A hybrid bis(amino-styryl) substituted Bodipy dye and its conjugate diacid: synthesis, structure, spectroscopy and quantum chemical calculations.

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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 10187

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A hybrid bis(amino-styryl) substituted Bodipy dye and its conjugate diacid: synthesis, structure, spectroscopy and quantum chemical calculations† Adela Nano,a Pascal Retailleau,b Jerry P. Hagon,*c Anthony Harriman*c and Raymond Ziessel*a A new type of fluorescent pH indicator has been developed whereby two dissimilar amino-styryl units are attached to a boron dipyrromethene (Bodipy) dye. The photophysical properties of this hybrid dye, and its simpler counterparts bearing only a single amino-styryl residue, depend on the polarity of the surrounding medium. Of the two terminal amines, DFT (B3LYP/6-31G**) calculations and spectroscopic measurements support the notion that julolidine is oxidised and protonated under milder conditions than is N,N-dimethylaniline. For the hybrid dye, similar DFT calculations carried out for the monoprotonated analogues indicate that the julolidine residue is the stronger base while the resultant conjugate acid is the weaker one. Absorption and fluorescence spectroscopic titrations show that protonation of the hybrid dye occurs in two well-resolved steps, whereby addition of the first proton

Received 28th November 2013, Accepted 24th March 2014 DOI: 10.1039/c3cp55021d

introduces a thermodynamic barrier for entry of the second. In the hybrid dye, the pKA values for the respective conjugate acids differ markedly from those derived for the mono-amino-styryl dyes and display negative co-operativity. Effectively, this means that electronic interactions running along the molecular backbone make it more difficult, relative to the individual dyes, to protonate both amino sites.

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As such, this dye operates as a probe over an unusually wide pH range.

Introduction The in situ measurement of proton concentration is one of the most important analytical protocols and has widespread applications in many diverse areas such as medical diagnosis, water treatment and purification, food science, drug analysis and environmental monitoring. In the vast majority of cases, the glass electrode suffices for all necessary pH measurements but it suffers somewhat from limited performance in strongly acidic or basic media.1 Thus, the development of alternative pH sensors continues to attract significant attention, especially in terms of devising improved protocols for harsh or hazardous conditions, a

Laboratoire de Chimie Organique et Spectroscopies Avanceés (ICPEES-LCOSA), UMR 7515 au CNRS, Ecole Europeénne de Chimie, Polyme`res et Mate´riaux, Universite´ de Strasbourg, 25 rue Becquerel, 67087 Strasbourg Cedex 02, France. E-mail: [email protected] b Centre de Recherche de Gif, Institut de Chimie des Substances Naturelles, ˆtiment 27, 1 avenue de la Terrasse, UPR2301-CNRS, Ba 91198 Gif sur Yvette Cedex, France c Molecular Photonics Laboratory, School of Chemistry, Newcastle University, Bedson Building, Newcastle upon Tyne, NE1 7RU, UK † Electronic supplementary information (ESI) available: Additional spectroscopic data and figures, structures of reference compounds, DFT output for BOD-DMA, information about derivation of pKA values. CCDC 934637 and 934639. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cp55021d

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inaccessible regions and in microscopic domains. Among the vast collection of fluorescent dyes that might be considered as suitable modules with which to devise next-generation pH probes, the 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (Bodipy) scaffold stands out as an attractive starting point. Such dyes are well known for their high molar absorption coefficients, almost unitary fluorescence quantum yields, narrow absorption and emission spectral bands in the visible region and high levels of photo-stability.2 Their photophysical and electrochemical properties can be tuned by simple substitution and there are many effective protocols for attaching secondary structural units to the Bodipy core. Generally, the pH-sensitivity for this type of fluorescent sensor is realized by attaching an (N,N-dialkyl)aniline or phenolic unit at the meso-position.3,4 Fluorescence is switched on or off by protonation/deprotonation of the aniline nitrogen or phenoxyl group, respectively, with the off signal being controlled by light-induced intramolecular charge transfer. A critical feature for an advanced fluorescence-based pH sensor relates to the spectral shift between neutral and charged species in the medium of interest. In particular, this shift should be as large as possible so that the monitoring of pH changes is not restricted to changes in emission intensity. To improve the optical properties of the generic Bodipy dye, a popular strategy is to attach styryl residues at the 3,5-positions

Phys. Chem. Chem. Phys., 2014, 16, 10187--10198 | 10187

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Chart 1 Molecular formula and energy-minimized conformation for the asymmetrical hybrid dye investigated in this work. Scheme 1 Keys: (i) p-TsOH cat., piperidine, toluene, reflux, 4 h, 66%. (ii) p-TsOH cat., piperidine, toluene, reflux, 75%.

so as to extend the p-conjugation length. Additional red shifts are available by building in some degree of charge-transfer character that can be extinguished by virtue of the acid/base transformation. This leads to the concept of styryl-based Bodipy dyes in which an amino residue is inserted into the conjugation pathway. This simple approach has not been used extensively although Boens et al.5 have made pioneering discoveries in this area using the N,N-dimethylanilino derivative. Importantly, the required functionality can be introduced by condensation of Bodipy with an appropriate aldehyde under Knoevenagel conditions.6 This route was followed earlier to develop highly porous polyacrylate beads coated with amino-styryl Bodipy dyes able to detect trace amounts of acids and bases in the atmosphere.7 We now extend this work to encompass styryl-Bodipy dyes equipped with a tertiary cyclic amine residue formed from 9-formyl-10-butoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5Hpyrido-[3,2,1-ij]quinoline.8 This particular molecular fragment is often termed ‘‘julolidine’’ and is a common constituent of push– pull electronic systems since it is a powerful electron donor. Literature reports can be used to argue that the corresponding ammonium salts of julolidine and N,N-dimethylaniline possess comparable pKA values in organic solvents, taking due account of the difficulty associated with quantifying protonation in nonaqueous media. This realization led us to prepare a mixed-amino derivative bearing both julolidine and N,N-dimethylaniline residues attached to a 3,5-bis-styryl Bodipy dye (Chart 1). The target compound, which absorbs and emits in the far-red region and has good solubility in most organic solvents, is considered to be a prototype for a new class of hybrid pH sensors suitable for weakly acidic conditions. The lowest-energy conformation points to a relatively flat geometry with extensive p-conjugation running throughout the molecular backbone (Chart 1). To the best of our knowledge, this is the first example of a Bodipy dye equipped with two disparate amino-based appendages.

Results and discussion

julolidine9 leads to isolation of the mono-functionalised compound as the exclusive product, despite using excess of aldehyde 2. This interesting observation opens numerous possibilities for further preparation of asymmetric dyes. It’s origin might arise from the strong electron density imported by the julolidine moiety that affects the acidity of the 5-methyl group or from deactivation of the carbonyl function. In fact, a similar observation was made with respect to the mono-condensation of certain Bodipy dyes with electron-rich triazatruxene-based modules.10 Moreover, we note that condensation of the mono-styryl species 4 with a large excess of N,N-dimethylbenzaldehyde gave the mixed-styryl derivative 5 in 75% yield. This latter compound (Chart 1), which is the main target of the present work, contains two distinct aminostyryl functions appended to the Bodipy core and retains an exposed iodo-group for attachment to macroscopic supports. The main point of interest here is to establish to what extent these two amino residues cooperate in an electronic sense. Single crystals suitable for X-ray analyses were obtained for compounds 4 and 5 by slow diffusion of pentane into a concentrated solution of the dye in dichloromethane. The corresponding molecular structures are shown in Fig. S1 and S2 (ESI†), together with selected data. The mono-styryl Bodipy compound 4 has the most planar dipyrrin core (12 atoms for a least-mean squares plane of 0.04 Å) and the most orthogonal iodophenyl group (dihedral angle of 86.41). However, slight curvature exists around the vinyl spacer of the two macrocycles, with their mutual dihedral angle being estimated at ca. 251. Such twisting is compensated in 5 by the additional substitution at C4 by N,N-dimethylaniline, (dihedral angle of (+)141 and of ( )181 with the aniline group). Also, there is less pronounced orthogonality with the iodophenyl group (dihedral angle of 79.71). Regardless of these minor structural changes, both derivatives can be considered to be roughly planar and to adopt comparable crystal lattices.

Synthesis

Spectroscopic studies conducted with the mono-amino-styryl derivative 4

The mono-styryl derivative 4 was prepared from the tetramethylsubstituted Bodipy dye 1 by way of a conventional Knoevenagel reaction, as depicted by Scheme 1. Such condensation with

The absorption spectrum recorded for 4 in dibutyl ether (Bu2O) is shown in Fig. 1 and exhibits a strong transition centred at 640 nm, together with weaker transitions at 462 and 408 nm.

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Fig. 1 Normalized absorption (black curve) and fluorescence (grey curve) spectra recorded for 4 in dibutyl ether at room temperature. The excitation wavelength was 570 nm.

Table 1 Summary of the photophysical properties determined for 4 in solvents of differing polarity at room temperature

Solvent

eSa

lABS/ lFLU/ nm nm FF

SS/ cm

C6H12 Bu2O EtOAcb CH2Cl2 C3H7CNc CH3CN

2.02 3.18 6.03 9.02 24.56 35.94

638 640 637 642 645 644

513 820 1010 2520 1890 2625

a

661 682 725 766 750 775

0.92 0.82 0.45 0.003 0.13 0.023

Static dielectric constant of the solvent.

b

1

tS/ ns

kRAD/ 108 s

3.7 3.9 3.15 1.1 1.6 0.88

2.50 2.10 1.43 0.25 0.81 0.26

1

kNR/ 108 s

1

0.22 0.46 1.75 9.2 5.4 11.0

Ethyl acetate. c Butyronitrile.

The lowest-energy absorption band is relatively broad but can be deconstructed into a series of five Gaussian-shaped components of common half-width (Dn = 1170 cm 1). The peak maximum (lABS) shows a slight dependence on the nature of the surrounding solvent (Table 1) and there is an obvious broadening of the spectrum with increasing solvent dielectric constant (eS). Indeed, in a strongly polar solvent, such as acetonitrile, the lowest-energy absorption band becomes more symmetrical and the absorption onset is red-shifted (Table 1). The higher-energy absorption transitions show similar solvent dependences. Fluorescence from 4 is readily detected in solution at room temperature (Fig. 1). In Bu2O, for example, the emission maximum lies at 682 nm and the spectral profile is relatively broad. Again, the total emission profile can be deconstructed into a series of four Gaussian-shaped components with a common half-width (Dn = 1040 cm 1). There is reasonable mirror symmetry with the lowest-energy absorption transition but the substantial Stokes shift (SS = 820 cm 1) points towards a significant geometry difference between relaxed ground- and excited-state species.11 There is excellent agreement between absorption and excitation spectra. In Bu2O at room temperature, the fluorescence quantum yield (FF) is 0.82 0.03 while the excited-singlet state lifetime (tS), derived from mono-exponential decay curves, is 3.9 0.1 ns. These values can be used to derive estimates for both the radiative (kRAD = 2.1 108 s 1) and nonradiative (kNR = 4.6 107 s 1) rate constants that characterize deactivation of the relaxed excited-singlet state.


The same information collected in a small series of solvents at 20 1C is provided in Table 1 and confirms that the emission behaviour is affected by changes in solvent polarity, as has been observed for related amino-styryl Bodipy derivatives (see Fig. S3, ESI†).5,12 It is especially informative to note the strong red-shifted and broadened fluorescence profile in polar media.12 It might also be noted that CH2Cl2 does not appear to fit with the other solvents, a feature ascribed to its mild acidic nature that will favour hydrogen bonding to the amino N atom. In CH2Cl2 solution, the absorption maximum occurs at 642 nm, where the molar absorption coefficient (eMAX) is 92 000 M 1 cm 1. This latter value is relatively high for a Bodipy dye,13 and by integration of the lowest-energy absorption band14 we find the relevant oscillator strength (f) to be 1.03. There is a small effect of solvent polarity on the magnitude of f but a more significant effect on eMAX (see Table S1, ESI†). In Bu2O, the radiative rate constant (kRAD) calculated from the Strickler–Berg expression15 is 1.8 108 s 1, compared to the experimental value of 2.1 108 s 1. According to DFT (UHF-B3LYP/6-31G** level) calculations, the ground-state dipole moment of 4 is 3.8 D and, on the basis of the Lippert–Mataga expression16 used in conjunction with a cavity radius of 5.5 Å as estimated from molecular modeling of the dye in a reservoir of solvent molecules having hypothetical dielectric constant of 10, the increase in dipole moment (Dm) upon formation of the relaxed excited-singlet state is ca. 7.2 D. In comparison, Dm estimated5 in the same way for BOD-REF (see Fig. S4, ESI,† for molecular formula) is 7.4 D based on a cavity radius of 5.25 Å. An approximation of the ionization potential (IP), made for the energy-minimized geometries for the neutral and cationic forms of 4 in vacuo,17 is 5.44 eV (an alternative estimate of IP made on the basis of Koopman’s theorem18 gives 4.31 eV). The same DFT calculations made for BOD-REF give m = 3.0 D and IP = 5.63 eV (note that in this case Koopman’s theorem gives an estimate for IP of 4.49 eV). A summary of the computed data is provided in Table 2.

Table 2 Summary of the computed (DFT-UHF-B3LYP/6-31G**) electronic properties for the various compounds

Compound

m/D

IPa/eV

IPb/eV

NMULc

NLOWd

4 BOD-DMA JL DMA 5e 5f 4H+ BOD-DMAH+ 5H+ g 5H+ h

3.8 3.0 3.9 2.8 3.64 3.64 2.2 2.2 NA NA

5.44 5.63 5.77 6.18 5.05 5.05 NA NA NA NA

4.31 4.49 4.38 4.72 4.03 4.03 NA NA NA NA

0.0242 0.1273 +0.1497 +0.1382 0.5374 0.5209 0.5447 0.5145 0.5075 0.5467

0.1028 0.0861 0.0312 0.0251 0.1247 0.1041 +0.0362 +0.0540 0.0923 0.1036

a

Ionization potential. b Ionization potential from Koopman’s theorem. Mulliken charge at the amino N atom. d Lowdin charge at the amino N atom. e For the hybrid dye, electronic charges refer to the JL-based N atom. f For the hybrid dye, electronic charges refer to the DMA-based N atom. g Mono-protonated analogue with the proton being resident on the JL unit, the data refer to the nonprotonated N atom (data for the protonated N atom are +0.0275 (Lowdin) and 0.5577 (Mulliken)). h Mono-protonated analogue with the proton being resident on the DMA unit, the data refer to the nonprotonated N atom (data for the protonated N atom are +0.05610 (Lowdin) and 0.5126 (Mulliken)). c


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Boens et al.5 have reported detailed investigations concerning how changes in solvent polarity affect the photophysical properties of BOD-REF. In summary, increasing polarity causes a significant red shift for the emission profile without serious effect on the absorption maximum. The absorption and fluorescence spectral profiles are broadened in polar solvents while increasing solvent polarity tends to decrease both FF and tS at ambient temperature. As noted for 4, polar solvents decrease kRAD but increase kNR. An important aspect of this prior literature study5 gives careful consideration as to what type of solvent polarity function best describes the experimental behaviour. Here, we make a cursory comparison of the photophysical properties of 4 and BOD-REF under ambient conditions with a view to establishing the relative donor strengths of the two amino residues. Table S1 (ESI†) reports the main photophysical properties of the two compounds as measured in a few aprotic solvents at room temperature. It is seen that these compounds display similar behaviour, certainly in terms of general trends. The most surprising disparity between the compounds relates to the red-shifted absorption maximum of around 45 nm found for 4 relative to BOD-REF. This latter finding can be interpreted in terms of julolidine being the more potent electron donor when used in conjunction with a styryl-Bodipy acceptor. It is also possible that the N lone-pair is better aligned in 4 than in BOD-DMA (Scheme 1) such that the p-conjugation length19 is slightly extended in the former. Examination of the computer-generated structures shows that the amine N to B distance is similar in both compounds (i.e., 9.084 Å for 4 and 9.065 Å for BOD-DMA) while the corresponding distance from the amine N atom to the styryl-substituted C atom on Bodipy is 8.024 Å for 4 and 8.040 Å for BOD-DMA. These differences are negligible. Likewise, bond lengths for the amine N-to-phenyl C atom (i.e., 1.384 Å in both cases) and the three bonds in the styryl linkage are identical to within experimental error (see Table S2, ESI†). Slight disparities exist in the angles around the amine-tostyryl linkage but there is no indication for a significant change in geometry. In contrast, the dihedral angle between the Bodipy core and the N,N-dimethylaniline unit is quite large at 10.91 while the corresponding dihedral angle for 4 is reduced to 2.81. This is the only significant geometry change between these two compounds but it could make a substantial difference in terms of effective p-electron delocalisation along the molecular backbone. As mentioned above, the DFT (B3LYP/6-31G**) calculations indicate that the ionization potential of 4 (IP = 5.44 eV) is slightly lower than that of BOD-DMA (IP = 5.63 eV). Identical calculations made for the corresponding molecules lacking the Bodipy unit (Fig. S4, ESI†) give rise to an IP of 5.77 eV (Koopman’s theorem gives 4.38 eV) and 6.18 eV (Koopman’s theorem gives 4.72 eV), respectively, for julolidine-(JL) and N,Ndimethylaniline-based (DMA) residues (Table 2). As expected, these smaller molecules are more difficult to oxidize than are the amino-styryl-Bodipy dyes but they retain comparable dipole moments to the mono-amino-styryl Bodipy dyes (Table 2). Cyclic voltammetry studies made for 4 in CH2Cl2 containing background electrolyte show the presence of a single, quasireversible reduction peak that corresponds to formation of the


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Electrochemical data for selected compoundsa

Compound

E1/2 (ox, soln) (V), DE (mV)

E1/2 (red, soln) (V), DE (mV)

1 2 4 4H+ b BOD-DMA BOD-DMAH+ b 5 H252+ b

+1.15 +0.86 +0.38 +0.67 +0.56 +0.83 +0.26 +0.53

1.24 — 1.17 1.16 1.17 1.16 1.09 1.08

(70) (60) (70); +0.79 (60) (60); +0.93 (60) (70); +0.88 (60) (70); +1.04 (60) (60); +0.39 (60); +1.27 (70) (80);c +1.31 (70)

(60) (70) (60) (70) (75) (70) (60)

a

Potentials determined by cyclic voltammetry in deoxygenated CH2Cl2 solution, containing 0.1 M TBAPF6, at a solute concentration of ca. 1.5 mM and at rt. Potentials were standardized vs. ferrocene (Fc) as internal reference and converted to the SCE scale assuming that E1/2 (Fc/Fc+) = +0.38 V (DEp = 60 mV) vs. SCE. Error in half-wave potentials is 10 mV. b Using slight excess of anhydrous HCl gas. c Two-electron process deduced from the integral.

Fig. 2 Computed Kohn–Sham distributions for the HOMO (left panel) and LUMO (right panel) isosurfaces for 4. The surface contains 90% of the orbital charge.

styryl-Bodipy p-radical anion (Table 3). The half-wave potential (E1/2 = 1.17 V vs. SCE) for this process is in line with the behaviour noted previously for related compounds. There are two peaks on oxidative scans, corresponding to E1/2 values of 0.38 and 0.79 V vs. SCE, respectively, both of which are quasireversible under these conditions. Examination of the DFT output (Fig. 2) leads to the conclusion that the LUMO computed for 4 is distributed around the styryl-Bodipy unit but with little penetration onto the aryl amine fragment. In contrast, the HOMO is distributed evenly around the entire molecule, most notably including the amine group. Likewise, HOMO( 1) is well distributed around the styryl-Bodipy core. Turning attention to BOD-DMA, we find the DFT output (Fig. S5, ESI,† for minimized geometries) is fully consistent with the LUMO being spread around the styryl-Bodipy unit and with the HOMO being distributed around the entire molecule. Cyclic voltammetry for BOD-DMA in CH2Cl2 solution gives the same half-wave potential (E1/2 = 1.1.7 V vs. SCE) for reduction and two quasi-reversible waves (E1/2 = 0.56 and 0.88 V vs. SCE) on oxidation (Table 3). All indications, therefore, point to 4 being easier to oxidize than is BOD-DMA despite the fact that the dipole moment is slightly higher for the former. Formation of the conjugate acid of 4 in acetonitrile Addition of HCl to a solution of 4 in CH3CN at room temperature causes a dramatic change in the absorption spectral profile (Fig. 3). The broad absorption band centred at 640 nm is replaced with a sharper band located at 566 nm. Isosbestic points are


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Fig. 3 Absorption spectral titration carried out for 4 (9 mM) in CH3CN by addition of HCl (effective pH decreasing from 8 to 1; see Fig. S6, ESI,† for actual pH values). The arrows indicate the course of reaction as increasing amounts of HCl are added to the solution. The spectra are corrected for minor effects of dilution.

preserved throughout the titration and, provided excess acid is avoided, the process can be reversed by addition of base. There are indications for a minor side-reaction that leads to formation of a species absorbing at around 740 nm and this particular species builds up on successive acid–base cycles or if a slight excess of acid is added. Similar spectral changes are observed following addition of HClO4 in CH3CN. Corresponding changes are observed by fluorescence spectroscopy (Fig. 4) in CH3CN solution. Here, emission from the neutral form of 4 is difficult to resolve from the baseline but, in the presence of low concentrations of HCl, the conjugate acid emits strongly at 573 nm. In the presence of a slight excess of HCl and following excitation at 525 nm, we note that FF is 0.97 while tS is 4.0 ns and that the Stokes shift decreases to the exceptionally small value of 215 cm 1. For the conjugate acid of 4,

Fig. 4 Fluorescence spectral titration carried out for 4 (2 mM) in CH3CN by addition of HCl (effective pH decreasing from 8 to 1; see Fig. S7, ESI,† for actual pH values). The arrows indicate the course of reaction as increasing amounts of HCl are added to the solution. The spectra are corrected for minor effects of dilution while the inset shows an expansion of the region where the neutral dye emits. Excitation was made at the isosbestic point around 490 nm.


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4H+, kRAD is 2.4 108 s 1 and kNR is less than 107 s 1. These fluorescence changes can be followed conveniently by excitation at one of the isosbestic points, although these occur at relatively low absorbing regions. Indeed, fitting the absorption and fluorescence titration data to standard Henderson–Haselbach equilibria20 leads to an estimate for the corresponding pKA value as being 3.55 0.08, while the Hill coefficient21 is essentially unity. These calculations, which are outlined in the ESI,† are based on complete dissociation of HCl in CH3CN and this is clearly not the case.22 The calculations are made solely to allow for comparison with the analogous BOD-DMA derivative where Boens et al.5 report a pKA of 2.25 in CH3CN solution. The conjugate acids of 4 and BOD-DMA possess comparable spectroscopic properties and it is notable that their absorption maxima, being 566 and 553 nm respectively for 4H+ and BODDMAH+,5 are not too dissimilar. Thus, the factors leading to the pronounced red-shifted absorption maximum for 4 relative to BOD-DMA are essentially absent for the conjugate acids. These acidic species share a small Stokes shift, negligible nonradiative decay channel and kRAD of ca. 2.4 108 s 1. This similarity points against there being a strong element of reverse charge transfer (i.e., charge transfer from Bodipy core to the ammonium cation23) since we might have expected more pronounced differences in the spectroscopic behaviour, as observed for the neutral species. Furthermore, DFT calculations carried out for 4H+ indicate both HOMO and LUMO are spread around the entire molecule but without penetrating onto the N atom (Fig. 5). The calculations show the expected (distorted) tetrahedral geometry, with an N–H bond length of 1.027 Å and arylC–N–H angle of 106.81 (the corresponding N–CH2 bond length is 1.512 Å and the arylC–N–C angle is 113.71). Cyclic voltammetry made with these conjugate acids shows that E1/2 for one-electron reduction is essentially unchanged from that of the neutral form but the lowest potential oxidation wave is moved to a more positive value by ca. 290 mV (Table 3). Thus, the electrochemistry resembles that found for monostyryl Bodipy dyes lacking the amino substituent. The potential difference between the two oxidation steps remains at ca. 0.4 eV, and this behaviour seems more consistent with p-systems where the HOMO is widely distributed rather than being localised at

Fig. 5 Computed Kohn–Sham distributions for the LUMO (left panel) and HOMO (right panel) isosurfaces for 4H+. The surface contains 90% of the orbital charge.


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the amino group. It is somewhat puzzling, however, to note that there is no apparent electrostatic effect for the reduction step. On the basis of the respective pKA values deduced for the two conjugate acids, 4H+ will appear at higher pH than will BODDMAH+. The discrepancy amounts to ca. 0.8 pH units in CH3CN, which might be relevant in certain sensing applications. This finding can be rationalized in terms of julolidine being a stronger base than is N,N-dimethylaniline, although attention is drawn to the apparent difference in dihedral angle that might affect electronic coupling along the molecular backbone. Previous computational work24 has concluded that pKB for aryl amines increases with increasing ionisation potential and electronic charge resident on the relevant N atom (Table 2). Our studies, therefore, are fully consistent with JL being the stronger base. It might also be stressed that, since the product of KB (for the amine) and KA (for the conjugate acid) is a constant (namely 10 14), the order of derived pKA values is as might be expected. Spectroscopic studies conducted with the hybrid amine 5 The presence of the second amino-styryl appendage, thereby giving rise to the hybrid derivative 5, causes a pronounced red shift for the lowest-energy absorption transition.25 The absorption maximum now appears at 720 nm in Bu2O, while eMAX is reduced slightly to 84 000 M 1 cm 1 and the oscillator strength14 falls to 0.70. This particular absorption band is somewhat better resolved than the corresponding spectral profile recorded for 4 (Fig. 6) and peak fitting protocols indicate that the best analytical fit involves two sets of overlapping transitions (see Fig. S8, ESI†). The emission spectrum is also fairly broad, with a maximum at 748 nm in Bu2O (Fig. 6), but can be resolved into 3 Gaussian-shaped components with a common half width of 930 cm 1, together with a small contribution from hot emission The Stokes shift is 520 cm 1, which is somewhat less than that of 4 in the same solvent. In Bu2O at room temperature, FF is 0.15 while tS has a value of 2.8 ns, giving rise to kRAD = 5.4 107 s 1 and kNR = 2.9 108 s 1. Time-resolved fluorescence decay profiles are mono-exponential, regardless of excitation or emission wavelength, and the excitation spectrum

Fig. 6 Normalized absorption (12 mM; black curve) and fluorescence (4 mM; grey curve) spectra recorded for 5 in dibutyl ether at room temperature. The excitation wavelength was 610 nm.


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agrees well with the absorption spectrum recorded over the entire visible region (see Fig. S9, ESI†). The shorter excited-state lifetime and reduced quantum yield are as might be expected on the basis of the energy-gap law.26 In turn, the red-shifted absorption and emission maximum can be attributed27 to the increased p-conjugation lengths arising from the dual styryl arms. The Stokes shift observed for 5 depends on solvent polarity and increases steadily on changing from Bu2O (SS = 520 cm 1) to CH2Cl2 (SS = 1260 cm 1) and to CH3CN (SS = 1355 cm 1). These solvent-induced shifts indicate a more modest increase in dipole moment upon excitation than recorded for either of the mono-amines. A further point of interest concerning the optical spectroscopy of these amino-styryl-Bodipy dyes is that the higherenergy transitions found in the near-UV region, and which are well reproduced in the excitation spectra, are both red-shifted and amplified in the hybrid-derivative. Thus, 5 in Bu2O exhibits a series of transitions centred at 516, 443 and 423 nm; for 4 the bands appear at 464, 410 and 383 nm. These absorption bands disappear on protonation of the amine but, for each band, eMAX is roughly doubled for 5 relative to 4. These transitions are believed to arise from intramolecular charge-transfer interactions and there is a clear inference that both terminal amines contribute towards the observed bands. Compared28 with the corresponding bis-styryl dye bearing terminal N,N-dimethylanilino groups, BOD-DMA2, the most significant point is that the red-shifted absorption and emission maxima persist at the bis-substitution level, the actual shift being ca. 35 nm. Cyclic voltammetry carried out in CH2Cl2 containing background electrolyte shows one quasi-reversible wave (E1/2 = 1.09 V vs. SCE) on reductive scans that corresponds to formation of the p-radical anion (Table 3). Thus, the presence of the second styryl unit makes the compound slightly (i.e., ca. 100 mV) easier to reduce: note that the DFT calculations are consistent with the LUMO being spread over the Bodipy unit and onto both styryl arms but not the amino groups (Fig. 7). Three peaks are observed on oxidative scans, each of which is quasi-reversible, corresponding to successive E1/2 values of 0.26, 0.39 and 1.27 V vs. SCE. It is tempting to ascribe these oxidative peaks to removal of an electron from each terminal amine before oxidation of the Bodipy core. However, DFT calculations show that the HOMO is evenly distributed around the entire molecule and involves both amino N atoms (Fig. 7); according to these calculations, the two amino-bearing arms retain closely comparable geometries (see Table S3 in the ESI†) A similar picture emerges for HOMO( 1) and HOMO( 2) and it is not obvious that there is a preference to oxidize one particular amine. The computed dipole moment (m = 3.64 D) and ionization potential17 (IP = 5.05 eV, cf. Koopman’s theorem18 of 4.03 eV) appear consistent with the anticipated lowering of the HOMO energy on adding the second styryl arm but do not address the issue of selective oxidation. Interestingly, a crude electrostatic calculation made on the basis of charge localization at the amine N atoms, where the separation is 12.9 Å, in a dielectric continuum with eS = 9 predicts a difference in the first two E1/2 values of 120 mV.


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Fig. 8 Absorption spectral titration carried out for 5 (15 mM) in CH3CN by addition of HCl (effective pH decreasing from 7.7 to 0.6; see Fig. S10, ESI,† for actual pH values). The arrows indicate the course of reaction as increasing amounts of HCl are added to the solution. The spectra are corrected for minor effects of dilution.

Fig. 7 Computed Kohn–Sham distributions for the HOMO (upper panel) and LUMO (lower panel) isosurfaces for 5. The surface contains 90% of the orbital charge.

Protonation of 5 in acetonitrile The hybrid dye, 5, exhibits a strong absorption peak in the farred region (lMAX = 736 nm) in CH3CN solution and a corresponding fluorescence peak centred at ca. 820 nm. Under these conditions, FF is reduced to 0.020 while tS has a value of 0.95 ns at room temperature. Addition of a slight excess of HCl in CH3CN causes a pronounced blue shift for both absorption (lABS = 630 nm) and fluorescence (lFLU = 635 nm) maxima (Fig. S12, ESI†). There is a concomitant increase in the fluorescence quantum yield (FF = 0.35) and lifetime (tS = 1.7 ns). The resultant conjugate diacid, H252+, displays red shifts of ca. 65 nm relative to 4H+, because of the increased conjugation, and ca. 10 nm relative to the corresponding diacid7 formed from BOD-(DMA)2 where lABS = 620 nm and lFLU = 628 nm. A remarkable feature of these conjugate diacids is the exceptionally small Stokes shifts. It is also apparent that the HOMO


and LUMO are evenly distributed around the entire molecule, without localisation along a particular arm (see Fig. S13 and S14, ESI†). Absorption spectroscopy provides firm evidence that protonation of 5 occurs in two distinct steps (Fig. 8). Addition of the first proton results in a modest blue shift, with lABS moving to 720 nm, but with retention of the broad spectral profile. In contrast, addition of the second proton causes the spectrum to sharpen, with an increased eMAX, and shift to 630 nm. Analysis of the titration data28 is fully consistent with successive pKA values of 5.75 and 2.75 for the conjugate acid and diacid (see ESI† for details). Comparison can be made with the conjugate acid formed from BOD-(DMA)2 where pKA values of 5.10 and 3.04 were recorded in acetonitrile. It is notable that the pKA difference is escalated for the hybrid dye and that 5 is slightly easier to protonate than is BOD-(DMA)2 in CH3CN. The same titration was followed by fluorescence spectroscopy, although there is no single isosbestic point apparent in the absorption spectral profiles. The most appropriate excitation wavelength was found to be 365 nm, where all three species absorb strongly. Addition of small aliquots of HCl in CH3CN caused the emission maximum to shift from 820 nm to 810 nm before undergoing a more substantive shift to 635 nm (Fig. 9). The intermediate species is ascribed to the monoprotonated form, H5+, as already identified by absorption spectroscopy. Analysis of the data in terms of stepwise addition of two protons to 5 results in estimates for the successive pKA values for the conjugate acids as 5.80 and 2.70. These derived values are in good agreement with those obtained from absorption spectroscopy. A key point now concerns the sequence of protonation events, bearing in mind the disparate nature of the two amino groups. To aid this mechanistic enquiry, DFT (B3LYP/6-31G**) calculations were carried out with the two mono-protonated forms of 5, where the proton was located at either terminal.


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Fig. 9 Fluorescence spectral titration carried out for 5 (3 mM) in CH3CN by addition of HCl (effective pH decreasing from 7.7 to 0.60; see Fig. S11, ESI,† for actual pH values used). The arrows indicate the course of reaction as increasing amounts of HCl are added to the solution. The spectra are corrected for minor effects of dilution while the inset shows an expansion of the region where the neutral dye emits. Excitation was made at 365 nm.

Prior to adding the proton, the computed Lowdin charges29 for the two N atoms (Table 2) indicate that protonation is expected to occur at the julolidinyl N atom. In fact, this site appears to be heavily favoured (NLOW = 0.125) over the corresponding N atom associated with the N,N-dimethylanilino group (NLOW = 0.104). Addition of the proton to the julolidinyl N atom affects the charge at the remaining N atom, pushing the Lowdin charge to a more positive value (NLOW = 0.104). This has the effect of suppressing the second protonation step such that the pKA for the conjugate diacid is significantly lower than that for the monoacid. The same situation holds if the first proton is added to the N,N-dimethylanilino site since the Lowdin charge for the opposite N atom is raised to 0.092. It appears therefore that the DFT calculations allow a good rationalisation for the protonation equilibria observed for 5 in CH3CN.

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respect to protonation, as long as excess acid or base is avoided. An interesting feature of these planar amino-styryl-Bodipy dyes is that the HOMO tends to be spread around the molecule rather than being concentrated near to the donor N atom. This effect is likely to lower the contribution of charge-transfer interactions, keeping the dipole moment to a modest level and thereby help to render the dyes highly fluorescent, especially after protonation. In terms of a practical device, it is important to note the distinctive spectral shifts that accompany the acid–base equilibria. Such shifts allow facile determination of the solution pH without recourse to calibration of fluorescence intensities. The compounds described here are insoluble in water and poorly soluble in many nonpolar organic solvents. After double protonation, the conjugate diacids do disperse in water but it is doubtful that these are true solutions. This is not seen as a problem, however, because our interest is not to add to the number of existing pH indicators for aqueous solution. Rather, we foresee any applications as involving the dyes adsorbed onto solid supports and maintained in contact with either a fluid or a gaseous stream. The hybrid pH probe could also be used as an electrochemical set-up since there is a massive shift in half-wave potential on oxidative scans when protons are added (Fig. 10). Thus, the two oxidative waves seen for 5 merge into a single wave upon formation of the conjugate diacid. As such, a simple pH probe could be fabricated by current measurement at fixed potentials

Conclusions This work has introduced a new class of fluorescent pH probes bearing disparate protonation sites. Negative cooperativity is displayed between these sites such that the probe operates over a fairly wide pH range (i.e., pH 2–7). Thus, relative to the individual dyes, it is more difficult to protonate both amino N atoms in the hybrid dye. Addition of the first proton causes small but easily monitored spectral changes that are best recognised through ratiometric methodologies.30 In contrast, attachment of the second proton induces dramatic changes in both absorption and fluorescence properties while minimising the Stokes shift. On the basis of DFT calculations, the sequence of protonation steps has been explored and conveniently explained in terms of Lowdin charges29 on the relevant N atoms. Throughout this small series of dyes, there is a smooth correlation between pKA for the conjugate acid and the calculated Lowdin charge on the N atom of the corresponding base. The new hybrid probe is stable and essentially reversible with


Fig. 10 (a) Cyclic voltammogram of 4 (1 mM): red trace under argon and blue trace under gaseous HCl. (b) Cyclic voltammogram of 5 (2 mM): red trace under argon and blue trace under gaseous HCl. With HCl the window was restricted to 0 to 1.5 V to avoid proton reduction.


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using rapid scan voltammetry.31 This is further confirmation that the protonated amino sites display comparable electronic properties, possibly aided of the extensive delocalisation of the HOMO (and LUMO) around the molecule. The effect of HCl on the cyclic voltammogram recorded for 4 is much less pronounced (Fig. 10) and in essence only the first oxidative wave is perturbed. In principle, this behaviour would allow the state of protonation to be monitored by a microscopic electrochemical cell. Similar properties have been reported before for conducting films based on polyaniline32 or polycarbazole33 and employed to develop ultra-small pH probes for medical applications. The ability to monitor proton concentrations with the hybrid dye persists in the solid state, at least with the probe dispersed in poly(methylmethacrylate), PMMA, films prepared by spin coating. Here, the dried films can be dipped into aqueous solution and used in much the same way as conventional pH paper. An alternative set-up has the dye impregnated (from CH2Cl2 solution) onto a microscopic porous glass frit maintained in contact with an aqueous solution and with monitoring via fluorescence spectroscopy. This particular protocol is improved by dispersing the dye in PMMA before adhering to the frit. Preliminary studies have demonstrated that the hybrid dye can be applied to conductive paper sheets by way of inkjet printing. Although the conditions were far from optimal, there are strong indications that the pH sensory behaviour is retained by the small islands of dye dispersed in standard inks. Using the four dyes referred to in this text, there is a unique signature for pH values in the range 1.5 to 7.5 such that extremely accurate pH monitoring is possible even with such a small matrix of dyes. This finding augers well for the development of novel inks able to detect changes in pH over a wide range by employing sensors that operate at discrete pH ranges. On-going work is aimed at extending these inkjet printed materials as sensitive probes for volatile explosives. Finally, we draw attention to the potential use of DFT calculations as a tool for developing similar pH sensors that work at predetermined pH regions. Our work shows that, for a closely related series of chemical structures, there is a good correlation between the pH for the conjugate acid and the Lowdin charge29 computed for the accommodating N atom. The computation has to be made at a high level but consistent results have been found here for relatively large molecular entities. It will be interesting to see how far this approach can be taken as a predictory tool. If a suitable protocol can be identified, it might help cut down on the number of synthetic dyes needed to effect sensors to cover the full pH range.

Experimental section

Paper

room temperature using a Bruker Advance spectrometer. Chemical shifts for 1H and 13C NMR were referenced to residual signals from the solvent. The 128 MHz 11B NMR spectra were recorded at room temperature with boron in borosilicate glass as internal reference. Chromatographic purifications were performed using 40–63 mm silica gel. Routine absorption spectra were recorded with a Shimadzu UV-3000 spectrophotometer and infrared spectra were recorded as solid samples with a Perkin-Elmer Spectrum One instrument. All reagents were used as received from commercial sources. Compound 1 was synthesized34 according to the literature. Solutions of HCl in CH3CN were prepared fresh by saturating a fixed volume (e.g., 10 mL) of spectroscopic grade CH3CN with gaseous HCl and diluting the sample by a factor of 3-fold. The final solution was standardized by titration with aqueous NaOH solution. Electrochemical studies employed cyclic voltammetry with a conventional 3-electrode system using a BAS CV-50W voltammetric analyser equipped with a Pt microdisk (2 mm2) working electrode and a silver wire counter-electrode. Ferrocene was used as an internal standard and was calibrated against a saturated calomel reference electrode (SCE) separated from the electrolysis cell by a glass frit presoaked with electrolyte solution. Solutions contained the electro-active substrate in deoxygenated, anhydrous CH2Cl2 containing tetra-n-butylammonium hexafluorophosphate (0.1 M) as supporting electrolyte. The quoted half-wave potentials were reproducible to within 10 mV. Compound 2 An aliquot of NaH (46 mg, 1.918 mmol) was added slowly to a solution of 9-formyl-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5Hpyrido[3,2,1-ij]quinoline (314 mg, 1.279 mmol) in DMF under argon. The mixture was stirred for 1 h before addition of 1-bromohexane (0.2 mL, 1.535 mmol). After stirring for 2 days at 95 1C, the solution was quenched with water and extracted several times with dichloromethane. The isolated product was purified by column chromatography, eluting with dichloromethane/ petroleum ether (7/3, v/v) (388 mg, 85%). Rf = 0.19 (dichloromethane/petroleum ether). 1H NMR (CDCl3, 300 MHz) d: 0.9 (t, 2J = 6,9 Hz, 3H), 1.26 (s, 6H), 1.32–1.37 (m, 4H), 1.42 (s, 6H), 1.45–1.51 (m, 2H), 1.70 (dd, 3J = 12.1 Hz, 2J = 6.2 Hz, 4H), 1.9 (td, 3J = 11.0 Hz, 2J = 7.2 Hz, 2H), 3.22 (t, 2J = 5.8 Hz, 2H), 3.28 (t, 2J = 6.1 Hz, 2H), 3.95 (t, 2J = 6.9 Hz, 2H), 7.58 (s, 1H), 9.95 (s, 1H); 13C NMR (CDCl3, 75 MHz) d: 14.0, 22.6, 25.7, 29.7, 30.0, 30.3, 31.8, 32.1, 32.5, 35.7, 39.4, 46.9, 47.5, 79.0, 117.3, 120.7, 125.3, 126.1, 148.2, 162.2, 187.8; MS (EI neat matter): m/z (intensity, %) calc. for [M]: 357.2 found: 357.1 (100); 329.1 (35), [M CHO]; anal. calc. for C23H35NO2: C, 77.27; H, 9.87; N, 3.92 found: C, 77.04; H, 9.62; N, 3.77.

Generals methods

Compound 4

All reactions, except where indicated otherwise, were performed under a dry argon atmosphere. Tetrahydrofuran (THF) was distilled from sodium and benzophenone under Ar while N,Ndimethylformamide (DMF) was distilled from potassium hydroxide under reduced pressure. All 1H NMR (200, 300, 400 MHz) and 13 C NMR (50, 75, 100 MHz) spectra were recorded in CDCl3 at

In a round bottom flask, piperidine (around 4 mL), and a crystal of p-TsOH were added to a solution of 9-formyl-10-butoxy-1,1,7,7tetramethyl-2,3,6,7-tetrahydro-1H,5H-pyrido [3,2,1-ij ] quinoline (206 mg, 0.576 mmol) and 4,4-difluoro-8-(4-iodo)phenyl-1,3,5,7tetramethyl-4-bora-3a,4a-diaza-s-indacene (260 mg, 0.576 mmol) in toluene (20 mL). The solution was heated at 140 1C for 3–4 h.



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After evaporation of solvent, the crude product was purified by column chromatography (SiO2, dichloromethane/petroleum ether: 6/4, v/v). Recrystallization from a methanol–dichloromethane mixture gave the desired product (300 mg, 66%) as a dark blue solid. Rf = 0.61 (dichloromethane/petroleum ether: 6/4); 1H NMR (CDCl3, 300 MHz) d: 0.88 (t, 3J = 7.1 Hz, 3H), 1.33–1.38 (m, 10H), 1.42 (s, 9H), 1.46 (s, 3H), 1.50–1.52 (m, 2H), 1.73 (t, 3J = 5.7 Hz, 4H), 1.81–1.90 (m, 2H), 2.58 (s, 3H), 3.16 (t, 3J = 5.5 Hz, 2H), 3.23 (t, 3J = 6.0 Hz, 2H), 3.82 (t, 3J = 6.5 Hz, 2H), 5.95 (s, 1H), 6.59 (s, 1H), 7.08 (d, 3J = 8.2 Hz, 2H), 7.39 (d, 3J = 16.4 Hz, 1H), 7.45 (s, 1H), 7.5 (d, 3J = 16.4 Hz, 1H), 7.83 (d, 3J = 8.2 Hz, 2H); 13C NMR (CDCl3, 50 MHz) d: 14.2, 14.6, 14.7, 15.2, 22.8, 26.2, 30.2, 30.3, 31.0, 32.0, 32.4, 32.8, 36.6, 40.2, 47.1, 47.6, 76.3, 94.6, 113.3, 117.5, 118.4, 120.3, 122.0, 123.5, 126.9, 130.7, 133.2, 135.4, 136.4, 138.3, 139.5, 142.7, 144.7, 152.2, 156.6, 157.6; 11B NMR (CDCl3, 128 MHz) d: 1.04 (t, JB–F = 32.6 Hz); MS (EI neat matter): m/z (intensity, %) calc. for [M]: 789.3 found: 789.2 (100); 770.2 (20), [M F ]; IR (cm 1) n: 756, 977, 1058, 1154, 1191, 1257, 1292, 1463, 1493, 1586, 2850, 2925; anal. calc. for C42H51BF2IN3O: C, 63.89; H, 6.51; N, 5.32 found: C, 63.62; H, 6.37; N, 5.28. Compound 5 In a round bottom flask, piperidine and a crystal of p-TsOH were added to a solution of 4 (73 mg, 0.124 mmol) and N,Ndimethylamino-4-benzaldehyde, (43 mg, 0.130 mmol) in toluene. The solution was heated at 140 1C until dryness. The crude material was purified by silica gel chromatography, eluting with toluene/dichloromethane (3/7, v/v), and recrystallized from a pentane–dichloromethane mixture (73 mg, 75%). Rf = 0.76 (toluene/dichloromethane) 1H NMR (CDCl3, 300 MHz) d: 0.99 (t, 3J = 7.4 Hz, 3H), 1.37–1.46 (m, 18H), 1.52–1.59 (m, 2H), 1.72–1.77 (m, 4H), 1.82–1.89 (m, 2H), 3.03 (s, 6H), 3.20 (m, 4H), 3.84 (t, 3J = 6.6 Hz, 2H), 6.59 (s, 2H), 6.71 (d, 3J = 8.7 Hz, 2H), 7.1 (d, 3J = 8.1 Hz, 2H), 7.17–7.20 (m, 1H); 7.45–7.56 (m, 6H), 7.83 (d, 3J = 8.1 Hz, 2H); 13C NMR (CDCl3, 50 MHz) d: 14.3, 14.9, 15.1, 19.6, 29.8, 30.2, 31.2, 32.4, 32.7, 36.7, 40.2, 40.4, 47.0, 47.6, 94.4, 112.2, 113.9, 115.1, 117.2, 117.3, 122.0, 123.6, 125.3, 126.9, 129.1, 131.1, 134.0, 135.6, 136.1, 138.1, 139.9, 140.9, 144.3, 150.9, 152.4, 154.7, 157.4; 11B NMR (CDCl3, 128 MHz) d: 1.31 (t, JB–F = 33.4 Hz); MS (EI neat matter): m/z (intensity, %) calc. for [M]: 892.4 found: 892.2 (100); 873.2 (25), [M F ]; IR (cm 1) n: 697, 760, 800, 953, 990, 1062, 1106, 1162, 1256, 1286, 1319, 1362, 1489, 1533, 1586, 2858, 2925, 2952; anal. calc. for C49H56BF2IN4O: C, 65.93; H, 6.32; N, 6.28 found: C, 65.72; H, 6.05; N, 5.93. Absorption spectra were recorded with a Hitachi U3310 spectrophotometer and fluorescence spectra were recorded with a modified Jobin-Yvon Fluorolog tau-3 spectrophotometer. In fluorescence mode, the microchannel plate PMT detector was gated for an 80 ms period in synchronization with the duration of the excitation pulse. Plane, ruled gratings were used for both excitation and emission monochromators. Emission spectra were recorded for optically dilute (i.e., absorbance less than 0.08 at the excitation wavelength) solutions after purging with N2. Fluorescence quantum yields were determined relative


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to Cresyl Violet35 in ethanol, Rhodamine 6G36 in ethanol and fluorescein37 in 0.1 M NaOH; secondary fluorescence standards included many closely-related Bodipy dyes studied earlier.38 For all compounds, the excitation spectrum was carefully matched to the absorption spectrum. Fluorescence lifetimes were measured at room temperature using several complementary experimental protocols. Routine measurements were made with a PTI EasyLife set-up using pulsed laser diodes (310, 440, 505, 525 or 635 nm output) as excitation source. Samples were optically dilute, absorbance being approximately 0.10 at the excitation wavelength, and 150 runs were signal averaged before data analysis. Fluorescence was isolated using a series of narrow band-pass filters. In each case, a minimum of three different time bases was used for data collection. In most cases, after iterative reconvolution of the instrument response function (IRF) and standard Marquardt analysis, single exponential fits were recovered as judged by the goodness-of-fit criteria established by Eaton39 and by O’Connor and Phillips.40 These analytical tests included minimization of chi-squared, visual inspection of weighted residuals, the Demas method of moments, and the phase plane method (for exponential decays only). The derived lifetimes, being well outside that of the IRF, were confirmed by frequency domain, phase modulation measurements made using the Fluorolog tau-3 spectrophotometer. In this latter mode, the emission wavelength was controlled by a monochromator/slit combination and data analysis was made by deconvolution against a reference compound having a mono-exponential decay. For this latter purpose, the reference compounds used after extensive purification were: 9-cyanoanthracene in cyclohexane (lEX = 310 nm, tS = 12.6 ns), coumarin-153 in methanol (lEX = 355 nm, tS = 4.2 ns), erythrosin B in methanol (lEX = 490 nm, tS = 0.50 ns), and rhodamine B in methanol (lEX = 520 nm, tS = 2.6 ns). Acid/base titrations were conducted at 20 1C using a thermostatted glass sample chamber containing a standard solution of the dye in CH3CN (20 mL). Small aliquots of HCl (gaseous) or NaOH (solid) in CH3CN were added to the stirred solution using an automatic syringe injector pump. The solution was incubated for 10 minutes after each addition and appropriate spectra recorded. Corrections were made for the small change in volume. Each titration was performed three times41 and the data averaged before calculation of the pKA values. Calculations were geometry optimized at the B3LYP42/ 6-31G**43 level for ground-state species in the gas phase. In some cases the gradient convergence criterion was relaxed from 10 4 Hartrees Bohr 1 to 2 10 4 Hartrees Bohr 1 to obtain geometry convergence within a reasonable time interval. An unrestricted Slater determinant was used with annihilation of the first spin contaminant where appropriate in anticipation that this gives improved performance for charged systems with respect to the more commonly used RHF determinant. A common determinant was employed throughout in order to compare the energy differences between charged and neutral systems. No subsequent calculations were performed to further optimize the energy. The vertical ionisation potentials were also calculated at the ab initio level employing the outer valence


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Green function (OVGF) method, which includes electron correlation and electron relaxation effects. Calculations were run on an Intel Xeon processor with 8 cores using the parallel version of the GAMESS code of Gordon et al.44 Output was visualized using the VMD system.45 Listed in the ESI† are Cartesian coordinates for the optimized structures computed for neutral and protonated molecules. All reported structural minima were supported by appropriate frequency computations. Estimation of the size of the solvent cavity, used for the dipole moment change on excitation, were made using a polarizable continuum model46 with the static dielectric constant set to 10.

Acknowledgements ´ de Strasbourg and Newcastle We thank CNRS, EPSRC, Universite University for financial support of this work. We gratefully acknowledge IMRA Europe S.A.S. (Sophia Antipolis, France) for ´phane Jacob and Gilles awarding a PhD fellowship to AN. Drs Ste Dennler (IMRA Europe) are thanked for helpful discussions.

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