations extracted from calculated energies of their groundstate structures, however, warrant caution because the inherent error of these computational methods can exceed 0.2 eV. This uncertainty offers an alternative explanation, i.e., the energy differences between the anti syn conformers of 7-BgCoum can very well be significantly smaller than 10k B T as the computational results show.
Implementing vertical excitations on 8-BgCoum structures in the gas phase and DCM reveals allowed S 0 → S 1 (FC) transitions with oscillator strengths, f, exceeding 0.5 and similar energies for the two conformers (Figure 4e,f), which is consistent with the large single absorption band observed between 400 and 500 nm (Figure 2c). The relaxation of the S 1 (FC) state of the low-energy syn conformer leads to breaking of the C−O bond of the lactone and opening of the pyrone ring (Figure 4e,f). Coumarins are prone to such a ring-opening photoreaction, which provides a pathway for non-radiative decay competing with the radiative deactivation. The pyrone

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pubs.acs.org/jacsau Article ring in unsubstituted coumarin is the most susceptible to this photoreaction according to calculations and spectroscopic evidence. 78 Introducing DCM solvation medium does not prevent the ring opening of the excited-state syn-8-BgCoum. Nevertheless, the solvation substantially stabilizes the relaxed anti-8-BgCoum S 1 structure which, with an electric dipole moment μ of 32 D, has a substantial CT character. This polarityinduced drop of the energy of the S 1 (CT) state of anti-8-BgCoum below that of syn-8-BgCoum S 1 state opens pathways for preventing the ring-opening reaction by syn-toanti rotation of the ester group in the S 1 manifold.
These computational finding are in an excellent agreement with the spectroscopic results. For n-hexane, offering solvating conditions close to those of the "non-polar" gas phase, the relatively small Φ f of 8-BgCoum (Φ f ≈ 0.1, Table 1) is consistent with an efficient ring-opening pathway responsible for the fluorescence quenching. Even a slight increase in solvent polarity, decreases k nr by 3 orders of magnitude resulting in quantitative Φ f (Table 1). A further increase in medium polarity lowers the energy of the highly polarized S 1 (CT) state resulting in the observed bathochromic shifts in the fluorescence of 8-BgCoum. This polarity-induced drop of the S 1 (CT) energy brings it close to that of the ground state and opens efficient non-radiative IC pathways via back CT, which is consistent with the observed decrease in Φ f for polar solvents. In order to evaluate if the replacement of methyl group with other substituents can alter ring-opening pathway responsible for fluorescence quenching, we computed analogs of 8-BgCoum possessing alternative ester groups (see Supporting Information). It turned out the same trend is also predicted for ethyl or benzyl esters for which only anti-8-BgCoum are expected to be emissive.
Computationally, the other type i dye, 6-BgCoum, shows similar behavior to that of 8-BgCoum. For the syn-6-BgCoum, the oscillator strength of the S 0 → S 1 transition is about twoto-four times smaller than that for the S 0 → S 2 one ( Figure  4a,b). This finding is consistent with its absorption spectra of 6-BgCoum where the UV features dominate, showing larger amplitudes than the band at around 400 nm that extends into the visible region (Figure 2a). The transitions from the ground to the S 1 (FC) states for both conformers lead to doubling their dipoles, which further increase upon relaxation to the emissive S 1 (CT) structures. Upon relaxation of their S 1 (FC) states, however, neither of the 6-BgCoum conformers undergoes ring opening. Instead, the relaxed S 1 (CT) states of 6-BgCoum are prone to radiative deactivation to S 0 with f > 0.1 and similar transition energies for the syn and anti conformers. The two type ii dyes, 7-BgCoum and 9-BgCoum, show quite weak S 0 → S 1 (FC) vertical transitions with f between 0.01 and 0.05 (Figure 4c,d,g,h), which is consistent with the immensely small amplitudes observed for the 400 nm bands in their absorption spectra (Figure 2). That is, S 0 → S 2 (FC) and S 0 → S 3 (FC) transitions dominate the absorption spectra of 7-BgCoum and 9-BgCoum (Figures 2a,d and 4c,d,g,h). In the gas phase, each of these two dyes show only a single radiative transition, corresponding to a single fluorescence band for nonpolar solvents: (1) from the S 1 (CT) → S 0 (FC) of syn-7-BgCoum, because the S 1 (CT) state of anti-7-BgCoum is energetically unaccusable; and (2) from the S 1 (CT) → S 0 (FC) of anti-9-BgCoum, because the S 1 state of syn-9-BgCoum undergoes ring opening (Figure 4c,g).
Introducing DCM solvation to 7-BgCoum and 9-BgCoum stabilizes their S 1 (CT) states and makes the energies of the S 1 (CT) → S 0 (FC) transitions substantially different for the syn and anti structures. The energy gap between the S 1 (CT) and S 0 (FC) states of the syn-7-BgCoum is smaller than that of the anti-BgCoum (Figure 4d). Although the ground-state population of the anti-7-BgCoum appear to be negligibly small, excited-state syn−anti transformation allows access to the anti S 1 (CT) state from the syn manifold. For the 9-BgCoum, on the other hand, the S 1 (CT) → S 0 (FC) transition energy of the anti conformer is smaller than that of the syn. These computational results for the type ii dyes in DCM agree well with the observed double fluorescence that they produce.
Overall, the type ii dyes, 7-BgCoum and 9-BgCoum, are weakly fluorescent, as apparent from the small Φ f and k f values obtained for them from steady-state and time-resolved spectroscopy studies (Table 1). Consistently, the calculated oscillator strengths for the radiative−deactivation transitions of 7-BgCoum and 9-BgCoum (with and without solvation) are relatively small, i.e., the values of f range between 0.03 and 0.07 (Figure 4c,d,g,h). Indeed, the rate of radiative deactivation of the higher energy conformer has to be comparable to the rate of transitions between the syn and anti structures for the double fluorescence to be detectable.
Examining the NTOs for the S 1 (CT) → S 0 (FC) transitions reveals that for the type i dyes the electron and the hole orbitals extend over the NMe 2 substituent. For the type ii dyes, however, only the hole has distribution over NMe 2 and the electron does not ( Figures 5, and S14). Hence, enhancing the CS between the amine and the coumarin rings for the type ii BgCoum derivatives decreases f, k f , and Φ f .
Overall, the position of the amine, changing its electronic coupling with the coumarin rings, governs the fluorescence efficiency of the π-expanded aminocoumarins. Conversely, the conformational degrees of freedom of the ester substituent on the pyrone ring govern the formation of multiple populations of emissive CT excited states responsible for the observed dual fluorescence.
Even though the quantum yields of the short-wavelength fluorescence, Φ f SW , are small (Table 1), the detectable emission from the upper excited states implies that they live long enough to drive useful processes such as charge transfer and energy transfer. The estimated radiative-decay rate constants, k r , hardly exceeds 10 7 s −1 for the regioisomers that manifest dual emission. The oscillator strengths for the S 1 → S 0 transitions are relatively small and similar for the syn-and anticonformers of each compound (Table 1, Figure 3). These considerations suggest sub-nanosecond non-radiative deactivation of the conformers with higher lying S 1 states, rendering them feasible for driving picosecond reactions efficiently.

Effects of Rigidity of the Solvating Micro Environment
To elucidate which photoinduced transitions depend on largeampliated conformational changes, we examine the effects of JACS Au pubs.acs.org/jacsau Article the rigidity of the solvating media on the dye photophysics. Sucrose octaacetate (SOA) forms transparent glass at room temperature making it a good choice as a solid solvent for optical spectroscopy tests. 15,79 The polarity of SOA is similar to that of PrBu, which induces double fluorescence as a solvent of 6-, 7-, and 9-BgCoum ( Figure 6, Table 1). Transferring these dyes from the liquid PrBu to the solid SOA medium enhances the quantum yields not only of their principal long-wavelength fluorescence, ϕ f (LW) , but also of the short-wavelength emission, ϕ f (SW) . Twisting the dimethylamine substituents away from the plane of the aromatic ring decreases the NTO overlap and the rates of radiative deactivation. Therefore, the SOA-induced enhancement of ϕ f (LW) and ϕ f (SW) can originate from suppressing the NMe 2 dihedral rotation around its bond with the aromatic ring. Suppressing the rotation of the ester substituent in the solid medium presents another cause for the observed increase in ϕ f (SW) , but not in ϕ f (LW) , because transitions between the anti and syn conformers provide a non-radiative pathway for the decay of the upper emissive CT state.
The photophysical characteristics 8-BgCoum, i.e., Φ f , τ, k f , and k nr , are quite similar for SOA and PrBu ( Table 1). The SOA-induced suppression of the NMe 2 twist should enhance k f . The prevention of the ester rotation and the syn-to-anti transition in the solid SOA, on the other hand, should leave the ring opening as the only pathway of deactivation resulting in substantial fluorescence quenching, which is not what the experimental results show. The rigid solvating medium, however, can also affect the efficiency of the ring opening and prevent the breaking of the lactone bond, which opens a pathway of radiative deactivation of the syn conformer responsible for the observed fluorescence of 8-BgCoum in SOA ( Figure S4e).
For 6-BgCoum, on the other hand, SOA enhances ϕ f (SW) by an order of magnitude and ϕ f (LW) by a factor of 5 and decreases k nr of the lower emissive CT state by a factor of 4 ( Table 1). The effects of SOA on the photophysics of 7-BgCoum are comparable to those on 6-BgCoum (Table 1). Therefore, the dihedral rotation of NMe 2 and the ester appear to play a key role in the excited-state dynamics of 6-BgCoum and 7-BgCoum.
The effects of the solid medium are the most pronounced for the weakly fluorescent 9-BgCoum where the enhancement of ϕ f (SW) and ϕ f (LW) exceed an order of magnitude (Table 1). That is, large-amplitude conformational changes contribute significantly to the non-radiative deactivation of 9-BgCoum in liquid solvents.
The observed short-wavelength fluorescence of 7-BgCoum for SOA appears to contradict the computational findings. The energy level of the anti conformer is more than 10k B T above that of the syn one for room temperature (Figure 4d). Therefore, the photophysical phenomena should originate from excitation of syn-7-BgCoum, which dominates the ground-state population. The solid SOA medium precludes any syn to anti conformational transitions within the short lifetimes of the excited states of 7-BgCoum. Indeed, the calculations are not for SOA solvent. Nevertheless, the results for the gas phase and for DCM are quite similar and SOA is less polar than DCM. Hence, it is safe to assume that the results for the S 0 energies in SOA should be similar to those in

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pubs.acs.org/jacsau Article DCM and in the gas phase. Another consideration involves the preparation of the SOA samples that require melting of the medium at about 90°C, which increases k B T, but only by about 6 meV, which would hardly affect the change the populations with energy differences exceeding 0.2 eV. Based on these considerations, recalling that the inherent error of the employed ab initio methodology can exceed 0.2 eV, it is safe to assume that the calculated energy difference between the anti and syn ground states is considerably less than 0.26 eV.

Solid-State Photophysics
In addition to the restrictive nature of solid media, excitonic coupling plays an important role when the dyes are packed together in crystals. In the solid state, each of the four dyes shows a single fluorescence band with Φ f comparable to that for toluene (Table 2, Figure 7). Specifically, 7-BgCoum exhibits the smallest Φ f among the four dyes, and 6-BgCoum�the highest, which is similar to their behavior in liquid solvents ( Table 1).
The relatively large Φ f values that we obtain for the powder samples are consistent with the rigidity of the crystal packing, along with the inherently low polarity of the solid media because of the elimination of the orientational polarization. This polarity decrease in the solid state is consistent with the hypsochromically shifted emission of the powder samples in comparison with their fluorescence in liquid solutions. For example, 6-BgCoum in acetonitrile shows an emission maximum at 755 nm, while in the solid-state, it is at 625 nm (Tables 1 and 2). The same is the case for 7-BgCoum, i.e., λ em (max) = 700 nm for THF, and λ em (max) = 668 nm for solid state.
It is important to keep in mind that the crystal packing does not necessarily represent the conformational-equilibrium preference in solution phase. While the crystals of 9-BgCoum contain its syn conformer reflecting the liquid-phase thermodynamics, the crystals of 6-BgCoum comprise its anti conformer (Figure 8). The ethyl substituent of the anti-6-BgCoum in the crystals, however, points away from the lactone carbonyl oxygen avoiding steric hindrance, which is different from what the computational results show for the gas phase and the DCM medium. Overall, the crystals of each of the compounds contain a single conformer, precluding the possibility for dual fluorescence from these solid-state samples, which concurs with the observed emission spectra. This finding further confirms that the observed dual fluorescence for solvents with intermediate polarity originates from populations of conformers with different S 1 (CT) → S 0 (FC) transition energies.
In addition to the increased rigidity and the decreased polarity of the solvating media in solid state, along with the "conformational purity," the crystalline packing provides conditions for intermolecular excitonic coupling that affects the observed spectral features. As previously reported for the parent compound 7-BgCoum, stacking these molecules in the head-to-tail arrangements produces aggregates with overall C i symmetry. 65 The excitonic-coupling (Davydov) splitting for dimer models of such aggregates is estimated from the S 0 → S 1 and S 0 → S 2 vibrationless bands. For 7-BgCoum and 9-BgCoum, the excitonic coupling is 0.5 and 0.3 eV, respectively, suggesting a larger stabilization of the 1 A g state for these compounds. This state has a vanishing transition dipole moment to the ground state due to symmetry rules. Thus, for these dimers the lowest absorbing/emitting state is the 1 A u one, with low oscillator strength (less than 0.03). For 6-BgCoum, however, the excitonic coupling is estimated to be small, and the emission from S 2 ( 1 A u ) is expected at about 2.0 eV (f ≈ 0.2). This finding may correlate to the intense emission band around 700 nm observed experimentally for this benzo[g]coumarin. In contrast to the type i dyes in crystalline state, therefore, the type ii BgCoum derivatives appear to show strong excitonic coupling.

General Procedure for the Preparation of BgCoums from Aldehydes
Diethyl malonate (245 μL, 1.2 equiv, 1.56 mmol) was added dropwise via a syringe to a vigorously stirred solution of 4 (280 mg, 1.3 mmol) in EtOH (5 mL). To the resulting mixture was added a catalytic amount of piperidine (20 μL) and the reaction was stirred at reflux for 4 h. After cooling to an ambient temperature, the solid was filtrated and washed with cold EtOH to afford the corresponding product as a powder.

Steady State Optical Spectroscopy
Spectroscopic grade solvents were purchased from Sigma-Aldrich and used as obtained. For optical studies, solutions of molecules at low concentrations, about few micromoles per liter, were used to avoid dimerization or reabsorption effects. All absorption and fluorescence spectra were taken at room temperature. Steady-state absorption spectra are recorded in a transmission mode using Shimadzu UV-3600i Plus (Japan) and JASCO V-670 (Tokyo, Japan) spectrophotometers. Fluorescence spectra were recorded with the FS5 (Edinburgh Instruments, Edinburgh, UK), the FluoroLog-3 (Horiba-Jobin-Yvon, Edison, NJ, USA) spectrofluorometers, and the FLS 1000 Edinburgh Instruments (Edinburgh, UK) with integrating sphere and corrected for the spectral response sensitivity of the photodetector. The FluoroLog-3, which is equipped with a pulsed diode laser (λ = 406 nm, 200 ps pulse full width at half maximum, FWHM) and a TBX detector, was also employed for time-correlated single-photon counting (TCSPC) measurements.
Remaining detailed methods are found in the Supporting Information.