Effect of Proton Substitution by Alkali Ions on the Fluorescence

Article pubs.acs.org/JPCA

Effect of Proton Substitution by Alkali Ions on the Fluorescence Emission of Rhodamine B Cations in the Gas Phase Jean-François Greisch,†,* Michael E. Harding,† Wim Klopper,†,‡ Manfred M. Kappes,†,‡ and Detlef Schooss†,‡,* †

Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ‡ Institute of Physical Chemistry, Karlsruhe Institute of Technology, Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany S Supporting Information *

ABSTRACT: The photophysics of chromophores is strongly influenced by their environment. Solvation, charge state, and adduct formation significantly affect ground and excited state energetics and dynamics. The present study reports on fluorescence emission of rhodamine B cations (RhBH+) and derivatives in the gas phase. Substitution of the acidic proton of RhBH+ by alkali metal cations, M+, ranging from lithium to cesium leads to significant and systematic blue shifts of the emission. The gas-phase structures and singlet transition energies of RhBH+ and RhBM+, M = Li, Na, K, Rb, and Cs, were investigated using Hartree−Fock theory, density functional methods, secondorder Møller−Plesset perturbation theory, and the second-order approximate coupledcluster model CC2. Comparison of experimental and theoretical results highlights the need for improved quantum chemical methods, while the hypsochromic shift observed upon substitution appears best explained by the Stark effect due to the inhomogeneous electric field generated by the alkali ions.

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colored fluorescent zwitterionic RhB± and the colorless lactone RhB0 forms. The lactone fraction is negligible in water and other protic solvents while dominant in an aprotic solvent.7−10 The combination of gas-phase ion-beam/trapping techniques and of luminescence characterization can provide new insights into rhodamine B fluorescence properties. Single ionic species can be isolated allowing focusing on isomerically frozen cationic populations while excluding interferences due to neutral species or dynamical equilibria. Solvent effects are removed and phenomena otherwise masked, such as the effect of electrostatic interactions involving alkali metals on the energetics and dynamics of chromophore excited states, are highlighted. The use of mass spectrometric techniques to investigate the photophysics of rhodamine based cations has been reported by a number of research groups; see the introduction of reference 1 for a nonexhaustive list. The present work focuses on rhodamines where the acidic proton is substituted with alkali ions, so-called cationized species. The effect of proximal alkali ions on the electronic spectra has been studied for numerous compounds in the condensed phase. In the case of naphthalene, for example, low-mass cations such as Li+ and Na+ lead to fluorescence, while an increase in the mass of the cation (e.g., for Rb+ and Cs+) leads to a dramatic decrease in fluorescence intensity and a simultaneous increase in phosphorescence.11 An increase in

INTRODUCTION For a given chromophore, the positions and intensities of the bands associated with electronic transitions can strongly depend on the environment. When comparing effects due to different solvents, this phenomenon is called solvatochromism. It shares a common origin with electrochromism, i.e., the splitting or shift of spectral lines in an electric field. We have recently become interested in electrochromism in gas-phase ions with the goal of understanding how the optical properties of organic chromophores are influenced by proximal charges toward designing appropriate antenna systems for energy transfer. An initial focus has been on rhodamine fluorophores, which are well studied in the condensed phase. Whereas our previous work1,2 discussed the effect of the absence of solvent on the optical properties of rhodamine-type cations, the present work focuses on the effect of intramolecular charges located in the direct vicinity of a chromophoric unit. In order to discuss this internal electrochromism, rhodamine B (RhB)and in particular cations deriving from ithas been selected as model system. Since the developments that led to their use as laser dyes, rhodamines have attracted considerable attention. They have been used to demonstrate microscopy and spectroscopy of single fluorophores,3,4 lasing from meso-structured waveguides,5 as well as fluorescent chemical sensors.6 Rhodamine B is typically found in solution either as a cation RhBH+ ([9-(2carboxyphenyl)-6-diethylamino-3-xanthenylidene] diethylammonium) comprising a protonated carboxylate group or as a neutral RhB involved in an equilibrium between the intensely © 2014 American Chemical Society

Received: March 21, 2014 Revised: May 2, 2014 Published: May 2, 2014 3787

dx.doi.org/10.1021/jp502833c | J. Phys. Chem. A 2014, 118, 3787−3794

The Journal of Physical Chemistry A

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systems are therefore of general interest. Electronic spectroscopy is particularly useful in this regard. A further aim of the present study is to contribute to the correct interpretation of photochemical and photophysical phenomena connected with rhodamines, especially when studied in the presence of a large salt concentration. In the present work, we pursue a systematic characterization of the effects of alkali ion complexation on the fluorescence of rhodamine dyes. Dispersed fluorescence measurements are performed, focusing on rhodamine B with the carboxylic proton systematically substituted by alkali metal cations ranging from Li+ to Cs+.

phosphorescence and a reduction of the phosphorescence lifetime were similarly observed for xanthone molecules included within zeolite cavities upon exchanging the alkali metal cations in the cavities from Li+ to Cs+; thereby supporting an alkali-metal induced external heavy-atom effect on the transitions of xanthone molecules within zeolite cavities.12 In addition to heavy-atom effects on intersystem crossing, an alkali metal ion rigidly coupled to a chromophore also generates a strong inhomogeneous electric field with a strength on the order of 107 V/cm or larger. Interactions of the chromophore ground and excited-state dipoles with the electric field produced by proximal charges leads to the observation of Stark shifts reaching ∼1000 cm−1 for some electronic transitions.13 The action of the internal Stark effect is not limited to changes in the energies of the electronic states leading to shifts of the absorption and emission spectra. It can also modulate the probabilities and rates of excited-state reactions14 as well as affect the spin−orbit coupling strength between the singlet and triplet manifolds. Compounds that undergo a significant change in their dipole moment upon excitation are expected to be most sensitive to electric fields and, consequently, to display large solvatochromic shifts. Rhodamine dyes are such a class of molecules15 as shown in references 1, 2, 16−21. Zenobi and co-workers22 were the first to investigate, in the gas phase, the effect of the substitution of the carboxylic proton by alkali metals on the absorption of rhodamine Rh575 cations. They observed that these compounds absorb in the visible and that the absorption maximum undergoes a systematic blue shift upon exchanging the proton for Li+, Na+, K+, Rb+, and Cs+. This shift reaches approximately 850 cm−1 for Rh575Cs+ compared to Rh575H+. They inferred from their results that the metal cations bind to the carboxylic group and prevent the formation of a lactone ring.22 The increased blue shift of the absorption of rhodamine Rh575M+ as the alkali metal, M, is varied from Li and Na to K was explained by an increase in the dipole moment of the carboxyphenyl−alkali metal moiety.22,23 It is interesting to note that alkali cationization of organic moieties can have many consequences beyond changes to optical properties. Cationized molecular ions may undergo a McLafferty rearrangement upon collisional activation leading to fragmentation pathways differing significantly from their protonated analogues.24,25 The nature of the alkali metal involved may also play a role. For example, the butyl esters cationized by lithium rearrange, whereas those cationized by sodium do not.24 Another example is the binding of alkali metal cations to the amino or carboxylic groups of amino acids. In that case, stabilization of one form (zwitterionic or nonzwitterionic) is both dependent on the alkali metal size and on the amino acid involved. Gas-phase lithiated arginine, for example, is found to be both theoretically and experimentally nonzwitterionic, whereas sodiated arginine is predominantly zwitterionic with only a small fraction in the nonzwitterionic form. Increase of the alkali metal ion size stabilizes the zwitterionic form.26−28 Analogous trends with the alkali-metalion size have been reported for cationized lysine and ε-Nmethyllysine,29 whereas the opposite trend was reported for Na+ and Rb+ complexes with glycine, alanine, and analogues of these amino acids.30 Since the nature of the alkali metal and therefore the difference in the charge distribution at the binding site affects fragmentation channels and rearrangements,31 it can have both biological and sequencing implications. Cationization studies highlighting structural and electronic effects on model

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EXPERIMENTAL AND THEORETICAL METHODS A. Sample Preparation. Rhodamine B was obtained from Sigma-Aldrich (Steinheim, Germany) and used without further purification. A stock solution (10−2 M in MeOH) was diluted in dimethyl sulfoxide (DMSO) to reach 10−4 M. Cationization was achieved by adding 0.1 to 0.5% v:v of a saturated solution of the corresponding alkali chloride/iodide salt in DMSO before electrospraying. B. Trapped Ion Laser-Induced Fluorescence. The experimental setup described in reference 2 was operated as described in reference 1. Briefly, the gas-phase ions are produced using a nanoelectrospray source and stored in a temperature regulated Paul trap held at 85 K throughout the experiment. Mass selection is performed using the stored waveform inverse Fourier transform (SWIFT) method.32 The ion density in the trap center is then increased by raising the radio frequency amplitude from a trap parameter33 of qz = 0.3 to qz = 0.8. Thermalization of the trapped ions is achieved by collisions with helium at a pressure of ∼0.2 mbar. Trapped ions (∼104) are excited by the 488 nm line of a cw Ar+-laser (Spectra Physics 2080−15S) incident normal to the ion trap axis. Neutral density filters are used to attenuate the laser power while narrow band-pass filters (Semrock) are used to clean the laser beam spectrally. An electromechanical beam shutter provides synchronization with the experiment. A f = 500 mm lens focuses the laser beam which passes through 1.2 mm holes in the ring electrode to the center of the ion trap. The laser beam at the trap center is approximately 0.2 mm in diameter (1/e2). Ion fluorescence is collected perpendicularly to the excitation beam by a microscopy objective (Zeiss EC PlanNeofluor5x/0.15) through a 3 mm diameter aperture in one of the end-caps. The emitted light is first passed through a long-pass laser edge filter (Semrock) to remove scattered excitation light and then focused into a fiber and sent to a spectrograph (Triax 190, Jobin-Yvon, Horiba) equipped with an electron-multiplying charge coupled device for detection (Newton EMCCD, Andor). C. Structure and Energy Level Modeling Using Density Functional Methods and Wave Function Theory. Structure preoptimization of the cationized rhodamine B compounds was performed using OPENMOPAC.34 All other computations were carried out using the TURBOMOLE program package.35 All Hartree−Fock (HF), B3LYP,36−38 second-order Møller−Plesset (MP2), and second-order approximate coupled-cluster (CC2)39 computations were carried out in conjunction with the def2-SVP,40,41 def2SVPD40−42 and def2-TZVPP41,43 basis sets. In order to reduce the computational demands without compromising the targeted accuracy, the resolution-of-identity (RI)sometimes called density fitting (DF)approximation was applied in all 3788



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Figure 1. Fluorescence spectra of (a) RhBH+ and (b−f) RhBM+ with M = Li+, Na+, K+, Rb+, and Cs+ under 488 nm excitation. (−(x−xc)/w)

−(x−xc)/w+1] using extreme value functions y = y0 + Ae[−e  particularly well suited to asymmetric peak fittingwas used. The maximum of the main peak shows a significant shift to higher energies upon replacement of the acidic proton of RhBH+ by an alkali ion. In addition, the shift increases with the size of the alkali ion involved, from 0.06 eV for Li+ to 0.11 eV for Cs+. The systematic blue shift observed upon cationization is assessed by plotting the maxima of the emission spectra against the ionic radii of the cations involved. Although the ionic radii vary with coordination number and spin state and are therefore not an intrinsic property of the ions, their values are sufficiently transferable to highlight periodic trends. We have chosen the Pauling crystal ionic radii since they are typically preferred for electrochemical and solvation problems.47 In short, the smaller the alkali-metal cation the stronger the electric field at the RhB moiety and, consequently, the larger is the expected inductive effect on the ligated molecule. B. Structures. In solution, Rhodamine B typically exists as a cation RhBH+ or as a neutral RhB involved in equilibrium between the intensely colored fluorescent zwitterionic RhB± and the colorless lactone RhB0 forms. The (a)protic character of the solvent and not its polarity is found to determine the position of the zwitterion RhB± ↔ lactone RhB0 equilibrium.48,49 Dry DMSO solutions, for example, are found to be entirely colorless, indicating complete conversion to the lactone.48 The electrosprayed solution includes traces of a protic solvent (MeOH), both forms are therefore expected to coexist as supported by the coloration of the electrosprayed solutions. Upon electrospraying rhodamine B in absence of alkali metals, the detected ion signal corresponds to the cation RhBH+. Adding alkali metals to the electrosprayed solution leads to the partial substitution of the proton on the carboxylic group by a Li+, Na+, K+, Rb+, or Cs+ cation. Since the gas-phase species, RhBM+ with M = Li, Na, K, Rb, or Cs are all found to fluoresce, the lactone form appears to be ruled out in the gas phase.

B3LYP, MP2, and CC2 computations employing the corresponding RI basis sets.43−45 On the basis of the near quantitative agreement of the B3LYP/def2-SVP level found in reference 1, the preliminary OPENMOPAC structures were reoptimized using B3LYP. In addition to that, the ground state singlet geometries of cationized rhodamine B were also optimized at the HF and MP2 levels of theory employing the def2-SVP and (def2-TZVPP) bases. The latter (def2-TZVPP) basis is the def2-TZVPP basis, except for the hydrogens and the diethylamino side chains of rhodamine B, which are described by def2-SVP. Note that in the case of the parent compound (prior to alkali substitution) the carboxylic hydrogen is described with the full def2-TZVPP basis. All transition energies (i.e., energies corresponding to both the absorption and emission processes) have been evaluated by configuration interaction singles (CIS), in the framework of time-dependent density functional theory via TD-B3LYP, and via CC2 linear response theory. A self-consistent-field convergence threshold of 10 −9 hartree and geometry convergence thresholds of 10−8 hartree and 10−5 hartree/ bohr for the total energy and the Cartesian gradient, respectively, were used. In the B3LYP computations the numerical quadrature was performed on TURBOMOLE’s grid m4. All MP2 and CC2 computations have been carried out correlating only the valence-electrons. Point charges where obtained via a natural population analysis (NPA).46

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RESULTS AND DISCUSSION A. Fluorescence Emission Measurements. The fluorescence spectra of RhBH+ and RhBM+ with M = Li, Na, K, Rb, and Cs are displayed in Figure 1. The spectral profiles of the cationized species are similar to that of RhBH+.1,19 The spectra shown in Figure 1 result from the averaging of several sets of spectra corresponding to different ion numbers and excitation intensity, therefore their signal-to-noise ratios are not directly comparable. A quantitative comparison of the compounds’ brightness is beyond the scope of the present paper. To determine the maximum of the main peak, a two-component fit 3789



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Figure 2. Ground state structures (HF/def2-SVP) of (a) RhBH+ and (b) RhBNa+ comprising definitions of the distances d1 and d2 (see text)); (c) representation of carboxyphenyl−M+ moieties in the ground state S0 (HF/def2-SVP) and (d) in the singlet excited state S1 (CIS/def2-SVP) of the optimized geometries of RhBM+ (for M = Li+, Na+, K+, Rb+, and Cs+, green circles in order of increasing ionic radii).

In all computed ground-state structures of RhBM+, at all levels of theory considered, the alkali ion M+ is bound to the deprotonated carboxyphenyl group in a near-collinear geometry (see Figure 2) even when the alkali metal is large enough to potentially form a charge solvated structure with the xanthene oxygen. Systematic changes in the geometries largely reduce to a change in the position of the alkali metals relative to the carboxyphenyl and xanthene subunits, as defined by d1 and d2 in Figure 2. These changes directly correlate with the computed dissociation energies between H+, Li+, Na+, K+, Rb+, Cs+, and rhodamine B, RhB, as summarized in Table 1 (H+ > Li+ > Na+ > K+ > Rb+ > Cs+).

Table 2. Vertical Transition Energies (Computed at the S0 Geometry) in Comparison to Experimental Fluorescence Emission Energies (All Values in eV)

Table 1. Selected Structure Parameters d1 and d2 As Defined in Figure 2 of RhBH+ and RhBM+, at the HF/def2-SVP (S0) and CIS/def2-SVP (S1) levels, and Computed Vibration-Less Dissociation Energies (BDE) of H+, Li+, Na+, K+, Rb+, Cs+ with RhB at the HF/def2-SVP, B3LYP/def2-SVP, and MP2/ def2-SVP levels dist (pm) species

d1 (S0/S1)

d2 (S0/S1)

BDE HF

BDE B3LYP

BDE MP2

186/186 214/214 251/251 291/291 308/308 327/327

653/654 589/593 610/616 635/643 645/655 658/668

1112 351 255 181 159 137

1117 384 288 214 192 172

1094 340 248 182 167 148

method a

B3LYP

HF

MP2

MP2

method b

TD-B3LYP

CIS

CC2

CC2

2.73 2.76 2.79 2.80 2.80 2.80

3.82 3.89 3.93 3.98 3.98 3.98

2.48 2.52 2.55 2.58 2.57 2.58

2.47 2.50 2.52 2.54 2.55 2.55

species

expt

RhBH+ RhBLi+ RhBNa+ RhBK+ RhBRb+ RhBCs+

2.325(2) 2.39(1) 2.420(3) 2.438(2) 2.442(2) 2.437(2)

a

Method used for the ground-state optimization. bMethod used to compute the transition energy. cFor a description of the basis set, see text.

the S1 → S0 energy. This is also the case for the single point computations at the CIS S1 geometry using TD-B3LYP and, to a much lesser extent, for CC2. The CC2 computation at the CC2 excited state geometry, on the other hand, underestimates the experimental result. Considering a RhBH+ Stokes shift of ∼0.05 eV19 in the gas phase additionally allows comparison of the computed vertical transition energies at the S0 geometry (Table 2). Here all computational results overestimate the experimental ones with the CC2//MP2 results closest to the experimental values. Interestingly, TD-B3LYP performs worse than for the transition at the S1 geometry, which indicates fortuitous error compensation. Comparing CC2/def2-SVP and CC2/(def2TZVPP) only a very minor basis set dependence is observed. This also applies to the TD-B3LYP and CIS methods: computations employing the def2-SVPD basis, which includes a significant number of additional diffuse functions, indicate a similar (fast) convergence. Next we discuss the performance of different methods with regard to the cationized species RhBM+. For all methods, the computed vertical transition energies at the ground state geometry reproduce the trend in the blue shift observed for the dispersed fluorescence emission measurements. However, the magnitude of the computed transition energies varies significantly. Again, the experimental values are best reproduced at the CC2//MP2 level. The situation changes when vertical transition energies at the S1 geometry are considered. Here only

def2-SVP (kJ/mol)


(def2-TZVPP)c

def2-SVP

C. Transition Energies. Vertical transition energies at the optimized ground state (S0) geometry have been computed using time-dependent B3LYP (TD-B3LYP), configuration interaction singles (CIS), and CC2 response methods (Table 2). As fluorescence emission is expected to occur mainly from a minimum on the S1 potential energy surface, the ground state geometry has therefore been relaxed within the S1 state at the TD-B3LYP, CIS, and CC2 levels. The vertical transition energies at the optimized excited state geometry (S1) are summarized in Table 3. To compare experimental and computed results, we first focus on the (arguably) simplest case, RhBH+. As described before,1 the experimental emission and the TD-B3LYP vertical transition energy at the S1 geometry almost perfectly agree (see Table 3). Contrarily, the CIS result significantly overestimates 3790



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Table 3. Vertical Transition Energies (Computed at the S1 Geometry) in Comparison to Experimental Fluorescence Emission Energies (All Values in eV) (def2-TZVPP)c

def2-SVP method a

TD-B3LYP

CIS

CIS

CIS

CC2

CC2

method b

TD-B3LYP

CIS

TD-B3LYP

CC2

CC2

CC2

2.38 2.47 1.06 0.68 0.36 0.31

3.70 3.76 3.80 3.82 3.83 3.83

2.76 2.79 2.81 2.81 2.76 2.71

2.50 2.54 2.56 2.58 2.58 2.59

2.08 2.18 2.17 d d d

2.06 2.18 2.20 2.11 d d

species

expt


2.325(2) 2.39(1) 2.420(3) 2.438(2) 2.442(2) 2.437(2)

a

Method used for the excited-state optimization. bMethod used to compute the transition energy. cFor a description of the basis set see text. dThe corresponding minima of the S1 surface could not be located, see text.

transition energies obtained at the CIS geometries are able to reproduce the experimental trend. The CIS excited state equilibrium geometries are very similar to those obtained for the ground state at the HF level. CIS again overestimates the transition energies, whereas CC2 at the CC2 excited-state geometry underestimates them. Using the very small def2-SVP basis, the CC2 excited-state geometry optimizations are expected to suffer from basis set incompleteness and superposition errors. Here no minima could be located for the K+, Rb+, and Cs+ adducts. But even with the significantly larger basis set (def2-TZVPP) only a few minima could be located. Despite large overestimation of the transition energies by the CIS computations, and to a lesser extent by the CC2//CIS ones, these two methods reproduce the overall experimental trend well (see Figure 3 and Figure SI-1, Supporting

effects beyond the current calculations since we have found no evidence of the existence of a different isomer in the case of RhBCs+. In contrast, the TD-B3LYP results are consistently in disagreement with experimental values as well as with all other theoretical methods employed, regardless of the excitedstate structure used (see Table 3). D. Point-Charge Model. To shed more light on this peculiar behavior, we performed additional investigations of the dependence of the transition energy on the RhB−M+ distance, with the alkali ion modeled by a unitary point charge. To simulate the different alkali ions, the position of the point charge was varied in a stepwise fashion interpolating the RhBLi + and RhBCs+ CIS-optimized structures (see Figure 4 and Figure SI-2).

Figure 3. Comparison of the experimental and computed fluorescence emission of RhBH+ and RhBM+ (M = Li, Na, K, Rb, Cs): experimental transition energies (black, left scale), experimental shifts relative to RhBH+ (black, right scale), and shifts relative to RhBH+ computed using CIS/def2-SVP//CIS/def2-SVP (red, right scale) and CC2/def2SVP//CIS/def2-SVP (blue, right scale), respectively.

Figure 4. Comparison of the experimental and computed fluorescence emission of RhBH+ and RhBM+ (M = Li, Na, K, Rb, Cs): experimental transition energies (black, left scale), experimental shifts relative to RhBH+ (black, right scale), shifts relative to RhBH+ computed using a point-charge instead of the alkali ion employing CIS/def2-SVP (red, right scale) and CC2/def2-SVP (blue, right scale) at the interpolated geometries of RhBLi+ and RhBCs+ obtained with CIS/def2-SVP (see text).

Information). It is interesting to note that the blue shifts computed using the CIS method level off for the larger alkali ions as do the experimental data. On the basis of a Coulomb interaction (see below), this behavior is expected to occur for large distances between the alkali ion and the neutral (zwitterionic) chromophoric species. The fact that it already occurs for short distances suggests that other effects (well reproduced by CIS computations) may also play a role. The small decrease of the emission energy in the case of the Cs+ adduct compared to the Rb+ adduct likely involves further

Even when replacing the cation by a point charge, the experimentally observed blue shift is almost quantitatively reproduced. The latter results do not change significantly using the full def2-TZVPP basis and are therefore believed to be nearly independent of the basis set. These computations also revealed artificial avoided crossings, as displayed in Figure SI-3 in the Supporting Information, probably due to spurious charge transfer artifacts present at the TD-B3LYP level. In contrast, the CIS and CC2 results do not 3791



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ΔEFe,g⃗ both decrease from H+ to Cs+). Overall, a large part of the experimentally observed shift is found to be reproduced by the simple electrostatic model, thereby supporting the electrochromic nature of the observed phenomenon.

show any indication of an avoided crossing or a seam within the studied distance range (see Figure SI-4 and Figure SI-5). B3LYP transition energies very similar to those obtained at the CIS level can be produced by increasing the amount of HFexchange in the B3LYP functional. Of course the results acquired via tuning the HF-exchange parameter in B3LYP simply indicate that the original TD-B3LYP model cannot be applied to the computation of transition energies of the molecular systems under study. E. Simplified Electrostatic Model. In order to estimate to what extent the interaction between the alkali metal ion and the chromophore results from the strong intramolecular electric field, one can resort to an even simpler electrostatic model considering only Stark shifts. They can be assessed by evaluating the electrostatic interaction energy in the ground (g) and excited (e) states in the presence of the electric field F⃗ generated by a point charge representing the alkali ion.50 The overall energy shift of the emission band is then given by Δeg = ΔEFe ⃗ − ΔEFg ⃗ where ΔEFg ⃗ and ΔEFe ⃗ are the ground and excited state Stark shifts of the neutral RhB species. Because of the inhomogeneous nature of the intramolecular electric field, the charge distribution in the molecule must be explicitly considered. The ground state and excited state shifts are then given by ΔEFe,g⃗ = Q.∑i((qe,g i )/(|ri − R|)) where Q is the charge modeling the electric field; qe,g i is the charge on the ith atom of the molecule in states e and g; ri and R are the position vector of the ith atom of the molecule and the field source Q, respectively.13 The values of ri for RhBM+ were obtained from our computations at the HF and CIS levels employing the def2SVP and def2-TZVPP basis sets, the reference center was set at the position of the alkali ion. The field source Q corresponding to the alkali ion was taken as the elementary positive charge (Q = 1) and the charges qe,g i were taken from a natural population analysis. The RhBH+ shift is then subtracted from Δeg in order to obtain the shifts of the electrostatic model displayed in Figure 5. Within the electrostatic model, the electric field strength acting on the chromophore decreases from H+ to Cs+, leading to a (relative) destabilization of both S0 and S1. However, because of different Stark shifts in the ground and excited states (here ΔEFe ⃗ < ΔEFg ⃗ < 0), the energy separation between S1 and S0 increases from H+ to Cs+ leading to the observed blue shift (the

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CONCLUSION Mass spectrometric methods can be effectively used to manipulate ions in order to study their intrinsic properties. An important advantage of gas-phase measurements is the selective study of perturbations, for example involving solvent molecules, adducts, complexes, and so on, composing the total environmental interaction. This allows deducing the relative contributions of each of these interactions, for instance on the structure and electronic states. Gas-phase measurements can be particularly illuminating in the case of molecules cationized by metal ions for which solution measurements are typically hindered by solvent and dynamical equilibrium effects. Mass spectrometry allows isolating ions and, most importantly for the present study, to separate species cationized by a series of different alkali ions. In this work, the effect of the different alkali ions on the luminescence properties of rhodamine B have been probed experimentally and described theoretically. Using trapped-ion laser-induced-fluorescence spectroscopy we have studied the dispersed emission features of RhBH+ and RhBM+ with M = Li+, Na+, K+, Rb+, and Cs+ excited at 488 nm. The emission maximum shows a significant shift to higher energies upon replacement of the acidic proton of RhBH+ by an alkali ion. The shift increases with the ionic radius. Although none of the applied methods (ranging from TD-B3LYP, CIS, to CC2) is able to quantitatively reproduce the observed effect, the trend is best reproduced by CIS computations. Additionally, it was found that special caution should be exercised if states of compounds cationized by Na+−Cs+ are modeled with TD-B3LYP. Finally, it is clear from the comparison of the experimental and theoretical results presented herein that the systematic and significant blue shifts observed result mainly from an intramolecular Stark effect.

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ASSOCIATED CONTENT

S Supporting Information *

Figures SI-1 and SI-2 showing the absolute transition energies measured and computed and Figures SI-3, SI-4, and SI-5 displaying the energy levels of single point computations at the TD-B3LYP/def2-TZVPP, HF/def2-TZVPP, and CC2/def2TZVPP levels at the CIS/def2-SVP geometries where the alkali metal has been replaced by a point charge. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Authors

*(J.-F.G) E-mail: [email protected]. * (D.S.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the Deutsche Forschungsgemeinschaft (DFG) as administered by the transregional collaborative research center SFB/TRR 88 “3MET” (C1 and C7). We are also grateful to the Bundesministerium für Bildung und Forschung (BMBF) through the Helmholtz Research Program POF “Science and Technology of Nano-systems” and

Figure 5. Comparison of the experimental and computed fluorescence emission of RhBH+ and RhBM+ (M = Li, Na, K, Rb, Cs): experimental transition energies (black, left scale), experimental shifts relative to RhBH+ (black, right scale), shifts relative to RhBH+ computed within the electrostatic model using CIS S1 geometries and point charge analysis (NPA) with two different basis sets: def2-SVP (red, right scale) and def2-TZVPP (blue, right scale), respectively. 3792



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to the State of Baden-Württemberg for providing the necessary infrastructure. J.-F.G would like to acknowledge support from the Alexander von Humboldt Foundation.

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Effect of Proton Substitution by Alkali Ions on the Fluorescence

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