Electronic Finetuning of 8‐Methoxy Psoralens by Palladium‐Catalyzed Coupling: Acidochromicity and Solvatochromicity

Abstract Differently 5‐substituted 8‐methoxypsoralens can be synthesized by an efficient synthetic route with various cross‐coupling methodologies, such as Suzuki, Sonogashira and Heck reaction. Compared to previously synthesized psoralens, thereby promising daylight absorbing compounds as potentially active agents against certain skin diseases can be readily accessed. Extensive investigations of all synthesized psoralen derivatives reveal fluorescence in the solid state as well as several distinctly emissive derivatives in solution. Donor‐substituted psoralens exhibit remarkable photophysical properties, such as high fluorescence quantum yields and pronounced emission solvatochromicity and acidochromicity, which were scrutinized by Lippert–Mataga and Stern–Volmer plots. The results indicate that the compounds exceed the limit of visible light, a significant factor for potential applications as an active agent. In addition, (TD)DFT calculations were performed to elucidate the underlying electronic structure and to assign experimentally obtained data.


Introduction
The development of biologically active small molecules has reachedi ncreasing importance for applications in medicine, [1] biology [2] and biochemistry, [3] in particular, in the fields of diagnosis and therapy [4] of certain diseases. As ac onsequencee xploration of novel pharmacophores and structures remains an ongoing major challenge in synthetic chemistry. [5] In particular, photophysical properties might significantly affect the effectivity of some active ingredients. Controlling exciteds tate properties by diversity orienteds ynthetic strategies, such as multicomponent processes, [6] is becoming increasinglyi mportant.
Psoralen( Figure 1) is ap rivileged pharmacophore with photosensitizing character that interacts [7] with human DNA and can be used to treat many different types of skin diseases. [8] The psoralen derivative 8-methoxypsoralen (8-MOP) can be used,f or example, for the treatmento fvitiligo [9] or T-cell lymphoma [10] and promotes the healing of psoriasis by ap hotochemotherapeutic approach. [11] The PUVAm echanism (psoralen + UVAr adiation) assumesa key role in this process. [12] Previous studiess uggest that a double [2+ +2] cycloaddition occurs between the furan and the pyrone moieties of psoralen and the DNA. [13] This crosslinking of the DNA structure induces apoptosis, which prevents the cell from reproducing. Recent studies also indicatet hat a photo-induced electron transfer competes with the cycloaddition reaction. [7a, 14] For further advancing previous investigations and for establishingc oherence between various psoralens, it is necessary to establish efficient routest on ovel electronically tunable psoralens. Most of the previously known synthetic routes of psoralens start from coumarin, umbelliferon or benzofuran (Scheme 1). [15] Here, we reportadiversity-oriented route of 8-MOPd erivatives starting from pyrogallol by applying cross-coupling methodologies to 5-bromo-8-MOP for accessing donor-acceptor substituted systems in which the psoralen core acts as a donor.F urthermore, photophysical properties are studied by absorption and emissions pectroscopy,a sw ell as observed solvatochromism and halochromism is reported.
Specificd eviations from standard conditions had to be implementedf or coupling of the pyridine derivative (Table 1, entry 4). 4-Pyridinylboronica cid (10 d)p ossesses ligand properties that can inhibitr eductive elimination and reduce the amount of the active catalyst in the final step of the Suzuki couplingc ycle. This assumption is confirmed by the fact that an increasei nc atalyst loading led to higher yields. Moreover, in ap articularc ase highery ields were achieved using tri-tertbutylphosphonium tetrafluoroborate as al igand and Pd(dba) 2 as ac atalyst with potassium hydroxide as ab ase (Table 1, entry 2). However,a pplying these conditions to the other boronic acids did not lead to increased yields. By using 4- Table 1. Suzuki synthesis of 5-(hetero)aryl substituted 8-methoxypsoralens 11.

Entry
Boronic acid, R 1 B(OH) 2 10 5-Substituted 8-methoxypsoralen 11 (yield) [ carboxyphenylboronic acid (10 g), it was necessary to switch to completely different conditions (Pd 2 (dba) 3 ,S Phos as al igand, and KF as ab ase) to reach conversion (Table 1, entry 7). Additionally,i tw as possible to corroborate the structure of 5-(hetero)aryl substituted 8-methoxypsoralens 11 by an X-ray crystal structure analysis of compound 11 a (Figure 2). [27] The brownish block-shapedc ompound crystallizes with ac entrosymmetric arrangement in the monoclinic space group P2 1 /c. The cyanophenyl moiety is twisted with the psoralen core by an angle of 57.01(4)8.F urthermore,a nalysiso ft he crystal packing reveals that the psoralen cores are self-oriented in ap lanar fashion.T hereby two furan (3.424 )a nd two pyrone units (3.708 )a re mutually stacked on top of each other.T he plane distance between furan and pyrone (3.208 )i se ven shorter, rationalizing p-stackingo ft he molecules.
Subsequently,v ariouse thynyl and vinyl aryl substrates 12 and 14 werec oupled to the 5-position of 8-methoxypsoralen under Sonogashiraa nd Heck conditions with the same catalyst and ligand system consisting of Pd 2 (dba) 3 and cataCXium PtB to give the corresponding 5-(hetero)aryl alkynyl substituted 8methoxypsoralens 13 and 5-(hetero)aryl vinyl substituted 8methoxypsoralens 15 (Scheme 5a nd 6). For both series four examples with electron-withdrawing groups and one example with ad imethylaminog roup as electron-donating substituents were synthesized (Table 2a nd 3). As previously shown for the Suzuki coupling, double amountso fc atalyst and ligand were used for successfult ransformation of pyridyl derivatives (Table 2a nd 3, entries 4). All obtainedp soralen derivatives 11, 13,a nd 15 were purified by precipitation or column chromatography and then recrystallized in various solvents. Structures and purity were confirmed by 1 H, 13 CNMR, mass spectrometry, high resolution mass spectrometry,H PLC and elemental analysis.
An X-ray crystal structure analysis of alkyne-linked compound 13 e waso btained. [27,28] The yellow acicularc rystalsw ith the monoclinic space group P2 1 /c crystallize planar due to the rigid character of the ethynyl bridge ( Figure 3). The centrosymmetric arrangement, supported by the short interplanar distances between the molecules (< 3.4 ), enables ac lose inter-action of the molecules in the crystalline solid state. These interactions cause ap ronounced p-stacking, which appearst o be relevant for the observed solid state luminescence (vide infra).

Entry
Arylalkyne 12 5-Substituted8 -methoxypsoralen 13 (yield) [ ceptor function depending on the electronic nature of the 5substituent. Based on al ibrary of 5-substituted 8methoxypsoralens systematic studies of the absorption and emission properties were conducted. The relative fluorescence quantum yield F F was determined with Coumarin 30 as as tandard. [28] All 5-acceptor-8-methoxypsoralens 8, 9,a nd 11 exhibit a shoulder as the longest wavelength absorption between 355 and 412 nm, with molara bsorption coefficients, e,b etween 2700 and 6700 Lmol À1 cm À1 (Table 4). The longest wavelength bandso fc yano and nitro psoralens 8 and 9 are more bathochromically shifted than those of aryl-substituted psoralens 11, plausiblyr ationalized by the expected twist between the aryl moiety and the psoralen core, resulting in aw eaker orbital overlap.
All other 8-methoxypsoralens 8, 9,a nd 11 only fluoresce in the solid state, however,o nly very weakly in solution ( Figure 5). Strongly acceptor-(9, 11 b)a nd donor-substituted derivatives (11 e)p ossess redshifted absorptions that correlate to HOMO-LUMOt ransitions as supported by TD-DFT calculations (vide infra and for further details, see Supporting Information).
Ethynyl substituted psoralens 13 differ from compounds 11 by pronounced redshifted maxima of the longest wavelength absorption bands in UV/Vis spectra (Table 5, Figure6). Also the molar extinctionc oefficients are substantially higher (between 8800 and 20 000 Lmol À1 cm À1 ). Compounds 13 b,w ith the strongesta cceptor, and 13 e,w ith the strongestd onor, exhibit the largest bathochromic shift probably due to ac harge transfer state. Interestingly,t he latter is the first psoralen derivative absorbing light in the visible.
[c] Dñ = 1 lmax; abs À 1 lmax; em  The vinyl-substituted psoralen derivatives 15 possess most redshifted longest wavelength absorption bands in these psoralen series (Table 6, Figure 8). Additionally,t he longest wavelength absorption bands of compounds 15 a-d can only be recognized as weak shoulders (Figure 8).
Despite the mostly dissociative nature of the nitro group, compound 15 b unexpectedly fluoresces with as ubstantial fluorescencequantum yield of 13 %(Ta ble6,entry 2). All psoralen compounds 15 particularly fluorescei nt he solid state. As in the series 11 and 13 dimethylamino-substituted psoralens reveal pronounced positive emissions olvatochromicity.M ost remarkably these compounds cover as pectral range from blue emission in cyclohexane to orange-red emission in acetonitrile ( Figure 9). The solvatochromicity of compound 15 e wass tudied in more detail.T herefore, absorption and emission spectra were recorded in solvents of different polarity.T he absorption solvatochromicity in the range from 392 to 413 nm turns out to be weak. In comparison, the emission solvatochromicity with ab athochromic shift of 499 to 684 nm is very strong (for details,s ee Supporting Information).
This peculiar behavior originates from the change of dipole moment of the molecule upon excitation by UV light and the concomitant relaxation of surrounding solventm olecules. [30] Quantitative calculation of this dipole moment changec an be performed with the Lippert-Mataga model. [31] Initially,t he orientation polarizability Df of different solvents is determined according to the followinge quation [Eq. (1)]: Df ¼ e r À 1 2e r þ 1 À n 2 À 1 2n 2 þ 1 ð1Þ e r describes the relative permittivity and n the refractive index of the respective solvent. [30] Subsequently the orientation polarizability Df can be plotted against the Stokes shift Dñ The regression correlates with an excellent goodness of fit (r 2 = 0.98, for further details, see Supporting Information).
The Stokes shift can be described using the Lippert-Mataga equation[ Eq. (2)] by the change of the dipole moment from the ground to the excited state.
The parameters ñ a and ñ f define the absorption and emission maxima( in cm À1 ). e o is the vacuum permittivity constant (8.8542·10 À12 AsV À1 m À1 )a nd h is the Planck's constant (6.6256·10 À34 Js). Furthermore, c describes the speed of light (2.9979·10 10 cms À1 )a nd a the radius of the solventcavity which occupiest he investigatedm olecule. Finally, m E and m G refer to the dipole momenti nt he ground and excited state. The parameter a could be determined by assuming as pherical dipole  [a] Recorded in CH 2 Cl 2, c(15) = 10 À5 m at T = 293 K. [b] Recorded in CH 2 Cl 2 , c(15) = 10 À7 m at T = 293 K, relative quantum yields were determined with Coumarin 30 as as tandard in acetonitrile (F F = 0.67 [28]    using DFT calculations in the optimized ground state. This Onsager radius a is 5.83 (5.83·10 À10 m). With the determined parameters and constants, for compound 13 e ac hange of dipole moment Dm of 13 D( 4.28·10 À29 Cm) results. For the other donor-substitutedp soralens 11 e and 15 e,t he change in the dipole moment Dm values amount to 12 D( 3.88·10 À29 Cm) and 19 D( 6.43·10 À29 Cm), respectively.T he differences in the change of dipole moment indicate the increasei nc harge transfer characterw ith extension of the p-system.
Protonation of chromophores 11 e, 13 e and 15 e in dichloromethane reveals another photophysical effect. The protonation of the compoundss ignificantly changes the absorption and emission behavior (Figure 10). Upon addition of trifluoroacetic acid the solutions' yellowish color disappears with concomitant fluorescenceq uenching. Upon addition of triethylamine this acidochromicity can be reversed and luminescence returns.
Thereby,i tw as also possible to determine the pK a value of the chromophores 11 e, 13 e and 15 e.A ssuming complete dissociationo ft rifluoroacetic acid in dichloromethane the pK a values wered etermined by recording the absorption spectra at different pH values. For compound 13 e ah ypsochromic shift of the absorption maximum at 403 nm to as houldera t 371 nm was monitored ( Figure 11). For 13 e-H + + ap K a value of 2.81 could be determined (for experimental details, see Supporting Information). Likewise the aryl derivative 11 e gives a pK a of 3.05, whereas for the styryl derivative 15 e ap K a of 3.28 can be determined.
In addition, monitoring the fluorescenceq uenching by trifluoroacetic acid the pK a values of the chromophores 11 e, 13 e and 15 e were alternativelyd etermined from the resulting Stern-Volmer [32] plotsr evealing linear correlations of the fluorescencei ntensities F 0 /F with the concentration of the trifluoroacetic acid solution c(TFA) (for details, see Supporting Information). The determined Stern-Volmer constant K sv of compound 13 e is 155.93 Lmol À1 ,c orresponding to ap K a value of 2.15, which is in good agreementw itht he pK a value determined by absorption spectroscopy.T he pK a values by Stern-Volmer plots of compounds 11 e-H + + and 15 e-H + + were also determined to 3.31 and 3.45, respectively,c orresponding very well with the previously determined values by absorption spectroscopy.T he obtained values are typical of para-substituted amines, [33] therefore it can be assumed that the protonation occurs at the dimethylamino nitrogen atom, which is additionally supported by NMR spectra of the unprotonated and protonated species (for further details, see Supporting Information).
The comparison of the three p-dimethylamino phenyl derivatives 11 e, 13 e,a nd 15 e,g iving the highest fluorescence quantum yields in dichloromethane within all three consanguineous series, reveals that the emission maximal ie in av ery narrowm argin between 553 and 557 nm. This accountsf or a very similar electronic structure of the vibrationally relaxed excited state. For the alkynyl derivative 13 e the solid state spectrum was detected at 557 nm, that is, at av ery similar energy. In addition the chromophores 11 e, 13 e,a nd 15 e were embeddedi nP MMA (polymethylmethacrylate) films at 1wt% and their emission maximaa ppear at 500 (11 e), 523 (13 e), and 547 nm (15 e), that is, hypsochromically shiftedi nc omparison to the solution emission maxima. This slight blue shift can be rationalized by the polarity effect of the PMMA matrix (for spectra, see Supporting Information).

Calculated electronic structure
For gaining an insight in the electronic structureo ft hese Tshaped 8-methoxy psoralen chromophores, in which the psoralen moiety and the 5-substituents adopt rectangular orientations, TD-DFTc alculations were performed for the chromophores 11 a, 11 e, 13 a, 13 e, 15 a and 15 e.T he geometry of the electronic ground state structures was optimized using Gaussian 09, [34,35] with the PBE1PBE [35] functional andt he Pople 6-311G(d,p) [36] base set. Since all photophysical measurements were carriedo ut in dichloromethane solutions, the polarizable continuumm odel (PCM) with dichloromethane as as olvent was used. [37] Geometry optimization shows that the torsional angle between the aryl moiety and the psoralen core lies between 55 and 578 for all molecules synthesizedb yS uzuki cou-  pling. This is in good agreement with the torsionala ngles extracted from crystal structure analyses. Molecules synthesized by Sonogashira coupling are essentially coplanar due to the ethynyl bridge. The Heck derivatives possess torsional angles of the styryl substituents between 33 to 348.
Starting from the geometry optimized structures, the lowest energy electronic transitions of chromophores 11 a, 11 e, 13 a, 13 e, 15 a and 15 e were calculated on the TD-DFT level of theory with the Pople 6-21G basis set (Table 7). [38] The comparison considers in each series the cyano-substituted (acceptor) and the dimethylamino-substituted (donor) derivatives. The calculations confirm that the experimentally assessed longest wavelength absorption bands (maximaa nd shoulders)c an be clearly assignedt oH OMO-LUMO transitions.
8-MOP as an electronic amphiphile can adopte ither donor or acceptorf unctionality depending on the remote substituent's electronic nature.T his can be clearly visualizedb yt he molecules' FMOs, reflecting the Franck-Condon transition of the longest wavelength absorption band.
The calculated frontierm olecule orbitals (FMO) indicatet hat the coefficient densities in HOMOsp redominantly reside on the 5-substituents. The LUMOs, however,p redominantly localize coefficient density on the psoralen units ( Figure 12). The dominance of the HOMO-LUMO transitions clearly rationalize the charge transfer charactero ft hese dominant lowe nergy absorption bands, as well as the pronouncede mission solvatochromicity.I na ddition the T-shape of two constituting subchromophores, biaryl,t olane,a nd stilbene, andp soralen enables the design of rectangular excited state coupled chromophores with considerable alteration of dipole moment orientation. Furthermore the tunability of absorption and emission characteristics makest hese novel chromophores interesting candidates for photo-induced DNA-crosslinking.

Conclusions
An ovel route from pyrogallol to 5-bromo-8-methoxypsoralen was established. Several new chromophores with functional donor and acceptor groups were synthesized by different cross-coupling methodologies, such as Suzuki, Sonogashira or Heck reactions. These psoralen series absorb at wavelengths around4 00 nm and possess highlyi nterestinge missionp rop- Table 7. TD-DFT calculations (PBE1PBE/6-21G) of the UV/Vis absorption maximao f11 a < 11 e < 13 a < 13 e < 15 a and 15 e using PCM with dichloromethane as solvent.  erties with relative fluorescenceq uantumy ields of up to 28 %. Besides pronounced positive emission solvatochromicity reversible fluorescence quenching by acidochromicity can be assessed.E xperimentally the highly polar nature of the excited state was supported by determining the change of dipole moments according to the Lippert-Mataga model.T he acidochromicity and protic emission quenching was quantitatively investigated by determining pK a values of these psoralen chromophores by absorptionp hotometry andb yS tern-Volmer plots. TD-DFT calculations using the PBE1PBEf unctional can successfully applied to elucidate the nature of the longest wavelength absorption bands.
With the embedded multifunctionality these novel psoralen derivativesa re promising candidates for PUVAt herapy at lower energies. Their bathochromic absorption does not requiret he use of ultraviolet light, potentially also daylight suffices. In addition, these sensitivity to polar and protic environments encourage to scout for applicationsi nb iophysical analytics as well as theranostic agents. [39] Experimental Section All experimental details, such as preparations, typical procedures, and all 1 Ha nd 13 CNMR spectra, absorption and emission spectra, solvatochromicity and acidochromicity studies as well as crystal structures and quantum chemical calculations are included in the Supporting Information.