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Solvent impact on the photophysical properties and excited state behavior of p-aminobenzoic acids
Journal of Photochemistry and Photobiology A: Chemistry 305 (2015) 60–66
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Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Solvent impact on the photophysical properties and excited state behavior of p-aminobenzoic acids Jacob A. Boroff, Zachery D. Matesich, Daniela Canache Stuetzer, Sarah J. Schmidtke Sobeck * College of Wooster, Department of Chemistry, 943 College Mall, Wooster, OH, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 November 2014 Received in revised form 4 February 2015 Accepted 4 March 2015 Available online 7 March 2015
The photophysical properties of p-aminobenzoic acid and dimethyl-p-aminobenzoic acid are compared in a range of solvents. The electronic spectra are used to determine quantum efficiencies and evaluate excited state behavior. The latter system is found to undergo a charge transfer following photoexcitation and the thermodynamics for the process are assessed. The strength and type of solvent–solute interaction are important in determining the driving force for the charge transfer, with entropy being dominant in protic solvents and enthalpy in aprotic solvents. Vibrational spectroscopy and calculations provide further insight into the nature of the specific solvent–solute interactions, with shifts in the infrared spectrum observed due to hydrogen bonding interactions with the solvent. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Charge transfer Photochemistry Quantum efficiency Solvent effects
1. Introduction Photo-induced charge transfer reactions are a fundamental class of chemical reaction found in a variety of complex materials and biological systems, as well as small molecules [1–4]. Following photoexcitation charge rearrangements may occur in a system and such rearrangements can have practical applications in materials, for example providing a means to tune the wavelength of emitted light [5,6]. In biological systems electron and proton transfer reactions are fundamental in the working of many enzymes [7]. Within a variety of systems it is found tuning the solvent and types of intermolecular interactions in the system can alter the photochemistry observed. Properties of the local environment, such as ability to form strong hydrogen bonding interactions or polarity, may turn on or off a reaction pathway [1,4,8]. In this study we seek to gain a more complete understanding of the role of the local solvent environment upon photochemistry of a small model system. Small organic molecules, with relevant electron donor and acceptor moieties, provide a means to better understand how electronic character and interactions with the solvent change the observed physicochemical properties of a system. In this investigation we are focused upon para-aminobenzoic acid (PABA) derivatives, where the electron donor moiety is an amine and
* Corresponding author. Tel.: +1 330 263 2359; fax: +1 330 263 2386. E-mail address: [email protected] (S.J. Schmidtke Sobeck). http://dx.doi.org/10.1016/j.jphotochem.2015.03.005 1010-6030/ ã 2015 Elsevier B.V. All rights reserved.
the acceptor a carboxylic acid group. These compounds are similar to the extensively studied dimethylaminobenzonitriles (DMABN), which contain a cyano acceptor moiety [4,8–11]. When the electron donor group is a tertiary amine it may undergo an intramolecular charge transfer (ICT) to form a zwitterionic (ZI) product following photoexcitation, as shown in Fig. 1 [12,13]. As in the case of DMABN, the reaction proceeds via a twisted intramolecular charge transfer (TICT) mechanism in which the amine donor twists to a perpendicular conformation relative to the phenyl during the course of the electron transfer [4]. The resulting ZI product is unique electronically and structurally from the LE state. The ICT can be traced using fluorescence, as there are unique emission bands from the locally excited (LE) reactant and TICT product (ZI). The ZI emission occurs at a significant Stokes shift, relative to the LE emission, as illustrated in the potential energy surface for the reaction shown in Fig. 2(I). The differences in the viability of the charge transfer based upon the electron donor is attributed to the sp2-like nature of the tertiary amine, which makes it a more favorable reaction surface to form the ZI product in accordance with the Hammond postulate [14]. Calculations indicate a difference of about 5 degrees for the R– N–R bond angle and 15 degrees for the planarity of the amine moiety to the phenyl ring in the ground state of the two compounds. PABA has a nearly ideal sp3 configuration, whereas DABA deviates to a more planar configuration and larger R–N–R angle [13]. This is consistent with past studies of DMABN derivatives where no ICT is observed for the corresponding primary and secondary amines [4]. Additionally for DABA,
J.A. Boroff et al. / Journal of Photochemistry and Photobiology A: Chemistry 305 (2015) 60–66
R
O
O R
N R
OH
LE
N
R
OH
ZI
Fig. 1. Proposed twisted intramolecular charge transfer (TICT) reaction from the locally excited (LE) to zwitterionic (ZI) state for PABA (R = H) and DABA (R = CH3).
adjusting the pH of the solvent environment can also effectively turn off the ICT reaction due to protonation of the amine donor [13]. Mitchell et al. provide another explanation for absence of ICT in PABA [15]. A combination of experimental and computational methods is used to evaluate the effective push–pull nature of PABA. Both the amino donor and carbonyl acceptor contribute electron density to the phenyl upon excitation. The contribution from the carbonyl acceptor group counteracts the expected dipole change upon excitation and weakens the push–pull nature of the system [15]. Differences due to the amine substituent can be assessed using the Hammett parameter for DABA and PABA, 0.83 and 0.66, respectively [16]. The tertiary amine is a stronger electrondonating group and as such creates a stronger “push” in the donor– acceptor system. For this reason ICT is predicted to be more favorable for a tertiary amine donor, such as DABA, relative to the primary amine, PABA. Extensive studies have been carried out to explore the impact of both structural modifications upon the photochemistry, and viability of ICT, in the DMABN system with a nitrogen donor and acceptor [4,17,18]. The studies have shown that the ICT reaction dynamics for DMABN and its derivatives are solvent-dependent [4,10,19–23]. Additionally, there are variations in the dynamics observed when the electron acceptor is adjusted from a cyano to a carbonyl or fluoro acceptor [4,24]. There is a greater extent of ICT in polar solvents due to the stabilization of the ZI. The relative stability of the LE and ZI states impacts the vertical position of the excited state potential energy wells, due to differences in the dipole moments of the two states and stabilization of the polar ZI by the solvent, as shown in Fig. 2(II). Stabilization of the ZI, relative to the LE state, leads to a decreased ICT barrier. This figure also illustrates that the relative horizontal separation of the LE and ZI states can be impacted by the solvent, as stronger solvent–solute interactions result in a greater solvent reorganization accompanying the ICT reaction and leads to an increased reaction barrier. The fluorobenzene derivatives are less sensitive to solvent effects
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than the other DMABN derivatives, due to similar dipole moments for both excited states [24]. The impact of the different stabilization and solvent reorganization energies is quantified by examination of the excited state thermodynamics. Druzhinin et al. reported the ICT activation energy and overall thermodynamics for DMABN using temperature- and time-dependent spectroscopies. The data is fit with a Stevens–Ban plot, accounting for quantum efficiencies and lifetimes of the excited state species, to yield thermodynamics. In acetonitrile, at low temperatures, there is an activation energy of 7.7 kJ/mol. In the high-temperature regime the reaction appears barrierless and the ICT reaction is exothermic by 23.2 kJ/mol [8]. The excited ZI state lifetime has a direct relationship with solvent polarity, with the longest lifetimes in the most polar solvents [23]. In this investigation the role of solvent environment on the physicochemical properties of the PABA and DABA systems is further explored. This expands the past DMABN studies to focus upon a system that has an oxygen-based, carboxylic acid, electron acceptor moiety. The quantum efficiencies of the two compounds are measured in a range of solvents with varying types of intermolecular interactions. Temperature-dependent fluorescence spectroscopy is used to evaluate the reaction thermodynamics for DABA in order to assess how the solvent impacts the reaction driving force. Analysis of the molecular vibrations, using experimental and theoretically predicted infrared (IR) spectra, allow us to gain an understanding of the relative strength of the intermolecular interactions for hydrogen bonding solvents, like methanol. Together this analysis provides insight into how solvent–solute interactions affect the photochemistry of this model system. 2. Materials and methods 2.1. Sample preparation 4-Aminobenzoic acid (Sigma, 99% pure) and 4-(dimethylamino) benzoic acid (Aldrich, 98% pure) were used as received. Solutions were prepared with the following ACS grade solvents: methanol (Pharmco-Aaper, 99.9%), acetonitrile (Pharmco-Aaper, 99.9%), and dichloromethane (Pharmco-Aaper, 99.99%). Stock solutions were diluted for emission studies to ensure the maximum absorbance did not exceed 0.1 absorbance units. A fluorescein standard solution was prepared by diluting 5 mL of fluorescein (Molecular Probes Reference Dye Sampler Kit 1 mM) to 10 mL with 0.1 M sodium hydroxide (BDH Chemicals, 1.00 N). A standard quinine sulfate solution was made by diluting 20 mL of quinine sulfate (Molecular Probes Reference Dye Sampler Kit 1 mM) to 10 mL with 0.5 M sulfuric acid (BDH Chemicals). A standard tryptophan solution was prepared by dissolving a small amount of DL-tryptophan (Alfa Aesar, 99%) in 50 mL of distilled water. 2.2. Spectral measurements
Fig. 2. Potential energy surface for the ICT reaction showing the absorbance (blue) and emission (green, red) processes (I). Solvent effects on the relative positioning of the LE and ZI excited states are shown (II). The initial ZI well (solid black) relative to the LE state (blue) is compared to a case where a polar solvent preferentially stabilizes the ZI (dashed black) and a solvent with greater solvent reorganization accompanying the ICT (ZI dotted black). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Absorbance spectra were collected at room temperature on a Varian Cary 50 Bio Spectrophotometer. The instrument was blanked with the respective solvent for each sample, and the absorbance was measured from 200 to 400 nm with a scan rate of 600 nm/min in the dual beam mode. Fluorescence spectra were collected on a Varian Cary Eclipse Spectrofluorimeter. Samples were excited at the wavelength of maximum absorbance and the PMT setting was adjusted to achieve sufficient signal to noise (600 V for PABA and 800 V for DABA). Fluorescence spectra were recorded with a 120 nm/min scan rate, 5 nm slit width, and 1 nm step size. Variable temperature scans were taken at 5 C increments, using a Peltier-thermostatted cell. The sample was allowed to equilibrate for 5 min, with continual stirring in the cell, after the desired temperature was reached to
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ensure thermal equilibrium. A blank sample, of pure solvent, was measured under identical settings to ensure no significant solvent background was present. The emission profiles for the three fluorescence standards were used to determine the fluorimeter correction factor by comparison to standard spectra [25]. The wavelength-dependent correction factor was applied to all emission spectra prior to analysis to account for variability in the detector sensitivity across the spectral range. Infrared (IR) spectra were obtained using a Nicolet 6700 FTIR with ATR accessory. Spectra were taken for each compound in its solid form. For the solvated compound a few drops of solvent was put on the solid sample prior to recording the spectra. 2.3. Quantum yield analysis Quantum yields were determined based upon literature protocols [26,27]. Briefly, for each quantum standard five dilutions were prepared with a range of absorbance values up to a maximum of 0.08 absorbance units. An absorbance and emission spectra was taken for each solution. This was carried out at both PMT settings for each standard. The same procedure was carried out for the PABA and DABA solutions, in each solvent, at the appropriate PMT setting. The temperature was controlled using a Peltier-thermostatted cell. 2.4. Spectral fitting The fluorescence spectra were analyzed using the OriginPro 81 software. Spectral wavelengths were converted to frequency (cm1). Intensity (I) was scaled according to the relationship between the wavelength and frequency-dependent measurements for a wavelength-based grating instrument: IðvÞ ¼ l2 IðlÞ [25]. Spectra were fit using the Peak Analyzer application modeling each
peak as a Gaussian function to determine the area under each emission band for analysis. 2.5. Computational detail PABA and DABA were optimized in the protonated and deprotonated forms of the carboxylic acid in Gaussian 09 [28]. The optimizations were performed using the hybrid DFT B3LYP level of theory with the 6-31G + (d,p) basis set [29–31]. Calculations were performed with and without implicit solvation using the default polarizable continuum solvent model (IEFPCM) with solvent parameters for methanol [28]. Complexes of PABA and DABA with a single methanol hydrogen bonded at the acid moiety were optimized to model explicit solvation effects. Frequency calculations were performed for all optimizations to verify the nature of the stationary points, and to predict infrared spectrum for the free solute and explicit solvent–solute complex. 3. Results 3.1. Quantum yields Quantum yields were determined by comparison to a set of three fluorescence standards (fluorescein, quinine sulfate, tryptophan) and values reported are an average. Briefly, the integrated fluorescence was plotted as a function of absorbance and the slope of the line was found [26,27]. The slope was compared to the data for the fluorescence standards, using the following equation, to evaluate the quantum efficiency: ! Gradx h2x fx ¼ fSt (1) GradSt h2St
Fig. 3. Absorbance (I) and emission (II) data for quantum yield determination of PABA in methanol at room temperature, arrow indicates increasing concentration. The quantum yield was found as the slope of the line of emission area versus absorbance area (III).
J.A. Boroff et al. / Journal of Photochemistry and Photobiology A: Chemistry 305 (2015) 60–66
where x denotes the unknown, St the standard, f is the quantum yield, Grad is the slope of the line, and h the index of refraction of the solvent [28]. For PABA there is a single emission band, allowing direct analysis of the data using Eq. (1). Sample absorbance and emission scans, as well as the resulting quantum efficiency plot, for PABA in methanol is shown in Fig. 3. For DABA there is dual emission, with the LE emission at a higher energy than that from the ZI state. Modeling the spectra allowed determination of the contribution individually from each of the two excited states, and the quantum efficiency plots were made for both the LE or ZI species. Fig. 4 shows a representative fitting of the DABA spectrum to evaluate the quantum efficiencies of the LE and ZI states independently. Values for the quantum efficiencies for both compounds in the different solvents are reported in Table 1. 3.2. ICT reaction and thermodynamics Static absorbance and emission spectra were used to indicate the presence of ICT. A large Stokes shift in the emission is indicative of the presence of the ZI species. Fig. 5 shows the static absorbance and emission spectra for PABA and DABA in the three solvents at room temperature. As illustrated in this figure only DABA exhibits ICT. The magnitude of the ZI band relative to the LE state is greatest in acetonitrile and the least in methanol. The temperature-dependent emission spectra were modeled and the spectral changes were assessed using a Stevens–Ban plot to yield the ICT thermodynamics [32]. The analysis is based upon the following equation: DH DS þ (2) lnðK ICT Þ ¼ RT R
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Table 1 Solvent- and temperature-dependent quantum yields for PABA and DABA. Statistical error from linear fits in parentheses. Solvent
Temp (K)
PABA/102
DABA (LE)/105
DABA (ZI)/102
CH3OH
295 323
3.5 (0.1) 3.8 (0.2)
7.3 (0.5) 15 (2)
0.122 (0.003) 0.11 (0.01)
CH3CN
295 323
18 (3) 11.6 (0.3)
7.0 (0.8) a
0.218 (0.009) 0.54 (0.02)
295 303
9.6 (0.7) 9.7 (0.2)
71 (16) 89 (9)
1.28 (0.08) 1.47 (0.06)
CH2Cl2 a
Emission from LE state was not quantifiable.
where A indicates the area under the emission band, and f is the quantum efficiency of the given state. The area under each emission band is determined from the spectral fit, with each emission band modeled as a Gaussian curve, and using the average quantum efficiency in the temperature range. Representative spectral fits for all solvents are found in the supplementary data. The Stevens–Ban plots for DABA in all solvents are shown in Fig. 6 and the resulting thermodynamics summarized in Table 2. A positive enthalpy change indicates activation thermodynamics, whereas a negative enthalpy change indicates a barrierless reaction [8]. In both acetonitrile and dichloromethane the ICT was found to be barrierless with a decrease in the enthalpy and entropy accompanying the reaction. In methanol there is a reaction barrier and the activation thermodynamics are measured, with a positive enthalpy and entropy of activation.
where KICT is the equilibrium constant for the ICT, DH the reaction enthalpy, DS the reaction entropy, R the gas constant, and T the temperature. A plot of the equilibrium constant as a function of temperature yields the reaction thermodynamics. The equilibrium constant was calculated based upon the ratio of the ZI to LE populations as experimentally determined by the area under the LE and ZI emission bands, corrected for its quantum efficiency, K ICT ¼
ðAZI =fZI Þ ðALE =fLE Þ
(2)
Fig. 4. Representative fluorescence spectrum and fit for DABA in methanol used to evaluate the quantum efficiency. The black line is the raw data, green dashed lines the individual fit components for the LE and ZI emission, and the red line the total fitted spectrum. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Static absorbance (solid line) and emission (dashed line) spectra for PABA and DABA in methanol (blue), acetonitrile (red), and dichloromethane (green). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. Stevens–Ban plot for DABA in methanol (blue circles), acetonitrile (red squares), and dichloromethane (green triangles). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.3. Vibrational analysis Infrared spectroscopy was used to probe how solvent–solute interactions impact vibrational modes of the solute. Experimentally infrared spectra were measured for the pure solid in the absence (free) and presence (complex) of methanol solvent. The spectra were overlaid and shifts in the vibrational peaks for PABA or DABA were noted. Representative spectra are found in the Supplementary data. Computationally the solute was optimized alone and complexed with an explicit methanol molecule, as illustrated in Fig. 7. The predicted infrared active vibrational modes were compared between the free and complexed structures, and visualized in GaussView. Fig. 7 illustrates the primary normal mode that was affected by solvation for the deprotonated form (A) of PABA. The dominant vibration is in the carbonyl moiety that forms the hydrogen bond to the alcohol. Table 3 summarizes the experimental and calculated frequencies ðn Þ, spring constants (kf), and reduced masses (meff) for this mode with variations in methanol solvation. 4. Discussion Analysis of the spectral behavior of PABA and DABA illustrates that the solvent plays an important role in its deactivation pathways and photochemistry. In this study PABA is used as the control to understand the photochemistry in the absence of the charge transfer event and can be considered to have all deactivation from the LE state. PABA has greater fluorescence quantum yields, at least an order of magnitude, than DABA for all solvents. For DABA, the quantum efficiency of the ZI state is greater than that of the LE state. This is a direct result of the ICT deactivation pathway following photoexcitation. The solvent dependences of the quantum efficiencies illustrates that the distribution of deactivation pathways varies by solvent. Both compounds have the lowest quantum yields in methanol. This can Table 2 Thermodynamics for ICT in DABA. Statistical error from linear fits in parentheses. Solvent
DH (kJ/mol)
DS (J/K mol)
DG at 298 K (kJ/mol)
CH3OH CH3CN CH2Cl2
9.7 (0.8) 5.5 (0.7) 11.1 (0.3)
21 (3) 20 (2) 43 (1)
3.4 (1.1) 0.3 (1.0) 1.8 (0.4)
be attributed to strong hydrogen bonding to the solvent, particularly at the acid moiety, which allows for more efficient non-radiative cooling through energy transfer as heat to the solvent. Our previous investigations of PABA and DABA focused upon aqueous solutions buffered to acidic (pH 2), neutral (pH 7) and basic (pH 10) conditions [13]. It was shown that under acidic conditions the emission of DABA resembles that of PABA, with only LE emission. This was attributed to the protonation of the amine moiety, which eliminates the electron pair necessary for the ICT reaction [13]. The presence of dual emission, particularly the strong ZI band, is an indication that the amine moiety is not protonated in the organic solvents used. To assess the protonation state of the carboxylic acid spectral measurements for solutions of PABA and its potassium salt were compared [13]. Under the aqueous conditions (pH 2–10), the absorbance and emission spectra for both compounds were indistinguishable [13]. The dominant form of the acid is predicted to be the protonated state for acidic conditions (pH < pKa), and deprotonated under neutral or basic conditions. The pKa of the carboxylic acid increases in organic solutions relative to water, therefore the protonated state will be more dominant in nonaqueous environments. For example, DABA has a pKa of 5 in water, 10.4 in methanol, and 23 in acetonitrile [33,34]. Past studies show that acidic xanthene dyes exhibit shortened fluorescence lifetimes in hydrogen-bonding solvents, like water, and this is attributed to more efficient non-radiative decay in solvents with stronger intermolecular interactions [35]. Previous studies examined the fluorescence lifetime and quantum efficiency of PABA and report that both properties are solvent-dependent, with hydrogen bonding playing an important role. The past study finds, similar to our work, that the quantum efficiency of PABA is the lowest in a protic solvent (water) [36]. They assign the lower quantum yield to more efficient nonradiative decay in hydrogen bonding solvents and a reduced energy barrier for the decay process. The authors suggest a solvent-dependent structural change at the amine-group in protic solvents. Our thermodynamic values and observed vibrational shifts align with this suggestion of strong intermolecular solvent–solute interactions in protic solvents. We report the temperature-dependence of the quantum efficiencies across the experimental range used to assess the thermodynamics of the ICT reaction. Within this range there were minimal changes to the quantum efficiency observed for PABA. The quantum yield of the ZI state of DABA has a slight enhancement with increasing temperature, concurrent with a decrease in that of the LE state, in dichloromethane and acetonitrile. This is assigned to an increased ICT efficiency with the increased temperature. There may also be more efficient non-radiative cooling processes from the LE state at the higher temperature. In methanol, there is some variation of the quantum efficiency of the LE state with temperature, but no clear trend was observed. The variability is likely due to relatively low quantum efficiency for the state and error in the spectral fits for the minor (LE) peak component. For these reasons the quantum efficiency at a moderate temperature in the thermodynamic range was used for the quantum efficiency correction in Eq. (2). Druzhinin et al. have reported the quantum efficiencies for DMABN’s LE and ZI states in acetonitrile. They find similar weaker quantum efficiencies for the LE state relative to the ZI species, with a difference of approximately an order of magnitude [8]. Over a broader temperature range of 120 C the quantum yield of the LE state is found to be relatively flat at low temperatures, but steadily increases after 25 C. In contrast the ZI state increases linearly across the full temperature range, nearly doubling in value [8]. Scaling the observed temperature dependence of the ZI state to our
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Fig. 7. Vector picture of normal mode of interest for the free (I) and solvent complexed (II) PABA molecule. The calculations for the deprotonated acid moiety are presented.
more limited temperature range would result in a change in the quantum yield of approximately 10% from the low to high temperature used in this study. Future work may evaluate a broader temperature-range and incorporate kinetic rate constants into equilibrium expressions. The thermodynamics for the ICT reaction show a strong solvent dependence. This suggests dynamics similar to DMABN, where there is a significant dipole change accompanying the ICT reaction. Absence of a strong solvent effect, in the case of the fluorobenzene derivatives, is assigned to similar dipole moments in the LE and ZI [24]. In the protic solvent, methanol, the reaction activation thermodynamics are measured with a barrier to the reaction (positive enthalpy change) and increased entropy as the reaction proceeds through the transition state. This is attributed to the redistribution of the solvent environment as the electron transfer occurs resulting from the solvent reorganizing about the system, as there is a dramatic change in the solute’s dipole moment and charge distribution. In contrast this activation process is not observed in the others solvents, indicative of a barrierless process. Following excitation it is a downhill reaction and the overall thermodynamics of the ICT are measured. The reaction is most enthalpically favorable in dichloromethane, a polar solvent, with the least specific solvent–solute interactions. In the case of the barrierless reactions the entropy is found to decrease upon electron transfer. This is attributed to stronger (electrostatic) intermolecular interactions between the charge-separated species and the solvent. This induces are more ordered solvent shell about the zwitterion, due to ion–dipole interactions, than is initially present about the excited solute, with dipole–dipole interaction. The variations in the potential energy surfaces (PES) due to stabilization of the ZI state, relative to the LE state, and solvent reorganization is illustrated in Fig. 2. The overall and activation thermodynamics show similar trends to the past studies of DMABN [4,8]. Calculations predict an endothermic reaction in the gas phase, but overall exothermic ICT reaction in solution for DMABN [4]. The magnitude of the reaction barrier in acetonitrile is reported to range from barrierless to 20 kJ/
Table 3 Experimental (expt) and calculated (calc.) vibrational frequencies ðn Þ for PABA and DABA in the free and complex form with methanol. The calculated values are reported for the deprotonated (A) and protonated (HA) carboxylic acid. The first value includes implicit methanol solvation and value in parentheses is in the gas phase. The spring constant (kf) and effective mass (meff) for the calculated normal mode is reported.
PABA
DABA
Expt. n (cm1)
Form
Calc. n (cm1)
kf (N/m)
meff (amu)
Free
1520
Complex
1521
A HA A HA
1571 (1676) 1722 (1778) 1572 (1667) 1698 (1735)
15.2 (19.1) 16.1 (17.7) 13.7 (16.2) 9.3 (10.2)
10.5 (11.6) 9.2 (9.5) 9.4 (9.9) 5.5 (5.7)
Free
1527
Complex
1529
A HA A HA
1560 (1677) 1716 (1774) 1562 (1669) 1693 (1745)
13.9 (19.6) 15.9 (17.5) 12.7 (18.5) 9.2 (15.8)
9.7 (11.8) 9.1 (9.4) 8.8 (11.3) 5.4 (8.8)
mol. Experimentally, Druzhinin et al. found the overall reaction enthalpy and entropy changes are 27.0 kJ/mol and 38 J/Kmol, respectively, and the activation energy is 7.7 kJ/mol. In the broad temperature range a turnover was seen in which the reaction goes from having a barrier, at low temperature, to a barrierless reaction allowing for the determination of both activation and overall thermodynamics [8]. In our studies, we see only one temperature regime for all solvents examined. In the case of methanol there is a barrier, whereas acetonitrile and dichloromethane are in the hightemperature (barrierless) limit. For methanol, the line is less steep and exhibits more variance in the points at the high temperature (low 1/T) end of the Stevens–Ban plot. This suggests that there may be a turnover to a barrierless regime. The temperature-range is limited, however, by the solvent boiling point. Analysis of the vibrational modes for PABA and DABA, in the presence and absence of solvent, was used to explore the impact of intermolecular solvent–solute interactions. The calculated frequencies for the deprotonated acid (A) with implicit methanol solvation are most similar to the experimental results. In general smaller spectral shifts were observed for the calculated modes including implicit solvation, and the deprotonated acid (A) had smaller frequency shifts than the neutral form (HA). Experimentally there is a small increase in the frequency of the carbonylstretching mode in the presence of methanol. Calculations bring further light to this result, indicating the relatively small shift results from offsetting decreases in the reduced mass and spring constant due to intermolecular hydrogen bonding. Nonetheless there is an observable shift in the frequency due to the presence of the methanol, which is not observed in other IR-active modes. Additionally, we did not experimentally observe spectral changes in the normal modes of PABA or DABA with solvents that do not form strong, specific interactions such as hexane and dichloromethane. This result is similar to solvent-dependent shifts in the Raman spectra observed for a benzoate derivative of DABA, 4(dimethylamino) benzoate. Mitambo and Loppnow observed that Raman modes associated with the C¼O group were sensitive to solvent, and the C¼O stretch shifted to lower frequencies and split in alcohol solvents [37]. The solvent-dependence of the vibrational modes serves to support our assignment of differences in the ICT thermodynamics in methanol to the presence of stronger hydrogen bonding interactions, where the solute can be viewed as a solute–solvent complex. Stronger interactions between the solvent and solute lead to a greater energetic cost for the electronic rearrangement with the ICT and a reaction barrier. In contrast in solvents with nonspecific interactions the solvent is more fluid about the solute and undergoes less rearrangement as the electronic state of the solute changes. 5. Conclusions The impact of the solvent upon the viability and thermodynamics of ICT for PABA and DABA is explored. As previously reported in aqueous systems, no ICT is observed in PABA and the quantum efficiency of the LE state is reduced in the protic solvent.
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Similar solvent effects on quantum efficiency are observed for DABA, with the dominant emission observed from the ZI species. Using the quantum efficiency-corrected emission the thermodynamics for the ICT process were determined for DABA. In solvents where the dominant intermolecular interaction is electrostatic in nature there is no observed reaction barrier and the overall ICT reaction thermodynamics were measured. The reaction is enthalpically driven, as the entropy of the system decreases due to stronger ion–dipole interactions of the ZI product with the solvent. In the case of hydrogen bonding solvents, a different PES is predicted with a reaction barrier and the activation thermodynamics are found. Entropy drives the reaction, with a positive activation entropy change, due to the motion of the solvent as the ICT occurs. Conclusions regarding the strengths of the solvent–solute interactions and differences in the alcohol solvent are supported by experimental and theoretical analysis of the vibrational modes of the solute. Experimentally the IR spectra of PABA and DABA are unchanged in the presence of solvents lacking specific interactions, but show an observable shift in the normal mode associated with the carbonyl stretch of the acid moiety in the presence of methanol. Overall this work furthers our understanding of the role that solvent can play upon the ICT reactions. The PES for DABA’s ICT reactions has fundamental differences depending upon the strength and nature of solvent–solute interactions. In the long term understanding how the environment can control photochemical reactions, could provide insight to design systems with desired emission properties. Acknowledgements Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research (ACS PRF #53159-UR4). Support is also acknowledged from the College of Wooster Sophomore Research Program and Copeland Funds. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. jphotochem.2015.03.005. References [1] P.F. Barbara, T.J. Meyer, M.A. Ratner, J. Phys. Chem. 100 (1996) 13148–13168. [2] M.R. Wasielewski, Chem. Rev. 92 (1992) 435–461. [3] R.P. Sinha, D.-P. Häder, Photchem. Photobiol. Sci. 1 (2002) 225–236.
[4] Z.R. Grabowski, K. Rotkiewicz, W. Rettig, Chem. Rev. 103 (2003) 3899–4031. [5] J.A. Gan, Q.L. Song, X.Y. Hou, K. Chen, H. Tian, J. Photochem. Photobiol. A: Chem. 162 (2004) 399–406. [6] S.H. Jin, D.H. Kim, G.H. Jun, S.H. Hong, S. Jeon, ACS Nano 7 (2013) 1239–1245. [7] S. Hammes-Schiffer, Acc. Chem. Res. 34 (2001) 273–281. [8] S.I. Druzhinin, N.P. Ernsting, S.A. Kovalenko, L.P. Lustres, T.A. Senyushikina, K.A. Zachariasse, J. Phys. Chem. A 110 (2006) 2955–2969. [9] W. Rettig, E. Lippert, J. Mol. Struct. 61 (1980) 17–22. [10] W. Sudholt, A. Staib, A.L. Sobolewski, W. Domcke, Phys. Chem. Chem. Phys. 2 (2000) 4341–4353. [11] T. Gustavsson, P.B. Coto, L. Serrano-Andrés, T. Fujiwara, E.C. Lim, J. Chem. Phys. 131 (2009) 031101. [12] L.H. Ma, Z.B. Chen, Y.B. Jian, Chem. Phys. Lett. 372 (2003) 104–113. [13] M.P. Thayer, C. McGuire, E.M.S. Stennett, M.K. Lockhart, D. Canache, M. Novak, S.J. Schmidtke, Spectrochim. Acta Part A 84 (2011) 227–232. [14] E.V. Anslyn, D.A. Dougherty, Modern Physical Organic Chemistry, University Science Books, Sausalito, 2006, pp. 374–376. [15] D.M. Mitchell, P.J. Morgan, D.W. Pratt, J. Phys. Chem. A 112 (2008) 12597–12601. [16] D.H. McDaniel, H.C. Brown, J. Org. Chem. 23 (1958) 420–427. [17] Y.V. Il’ichev, W. Kühnle, K.A. Zachariasse, J. Phys. Chem. A 102 (1998) 5670–5680. [18] C.J. Jödicke, H.P. Lüthi, J. Am Chem. Soc. 125 (2003) 252–264. [19] K.A. Zachariasse, T. Harr, A. Hebecker, U. Leinhos, W. Kühnle, Pure Appl. Chem. 65 (1993) 1745–1750. [20] V.A. Galievsky, S.I. Druzhinin, A. Demeter, P. Mayer, S.A. Kovalenko, T.A. Senyushkina, K.A. Zachariasse, J. Phys. Chem. A 114 (2010) 12622–12638. [21] T. Fujiwara, M.Z. Zgierski, E.C. Lim, J. Phys. Chem. A 115 (2011) 586–592. [22] W.M. Kwok, C. Ma, M.W. George, D.C. Grill, P. Matousek, A.W. Parker, D. Phillips, W.T. Toner, M. Towrie, Photochem. Photobiol. Sci. 6 (2007) 987–994. [23] S.I. Druzhinin, S.R. Dubbaka, P. Knochel, S.A. Kovalenko, P. Mayer, T. Senyushkina, K.A. Zachariasse, J. Phys. Chem. A 112 (2008) 2749–2761. [24] T. Fujiwara, C. Reichardt, R.A. Vogt, C.E. Crespo-Hernández, M.Z. Zgierski, E.C. Lim, Chem. Phys. Lett (2013) 70–75. [25] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd ed., Springer, New York, 2006 [26] S. Fery-Forgues, D. Lavabre, J. Chem. Educ. 76 (1999) 1260–1264. [27] A Guide to Recording Fluorescence Quantum Yields. Horiba Scientific. http:// www.horiba.com/us/en/scientific/products/fluorescence-spectroscopy/ application-notes/quantum-yields/. [28] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, M.J. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, Ö. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Revision B.01, Gaussian, Inc., Wallingford CT, 2009 [29] C. Lee, W. Yang, R. Parr, Phys. Rev. Sect. B 37 (1988) 785–789. [30] B. Miehlich, A. Savin, H. Stoll, H. Preuss, Chem. Phys. Lett. 157 (1989) 200–206. [31] A. Becke, J. Chem. Phys. 98 (1993) 5648–5652. [32] B. Stevens, M.I. Ban, Trans. Faraday Soc. 60 (1964) 1515–1523. [33] F. Rived, M. Roses, E. Bosc, Anal. Chim. Acta 374 (1998) 309–324. [34] F. Eckert, I. Leito, I. Kaljurand, A. Kutt, A. Klmat, M. Diedenhofen, J. Comput. Chem. 30 (2009) 799–810. [35] D. Magde, G.E. Rojas, P.G. Seybold, Photochem. Photobiol. 70 (1999) 737–744. [36] A.M. Halpern, B.R. Ramachandran, Photochem. Photobiol. 62 (1995) 686–691. [37] M.M. Mitambo, G.R. Loppnow, Chem. Phys. Lett. 261 (1996) 691–697.
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Solvent impact on the photophysical properties and excited state behavior of p-aminobenzoic acids Jacob A. Boroff, Zachery D. Matesich, Daniela Canache Stuetzer, Sarah J. Schmidtke Sobeck * College of Wooster, Department of Chemistry, 943 College Mall, Wooster, OH, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 November 2014 Received in revised form 4 February 2015 Accepted 4 March 2015 Available online 7 March 2015
The photophysical properties of p-aminobenzoic acid and dimethyl-p-aminobenzoic acid are compared in a range of solvents. The electronic spectra are used to determine quantum efficiencies and evaluate excited state behavior. The latter system is found to undergo a charge transfer following photoexcitation and the thermodynamics for the process are assessed. The strength and type of solvent–solute interaction are important in determining the driving force for the charge transfer, with entropy being dominant in protic solvents and enthalpy in aprotic solvents. Vibrational spectroscopy and calculations provide further insight into the nature of the specific solvent–solute interactions, with shifts in the infrared spectrum observed due to hydrogen bonding interactions with the solvent. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Charge transfer Photochemistry Quantum efficiency Solvent effects
1. Introduction Photo-induced charge transfer reactions are a fundamental class of chemical reaction found in a variety of complex materials and biological systems, as well as small molecules [1–4]. Following photoexcitation charge rearrangements may occur in a system and such rearrangements can have practical applications in materials, for example providing a means to tune the wavelength of emitted light [5,6]. In biological systems electron and proton transfer reactions are fundamental in the working of many enzymes [7]. Within a variety of systems it is found tuning the solvent and types of intermolecular interactions in the system can alter the photochemistry observed. Properties of the local environment, such as ability to form strong hydrogen bonding interactions or polarity, may turn on or off a reaction pathway [1,4,8]. In this study we seek to gain a more complete understanding of the role of the local solvent environment upon photochemistry of a small model system. Small organic molecules, with relevant electron donor and acceptor moieties, provide a means to better understand how electronic character and interactions with the solvent change the observed physicochemical properties of a system. In this investigation we are focused upon para-aminobenzoic acid (PABA) derivatives, where the electron donor moiety is an amine and
* Corresponding author. Tel.: +1 330 263 2359; fax: +1 330 263 2386. E-mail address: [email protected] (S.J. Schmidtke Sobeck). http://dx.doi.org/10.1016/j.jphotochem.2015.03.005 1010-6030/ ã 2015 Elsevier B.V. All rights reserved.
the acceptor a carboxylic acid group. These compounds are similar to the extensively studied dimethylaminobenzonitriles (DMABN), which contain a cyano acceptor moiety [4,8–11]. When the electron donor group is a tertiary amine it may undergo an intramolecular charge transfer (ICT) to form a zwitterionic (ZI) product following photoexcitation, as shown in Fig. 1 [12,13]. As in the case of DMABN, the reaction proceeds via a twisted intramolecular charge transfer (TICT) mechanism in which the amine donor twists to a perpendicular conformation relative to the phenyl during the course of the electron transfer [4]. The resulting ZI product is unique electronically and structurally from the LE state. The ICT can be traced using fluorescence, as there are unique emission bands from the locally excited (LE) reactant and TICT product (ZI). The ZI emission occurs at a significant Stokes shift, relative to the LE emission, as illustrated in the potential energy surface for the reaction shown in Fig. 2(I). The differences in the viability of the charge transfer based upon the electron donor is attributed to the sp2-like nature of the tertiary amine, which makes it a more favorable reaction surface to form the ZI product in accordance with the Hammond postulate [14]. Calculations indicate a difference of about 5 degrees for the R– N–R bond angle and 15 degrees for the planarity of the amine moiety to the phenyl ring in the ground state of the two compounds. PABA has a nearly ideal sp3 configuration, whereas DABA deviates to a more planar configuration and larger R–N–R angle [13]. This is consistent with past studies of DMABN derivatives where no ICT is observed for the corresponding primary and secondary amines [4]. Additionally for DABA,
J.A. Boroff et al. / Journal of Photochemistry and Photobiology A: Chemistry 305 (2015) 60–66
R
O
O R
N R
OH
LE
N
R
OH
ZI
Fig. 1. Proposed twisted intramolecular charge transfer (TICT) reaction from the locally excited (LE) to zwitterionic (ZI) state for PABA (R = H) and DABA (R = CH3).
adjusting the pH of the solvent environment can also effectively turn off the ICT reaction due to protonation of the amine donor [13]. Mitchell et al. provide another explanation for absence of ICT in PABA [15]. A combination of experimental and computational methods is used to evaluate the effective push–pull nature of PABA. Both the amino donor and carbonyl acceptor contribute electron density to the phenyl upon excitation. The contribution from the carbonyl acceptor group counteracts the expected dipole change upon excitation and weakens the push–pull nature of the system [15]. Differences due to the amine substituent can be assessed using the Hammett parameter for DABA and PABA, 0.83 and 0.66, respectively [16]. The tertiary amine is a stronger electrondonating group and as such creates a stronger “push” in the donor– acceptor system. For this reason ICT is predicted to be more favorable for a tertiary amine donor, such as DABA, relative to the primary amine, PABA. Extensive studies have been carried out to explore the impact of both structural modifications upon the photochemistry, and viability of ICT, in the DMABN system with a nitrogen donor and acceptor [4,17,18]. The studies have shown that the ICT reaction dynamics for DMABN and its derivatives are solvent-dependent [4,10,19–23]. Additionally, there are variations in the dynamics observed when the electron acceptor is adjusted from a cyano to a carbonyl or fluoro acceptor [4,24]. There is a greater extent of ICT in polar solvents due to the stabilization of the ZI. The relative stability of the LE and ZI states impacts the vertical position of the excited state potential energy wells, due to differences in the dipole moments of the two states and stabilization of the polar ZI by the solvent, as shown in Fig. 2(II). Stabilization of the ZI, relative to the LE state, leads to a decreased ICT barrier. This figure also illustrates that the relative horizontal separation of the LE and ZI states can be impacted by the solvent, as stronger solvent–solute interactions result in a greater solvent reorganization accompanying the ICT reaction and leads to an increased reaction barrier. The fluorobenzene derivatives are less sensitive to solvent effects
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than the other DMABN derivatives, due to similar dipole moments for both excited states [24]. The impact of the different stabilization and solvent reorganization energies is quantified by examination of the excited state thermodynamics. Druzhinin et al. reported the ICT activation energy and overall thermodynamics for DMABN using temperature- and time-dependent spectroscopies. The data is fit with a Stevens–Ban plot, accounting for quantum efficiencies and lifetimes of the excited state species, to yield thermodynamics. In acetonitrile, at low temperatures, there is an activation energy of 7.7 kJ/mol. In the high-temperature regime the reaction appears barrierless and the ICT reaction is exothermic by 23.2 kJ/mol [8]. The excited ZI state lifetime has a direct relationship with solvent polarity, with the longest lifetimes in the most polar solvents [23]. In this investigation the role of solvent environment on the physicochemical properties of the PABA and DABA systems is further explored. This expands the past DMABN studies to focus upon a system that has an oxygen-based, carboxylic acid, electron acceptor moiety. The quantum efficiencies of the two compounds are measured in a range of solvents with varying types of intermolecular interactions. Temperature-dependent fluorescence spectroscopy is used to evaluate the reaction thermodynamics for DABA in order to assess how the solvent impacts the reaction driving force. Analysis of the molecular vibrations, using experimental and theoretically predicted infrared (IR) spectra, allow us to gain an understanding of the relative strength of the intermolecular interactions for hydrogen bonding solvents, like methanol. Together this analysis provides insight into how solvent–solute interactions affect the photochemistry of this model system. 2. Materials and methods 2.1. Sample preparation 4-Aminobenzoic acid (Sigma, 99% pure) and 4-(dimethylamino) benzoic acid (Aldrich, 98% pure) were used as received. Solutions were prepared with the following ACS grade solvents: methanol (Pharmco-Aaper, 99.9%), acetonitrile (Pharmco-Aaper, 99.9%), and dichloromethane (Pharmco-Aaper, 99.99%). Stock solutions were diluted for emission studies to ensure the maximum absorbance did not exceed 0.1 absorbance units. A fluorescein standard solution was prepared by diluting 5 mL of fluorescein (Molecular Probes Reference Dye Sampler Kit 1 mM) to 10 mL with 0.1 M sodium hydroxide (BDH Chemicals, 1.00 N). A standard quinine sulfate solution was made by diluting 20 mL of quinine sulfate (Molecular Probes Reference Dye Sampler Kit 1 mM) to 10 mL with 0.5 M sulfuric acid (BDH Chemicals). A standard tryptophan solution was prepared by dissolving a small amount of DL-tryptophan (Alfa Aesar, 99%) in 50 mL of distilled water. 2.2. Spectral measurements
Fig. 2. Potential energy surface for the ICT reaction showing the absorbance (blue) and emission (green, red) processes (I). Solvent effects on the relative positioning of the LE and ZI excited states are shown (II). The initial ZI well (solid black) relative to the LE state (blue) is compared to a case where a polar solvent preferentially stabilizes the ZI (dashed black) and a solvent with greater solvent reorganization accompanying the ICT (ZI dotted black). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Absorbance spectra were collected at room temperature on a Varian Cary 50 Bio Spectrophotometer. The instrument was blanked with the respective solvent for each sample, and the absorbance was measured from 200 to 400 nm with a scan rate of 600 nm/min in the dual beam mode. Fluorescence spectra were collected on a Varian Cary Eclipse Spectrofluorimeter. Samples were excited at the wavelength of maximum absorbance and the PMT setting was adjusted to achieve sufficient signal to noise (600 V for PABA and 800 V for DABA). Fluorescence spectra were recorded with a 120 nm/min scan rate, 5 nm slit width, and 1 nm step size. Variable temperature scans were taken at 5 C increments, using a Peltier-thermostatted cell. The sample was allowed to equilibrate for 5 min, with continual stirring in the cell, after the desired temperature was reached to
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ensure thermal equilibrium. A blank sample, of pure solvent, was measured under identical settings to ensure no significant solvent background was present. The emission profiles for the three fluorescence standards were used to determine the fluorimeter correction factor by comparison to standard spectra [25]. The wavelength-dependent correction factor was applied to all emission spectra prior to analysis to account for variability in the detector sensitivity across the spectral range. Infrared (IR) spectra were obtained using a Nicolet 6700 FTIR with ATR accessory. Spectra were taken for each compound in its solid form. For the solvated compound a few drops of solvent was put on the solid sample prior to recording the spectra. 2.3. Quantum yield analysis Quantum yields were determined based upon literature protocols [26,27]. Briefly, for each quantum standard five dilutions were prepared with a range of absorbance values up to a maximum of 0.08 absorbance units. An absorbance and emission spectra was taken for each solution. This was carried out at both PMT settings for each standard. The same procedure was carried out for the PABA and DABA solutions, in each solvent, at the appropriate PMT setting. The temperature was controlled using a Peltier-thermostatted cell. 2.4. Spectral fitting The fluorescence spectra were analyzed using the OriginPro 81 software. Spectral wavelengths were converted to frequency (cm1). Intensity (I) was scaled according to the relationship between the wavelength and frequency-dependent measurements for a wavelength-based grating instrument: IðvÞ ¼ l2 IðlÞ [25]. Spectra were fit using the Peak Analyzer application modeling each
peak as a Gaussian function to determine the area under each emission band for analysis. 2.5. Computational detail PABA and DABA were optimized in the protonated and deprotonated forms of the carboxylic acid in Gaussian 09 [28]. The optimizations were performed using the hybrid DFT B3LYP level of theory with the 6-31G + (d,p) basis set [29–31]. Calculations were performed with and without implicit solvation using the default polarizable continuum solvent model (IEFPCM) with solvent parameters for methanol [28]. Complexes of PABA and DABA with a single methanol hydrogen bonded at the acid moiety were optimized to model explicit solvation effects. Frequency calculations were performed for all optimizations to verify the nature of the stationary points, and to predict infrared spectrum for the free solute and explicit solvent–solute complex. 3. Results 3.1. Quantum yields Quantum yields were determined by comparison to a set of three fluorescence standards (fluorescein, quinine sulfate, tryptophan) and values reported are an average. Briefly, the integrated fluorescence was plotted as a function of absorbance and the slope of the line was found [26,27]. The slope was compared to the data for the fluorescence standards, using the following equation, to evaluate the quantum efficiency: ! Gradx h2x fx ¼ fSt (1) GradSt h2St
Fig. 3. Absorbance (I) and emission (II) data for quantum yield determination of PABA in methanol at room temperature, arrow indicates increasing concentration. The quantum yield was found as the slope of the line of emission area versus absorbance area (III).
J.A. Boroff et al. / Journal of Photochemistry and Photobiology A: Chemistry 305 (2015) 60–66
where x denotes the unknown, St the standard, f is the quantum yield, Grad is the slope of the line, and h the index of refraction of the solvent [28]. For PABA there is a single emission band, allowing direct analysis of the data using Eq. (1). Sample absorbance and emission scans, as well as the resulting quantum efficiency plot, for PABA in methanol is shown in Fig. 3. For DABA there is dual emission, with the LE emission at a higher energy than that from the ZI state. Modeling the spectra allowed determination of the contribution individually from each of the two excited states, and the quantum efficiency plots were made for both the LE or ZI species. Fig. 4 shows a representative fitting of the DABA spectrum to evaluate the quantum efficiencies of the LE and ZI states independently. Values for the quantum efficiencies for both compounds in the different solvents are reported in Table 1. 3.2. ICT reaction and thermodynamics Static absorbance and emission spectra were used to indicate the presence of ICT. A large Stokes shift in the emission is indicative of the presence of the ZI species. Fig. 5 shows the static absorbance and emission spectra for PABA and DABA in the three solvents at room temperature. As illustrated in this figure only DABA exhibits ICT. The magnitude of the ZI band relative to the LE state is greatest in acetonitrile and the least in methanol. The temperature-dependent emission spectra were modeled and the spectral changes were assessed using a Stevens–Ban plot to yield the ICT thermodynamics [32]. The analysis is based upon the following equation: DH DS þ (2) lnðK ICT Þ ¼ RT R
63
Table 1 Solvent- and temperature-dependent quantum yields for PABA and DABA. Statistical error from linear fits in parentheses. Solvent
Temp (K)
PABA/102
DABA (LE)/105
DABA (ZI)/102
CH3OH
295 323
3.5 (0.1) 3.8 (0.2)
7.3 (0.5) 15 (2)
0.122 (0.003) 0.11 (0.01)
CH3CN
295 323
18 (3) 11.6 (0.3)
7.0 (0.8) a
0.218 (0.009) 0.54 (0.02)
295 303
9.6 (0.7) 9.7 (0.2)
71 (16) 89 (9)
1.28 (0.08) 1.47 (0.06)
CH2Cl2 a
Emission from LE state was not quantifiable.
where A indicates the area under the emission band, and f is the quantum efficiency of the given state. The area under each emission band is determined from the spectral fit, with each emission band modeled as a Gaussian curve, and using the average quantum efficiency in the temperature range. Representative spectral fits for all solvents are found in the supplementary data. The Stevens–Ban plots for DABA in all solvents are shown in Fig. 6 and the resulting thermodynamics summarized in Table 2. A positive enthalpy change indicates activation thermodynamics, whereas a negative enthalpy change indicates a barrierless reaction [8]. In both acetonitrile and dichloromethane the ICT was found to be barrierless with a decrease in the enthalpy and entropy accompanying the reaction. In methanol there is a reaction barrier and the activation thermodynamics are measured, with a positive enthalpy and entropy of activation.
where KICT is the equilibrium constant for the ICT, DH the reaction enthalpy, DS the reaction entropy, R the gas constant, and T the temperature. A plot of the equilibrium constant as a function of temperature yields the reaction thermodynamics. The equilibrium constant was calculated based upon the ratio of the ZI to LE populations as experimentally determined by the area under the LE and ZI emission bands, corrected for its quantum efficiency, K ICT ¼
ðAZI =fZI Þ ðALE =fLE Þ
(2)
Fig. 4. Representative fluorescence spectrum and fit for DABA in methanol used to evaluate the quantum efficiency. The black line is the raw data, green dashed lines the individual fit components for the LE and ZI emission, and the red line the total fitted spectrum. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Static absorbance (solid line) and emission (dashed line) spectra for PABA and DABA in methanol (blue), acetonitrile (red), and dichloromethane (green). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. Stevens–Ban plot for DABA in methanol (blue circles), acetonitrile (red squares), and dichloromethane (green triangles). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.3. Vibrational analysis Infrared spectroscopy was used to probe how solvent–solute interactions impact vibrational modes of the solute. Experimentally infrared spectra were measured for the pure solid in the absence (free) and presence (complex) of methanol solvent. The spectra were overlaid and shifts in the vibrational peaks for PABA or DABA were noted. Representative spectra are found in the Supplementary data. Computationally the solute was optimized alone and complexed with an explicit methanol molecule, as illustrated in Fig. 7. The predicted infrared active vibrational modes were compared between the free and complexed structures, and visualized in GaussView. Fig. 7 illustrates the primary normal mode that was affected by solvation for the deprotonated form (A) of PABA. The dominant vibration is in the carbonyl moiety that forms the hydrogen bond to the alcohol. Table 3 summarizes the experimental and calculated frequencies ðn Þ, spring constants (kf), and reduced masses (meff) for this mode with variations in methanol solvation. 4. Discussion Analysis of the spectral behavior of PABA and DABA illustrates that the solvent plays an important role in its deactivation pathways and photochemistry. In this study PABA is used as the control to understand the photochemistry in the absence of the charge transfer event and can be considered to have all deactivation from the LE state. PABA has greater fluorescence quantum yields, at least an order of magnitude, than DABA for all solvents. For DABA, the quantum efficiency of the ZI state is greater than that of the LE state. This is a direct result of the ICT deactivation pathway following photoexcitation. The solvent dependences of the quantum efficiencies illustrates that the distribution of deactivation pathways varies by solvent. Both compounds have the lowest quantum yields in methanol. This can Table 2 Thermodynamics for ICT in DABA. Statistical error from linear fits in parentheses. Solvent
DH (kJ/mol)
DS (J/K mol)
DG at 298 K (kJ/mol)
CH3OH CH3CN CH2Cl2
9.7 (0.8) 5.5 (0.7) 11.1 (0.3)
21 (3) 20 (2) 43 (1)
3.4 (1.1) 0.3 (1.0) 1.8 (0.4)
be attributed to strong hydrogen bonding to the solvent, particularly at the acid moiety, which allows for more efficient non-radiative cooling through energy transfer as heat to the solvent. Our previous investigations of PABA and DABA focused upon aqueous solutions buffered to acidic (pH 2), neutral (pH 7) and basic (pH 10) conditions [13]. It was shown that under acidic conditions the emission of DABA resembles that of PABA, with only LE emission. This was attributed to the protonation of the amine moiety, which eliminates the electron pair necessary for the ICT reaction [13]. The presence of dual emission, particularly the strong ZI band, is an indication that the amine moiety is not protonated in the organic solvents used. To assess the protonation state of the carboxylic acid spectral measurements for solutions of PABA and its potassium salt were compared [13]. Under the aqueous conditions (pH 2–10), the absorbance and emission spectra for both compounds were indistinguishable [13]. The dominant form of the acid is predicted to be the protonated state for acidic conditions (pH < pKa), and deprotonated under neutral or basic conditions. The pKa of the carboxylic acid increases in organic solutions relative to water, therefore the protonated state will be more dominant in nonaqueous environments. For example, DABA has a pKa of 5 in water, 10.4 in methanol, and 23 in acetonitrile [33,34]. Past studies show that acidic xanthene dyes exhibit shortened fluorescence lifetimes in hydrogen-bonding solvents, like water, and this is attributed to more efficient non-radiative decay in solvents with stronger intermolecular interactions [35]. Previous studies examined the fluorescence lifetime and quantum efficiency of PABA and report that both properties are solvent-dependent, with hydrogen bonding playing an important role. The past study finds, similar to our work, that the quantum efficiency of PABA is the lowest in a protic solvent (water) [36]. They assign the lower quantum yield to more efficient nonradiative decay in hydrogen bonding solvents and a reduced energy barrier for the decay process. The authors suggest a solvent-dependent structural change at the amine-group in protic solvents. Our thermodynamic values and observed vibrational shifts align with this suggestion of strong intermolecular solvent–solute interactions in protic solvents. We report the temperature-dependence of the quantum efficiencies across the experimental range used to assess the thermodynamics of the ICT reaction. Within this range there were minimal changes to the quantum efficiency observed for PABA. The quantum yield of the ZI state of DABA has a slight enhancement with increasing temperature, concurrent with a decrease in that of the LE state, in dichloromethane and acetonitrile. This is assigned to an increased ICT efficiency with the increased temperature. There may also be more efficient non-radiative cooling processes from the LE state at the higher temperature. In methanol, there is some variation of the quantum efficiency of the LE state with temperature, but no clear trend was observed. The variability is likely due to relatively low quantum efficiency for the state and error in the spectral fits for the minor (LE) peak component. For these reasons the quantum efficiency at a moderate temperature in the thermodynamic range was used for the quantum efficiency correction in Eq. (2). Druzhinin et al. have reported the quantum efficiencies for DMABN’s LE and ZI states in acetonitrile. They find similar weaker quantum efficiencies for the LE state relative to the ZI species, with a difference of approximately an order of magnitude [8]. Over a broader temperature range of 120 C the quantum yield of the LE state is found to be relatively flat at low temperatures, but steadily increases after 25 C. In contrast the ZI state increases linearly across the full temperature range, nearly doubling in value [8]. Scaling the observed temperature dependence of the ZI state to our
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Fig. 7. Vector picture of normal mode of interest for the free (I) and solvent complexed (II) PABA molecule. The calculations for the deprotonated acid moiety are presented.
more limited temperature range would result in a change in the quantum yield of approximately 10% from the low to high temperature used in this study. Future work may evaluate a broader temperature-range and incorporate kinetic rate constants into equilibrium expressions. The thermodynamics for the ICT reaction show a strong solvent dependence. This suggests dynamics similar to DMABN, where there is a significant dipole change accompanying the ICT reaction. Absence of a strong solvent effect, in the case of the fluorobenzene derivatives, is assigned to similar dipole moments in the LE and ZI [24]. In the protic solvent, methanol, the reaction activation thermodynamics are measured with a barrier to the reaction (positive enthalpy change) and increased entropy as the reaction proceeds through the transition state. This is attributed to the redistribution of the solvent environment as the electron transfer occurs resulting from the solvent reorganizing about the system, as there is a dramatic change in the solute’s dipole moment and charge distribution. In contrast this activation process is not observed in the others solvents, indicative of a barrierless process. Following excitation it is a downhill reaction and the overall thermodynamics of the ICT are measured. The reaction is most enthalpically favorable in dichloromethane, a polar solvent, with the least specific solvent–solute interactions. In the case of the barrierless reactions the entropy is found to decrease upon electron transfer. This is attributed to stronger (electrostatic) intermolecular interactions between the charge-separated species and the solvent. This induces are more ordered solvent shell about the zwitterion, due to ion–dipole interactions, than is initially present about the excited solute, with dipole–dipole interaction. The variations in the potential energy surfaces (PES) due to stabilization of the ZI state, relative to the LE state, and solvent reorganization is illustrated in Fig. 2. The overall and activation thermodynamics show similar trends to the past studies of DMABN [4,8]. Calculations predict an endothermic reaction in the gas phase, but overall exothermic ICT reaction in solution for DMABN [4]. The magnitude of the reaction barrier in acetonitrile is reported to range from barrierless to 20 kJ/
Table 3 Experimental (expt) and calculated (calc.) vibrational frequencies ðn Þ for PABA and DABA in the free and complex form with methanol. The calculated values are reported for the deprotonated (A) and protonated (HA) carboxylic acid. The first value includes implicit methanol solvation and value in parentheses is in the gas phase. The spring constant (kf) and effective mass (meff) for the calculated normal mode is reported.
PABA
DABA
Expt. n (cm1)
Form
Calc. n (cm1)
kf (N/m)
meff (amu)
Free
1520
Complex
1521
A HA A HA
1571 (1676) 1722 (1778) 1572 (1667) 1698 (1735)
15.2 (19.1) 16.1 (17.7) 13.7 (16.2) 9.3 (10.2)
10.5 (11.6) 9.2 (9.5) 9.4 (9.9) 5.5 (5.7)
Free
1527
Complex
1529
A HA A HA
1560 (1677) 1716 (1774) 1562 (1669) 1693 (1745)
13.9 (19.6) 15.9 (17.5) 12.7 (18.5) 9.2 (15.8)
9.7 (11.8) 9.1 (9.4) 8.8 (11.3) 5.4 (8.8)
mol. Experimentally, Druzhinin et al. found the overall reaction enthalpy and entropy changes are 27.0 kJ/mol and 38 J/Kmol, respectively, and the activation energy is 7.7 kJ/mol. In the broad temperature range a turnover was seen in which the reaction goes from having a barrier, at low temperature, to a barrierless reaction allowing for the determination of both activation and overall thermodynamics [8]. In our studies, we see only one temperature regime for all solvents examined. In the case of methanol there is a barrier, whereas acetonitrile and dichloromethane are in the hightemperature (barrierless) limit. For methanol, the line is less steep and exhibits more variance in the points at the high temperature (low 1/T) end of the Stevens–Ban plot. This suggests that there may be a turnover to a barrierless regime. The temperature-range is limited, however, by the solvent boiling point. Analysis of the vibrational modes for PABA and DABA, in the presence and absence of solvent, was used to explore the impact of intermolecular solvent–solute interactions. The calculated frequencies for the deprotonated acid (A) with implicit methanol solvation are most similar to the experimental results. In general smaller spectral shifts were observed for the calculated modes including implicit solvation, and the deprotonated acid (A) had smaller frequency shifts than the neutral form (HA). Experimentally there is a small increase in the frequency of the carbonylstretching mode in the presence of methanol. Calculations bring further light to this result, indicating the relatively small shift results from offsetting decreases in the reduced mass and spring constant due to intermolecular hydrogen bonding. Nonetheless there is an observable shift in the frequency due to the presence of the methanol, which is not observed in other IR-active modes. Additionally, we did not experimentally observe spectral changes in the normal modes of PABA or DABA with solvents that do not form strong, specific interactions such as hexane and dichloromethane. This result is similar to solvent-dependent shifts in the Raman spectra observed for a benzoate derivative of DABA, 4(dimethylamino) benzoate. Mitambo and Loppnow observed that Raman modes associated with the C¼O group were sensitive to solvent, and the C¼O stretch shifted to lower frequencies and split in alcohol solvents [37]. The solvent-dependence of the vibrational modes serves to support our assignment of differences in the ICT thermodynamics in methanol to the presence of stronger hydrogen bonding interactions, where the solute can be viewed as a solute–solvent complex. Stronger interactions between the solvent and solute lead to a greater energetic cost for the electronic rearrangement with the ICT and a reaction barrier. In contrast in solvents with nonspecific interactions the solvent is more fluid about the solute and undergoes less rearrangement as the electronic state of the solute changes. 5. Conclusions The impact of the solvent upon the viability and thermodynamics of ICT for PABA and DABA is explored. As previously reported in aqueous systems, no ICT is observed in PABA and the quantum efficiency of the LE state is reduced in the protic solvent.
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Similar solvent effects on quantum efficiency are observed for DABA, with the dominant emission observed from the ZI species. Using the quantum efficiency-corrected emission the thermodynamics for the ICT process were determined for DABA. In solvents where the dominant intermolecular interaction is electrostatic in nature there is no observed reaction barrier and the overall ICT reaction thermodynamics were measured. The reaction is enthalpically driven, as the entropy of the system decreases due to stronger ion–dipole interactions of the ZI product with the solvent. In the case of hydrogen bonding solvents, a different PES is predicted with a reaction barrier and the activation thermodynamics are found. Entropy drives the reaction, with a positive activation entropy change, due to the motion of the solvent as the ICT occurs. Conclusions regarding the strengths of the solvent–solute interactions and differences in the alcohol solvent are supported by experimental and theoretical analysis of the vibrational modes of the solute. Experimentally the IR spectra of PABA and DABA are unchanged in the presence of solvents lacking specific interactions, but show an observable shift in the normal mode associated with the carbonyl stretch of the acid moiety in the presence of methanol. Overall this work furthers our understanding of the role that solvent can play upon the ICT reactions. The PES for DABA’s ICT reactions has fundamental differences depending upon the strength and nature of solvent–solute interactions. In the long term understanding how the environment can control photochemical reactions, could provide insight to design systems with desired emission properties. Acknowledgements Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research (ACS PRF #53159-UR4). Support is also acknowledged from the College of Wooster Sophomore Research Program and Copeland Funds. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. jphotochem.2015.03.005. References [1] P.F. Barbara, T.J. Meyer, M.A. Ratner, J. Phys. Chem. 100 (1996) 13148–13168. [2] M.R. Wasielewski, Chem. Rev. 92 (1992) 435–461. [3] R.P. Sinha, D.-P. Häder, Photchem. Photobiol. Sci. 1 (2002) 225–236.
[4] Z.R. Grabowski, K. Rotkiewicz, W. Rettig, Chem. Rev. 103 (2003) 3899–4031. [5] J.A. Gan, Q.L. Song, X.Y. Hou, K. Chen, H. Tian, J. Photochem. Photobiol. A: Chem. 162 (2004) 399–406. [6] S.H. Jin, D.H. Kim, G.H. Jun, S.H. Hong, S. Jeon, ACS Nano 7 (2013) 1239–1245. [7] S. Hammes-Schiffer, Acc. Chem. Res. 34 (2001) 273–281. [8] S.I. Druzhinin, N.P. Ernsting, S.A. Kovalenko, L.P. Lustres, T.A. Senyushikina, K.A. Zachariasse, J. Phys. Chem. A 110 (2006) 2955–2969. [9] W. Rettig, E. Lippert, J. Mol. Struct. 61 (1980) 17–22. [10] W. Sudholt, A. Staib, A.L. Sobolewski, W. Domcke, Phys. Chem. Chem. Phys. 2 (2000) 4341–4353. [11] T. Gustavsson, P.B. Coto, L. Serrano-Andrés, T. Fujiwara, E.C. Lim, J. Chem. Phys. 131 (2009) 031101. [12] L.H. Ma, Z.B. Chen, Y.B. Jian, Chem. Phys. Lett. 372 (2003) 104–113. [13] M.P. Thayer, C. McGuire, E.M.S. Stennett, M.K. Lockhart, D. Canache, M. Novak, S.J. Schmidtke, Spectrochim. Acta Part A 84 (2011) 227–232. [14] E.V. Anslyn, D.A. Dougherty, Modern Physical Organic Chemistry, University Science Books, Sausalito, 2006, pp. 374–376. [15] D.M. Mitchell, P.J. Morgan, D.W. Pratt, J. Phys. Chem. A 112 (2008) 12597–12601. [16] D.H. McDaniel, H.C. Brown, J. Org. Chem. 23 (1958) 420–427. [17] Y.V. Il’ichev, W. Kühnle, K.A. Zachariasse, J. Phys. Chem. A 102 (1998) 5670–5680. [18] C.J. Jödicke, H.P. Lüthi, J. Am Chem. Soc. 125 (2003) 252–264. [19] K.A. Zachariasse, T. Harr, A. Hebecker, U. Leinhos, W. Kühnle, Pure Appl. Chem. 65 (1993) 1745–1750. [20] V.A. Galievsky, S.I. Druzhinin, A. Demeter, P. Mayer, S.A. Kovalenko, T.A. Senyushkina, K.A. Zachariasse, J. Phys. Chem. A 114 (2010) 12622–12638. [21] T. Fujiwara, M.Z. Zgierski, E.C. Lim, J. Phys. Chem. A 115 (2011) 586–592. [22] W.M. Kwok, C. Ma, M.W. George, D.C. Grill, P. Matousek, A.W. Parker, D. Phillips, W.T. Toner, M. Towrie, Photochem. Photobiol. Sci. 6 (2007) 987–994. [23] S.I. Druzhinin, S.R. Dubbaka, P. Knochel, S.A. Kovalenko, P. Mayer, T. Senyushkina, K.A. Zachariasse, J. Phys. Chem. A 112 (2008) 2749–2761. [24] T. Fujiwara, C. Reichardt, R.A. Vogt, C.E. Crespo-Hernández, M.Z. Zgierski, E.C. Lim, Chem. Phys. Lett (2013) 70–75. [25] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd ed., Springer, New York, 2006 [26] S. Fery-Forgues, D. Lavabre, J. Chem. Educ. 76 (1999) 1260–1264. [27] A Guide to Recording Fluorescence Quantum Yields. Horiba Scientific. http:// www.horiba.com/us/en/scientific/products/fluorescence-spectroscopy/ application-notes/quantum-yields/. [28] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, M.J. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, Ö. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Revision B.01, Gaussian, Inc., Wallingford CT, 2009 [29] C. Lee, W. Yang, R. Parr, Phys. Rev. Sect. B 37 (1988) 785–789. [30] B. Miehlich, A. Savin, H. Stoll, H. Preuss, Chem. Phys. Lett. 157 (1989) 200–206. [31] A. Becke, J. Chem. Phys. 98 (1993) 5648–5652. [32] B. Stevens, M.I. Ban, Trans. Faraday Soc. 60 (1964) 1515–1523. [33] F. Rived, M. Roses, E. Bosc, Anal. Chim. Acta 374 (1998) 309–324. [34] F. Eckert, I. Leito, I. Kaljurand, A. Kutt, A. Klmat, M. Diedenhofen, J. Comput. Chem. 30 (2009) 799–810. [35] D. Magde, G.E. Rojas, P.G. Seybold, Photochem. Photobiol. 70 (1999) 737–744. [36] A.M. Halpern, B.R. Ramachandran, Photochem. Photobiol. 62 (1995) 686–691. [37] M.M. Mitambo, G.R. Loppnow, Chem. Phys. Lett. 261 (1996) 691–697.