Revisit of 4,4â²-Diaminodiphenyl Sulfone Photophysics in Different

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Revisit of 4,4′-Diaminodiphenyl Sulfone Photophysics in Different Solvents Prosenjit Bhattacharya,† Dibakar Sahoo,† and Sankar Chakravorti*,† †

Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India S Supporting Information *

ABSTRACT: This paper aims at getting the origin of dual emission of 4,4′-diaminodiphenyl sulfone (Dapsone) in various solvents using steady state and time-resolved emission spectroscopy in conjunction with ground and excited state optimized geometry calculations in gaseous phase and in solvents. The dual fluorescence in different solvents and a distinctive bathochromic spectral shift of the low energy band in polar solvents indicate the possibility of intramolecular charge transfer (ICT) from the donor site (−NH2) to the acceptor moiety in the excited state. Time-resolved emission spectra (TRES) and more precisely timeresolved area normalized emission spectra (TRANES) depict the evolution of the dynamics of excited state species and the charge transfer. Hindrance of rotational relaxation in restricted environments causes intensified charge transfer emission quantum yield. A plausible model for excited state deactivation through ICT has been proposed on the basis of both theoretical and experimental findings. The observed results also necessitate to conceive the involvement of a nonradiative deactivation channel in an unrestricted environment which seems to regulate the ICT emission and photophysics of the probe.

1. INTRODUCTION Among various photophysical and photochemical process charge transfer (CT) reactions between separate molecules (intermolecular) or between distinctive regions within the molecule (intramolecular) are fundamental and most frequently encountered processes. The photoinduced charge transfer in organic molecules plays a crucial role in photosynthesis and in many biological processes,1 molecular electronics, and organic photovoltaics and also in other applications.2−9 The engaging interest on CT excited states is therefore very natural. The electron donor and the electron acceptor moieties linked by formally electron bridge may result in intramolecular charge transfer either in the ground state or in the excited state. In polar and/or viscous solvents the charge transfer compounds do exhibit a characteristic dual fluorescence;10 one arises from locally excited (LE) state and the other involving an intramolecular charge transfer (ICT) state.11,12 There are some models of intramolecular charge transfer (ICT) process and the twisted intramolecular charge transfer (TICT) model.10−13 The intramolecular motion of the moieties leading to conformational change is strongly affected by local polarity and viscosity12,13 of the surrounding solvent. The application of ICT emission could be seen in living biological systems,14 and it is also an interesting subject of research.15−21 Owing to the growing concern for pollution by conventional solvents has given birth to green chemistry,22 which led to the realization room temperature ionic liquid (RTIL) as reaction media.23 The negligible vapor pressure, high stability, and in particular high polarity in room temperature may lead to novel photophysical properties of organic dyes,24 the knowledge of which is very limited. 4,4′-Diaminodiphenyl sulfone (Dapsone) is an interesting molecule, used in specific medical intervention (mainly leprosy), and could serve as a charge transfer system. The Xray analysis of Dapsone25 has shown that both phenyl rings are © 2012 American Chemical Society

approximately perpendicular to the N--S--N plane, having a perpendicular π-system. While discussing the charge transfer properties of similar compounds Rettig26 et al. mentioned that the d-orbital participation in Dapsone has a significant consequence on the sensitivity of the equilibrium of locally excited state (LE) and twisted intramolecular charge transfer states (TICT) and the decay kinetics of the probe as a whole. The fluorescence signature of Dapsone in aqueous solution27 and in aqueous β-cyclodextrin28 under different acidic conditions could be found in the literature. Some other works29,30 on the change of emission of Dapsone in different environments considering charge transfer emission is to be due to TICT could be found. While working with emission properties of Dapsone in restricted31 and microheterogeneous environments the nature of emission from charge transfer state was not found to be normal TICT behavior and also the work of Enoch and Swaminathan28 could not be explained if the origin of charge transfer emission of Dapsone to be due to TICT state. Surprisingly, a detailed elucidation of excited state photophysical properties of Dapsone could not exactly be found in the literature, and this is very important to resolve the ambiguity for the fundamental photophysical aspect and also for the application of this molecule. So we decided to revisit the origin of Dapsone photophysics in different solvents. We envisage in the present investigation looking into the nature, role, and dynamics of charge transfer emission of Dapsone in various media including RTIL with the help of time-resolved emission spectroscopy and also studied the emission at low temperature where the rotational motion ceases. To get an insight about the structural change and absorption spectra of Received: Revised: Accepted: Published: 2505

May 24, 2011 December 15, 2011 January 8, 2012 January 9, 2012 dx.doi.org/10.1021/ie201113b | Ind. Eng.Chem. Res. 2012, 51, 2505−2514

Industrial & Engineering Chemistry Research

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(TRANES) have been computed from the fluorescence decays of Dapsone across the emission spectrum at an interval of 5 nm.33−35 The ground state (GS) geometry optimization of Dapsone in vacuum has been obtained at DFT level employing the nonlocal exchange corelation functional (B3LYP)36 implemented in Gaussian 03. TDDFT formalism is used for transition energy calculation using B3LYP functional with the same set of basis. In all cases a 6-31G+ (d,p) basis set has been used. Long-range effects induced by solvent polarity on the predicted photophysical properties are taken into account by means of a dielectric continuum approach using Tomasi’s Polarizable Continuum Model (PCM).37 The PCM-TDDFT calculations were carried out with nonequilibrium solvation conditions, as we were interested in vertical excitation energy.

Dapsone the quantum chemical calculations using Hartree− Fock (HF) and Density Functional Theory (DFT) have been made. The simulated geometry change of Dapsone and the corresponding dipole moment and the energy of excited state in the orthogonal position may indicate the nature of charge transfer state. We will also try to propose a plausible model for charge transfer on the basis of both experimental and quantum chemical calculations. To supplement absorption spectra of Dapsone quantum chemical calculations using DFT and TDDFT methods have been employed.

2. MATERIALS AND METHODS Dapsone was procured from Aldrich Chemical (U.S.A.) and was further purified by vacuum sublimation. The spectral grade solvents ethanol (EtOH), methanol (MeOH), glycerol, chloroform (CHCH3), dimethyl formamide (DMF), and dimethyl sulfoxide (DMSO) were purchased from Aldrich and were used as supplied but only after checking the purity fluorimetrically in the wavelength range of interest. Acetonitrile (ACN), isobutanol (i-BuOH), isopropanol (i-PrOH), 1-chlorobutane, methyl cyclohexane (MCH), dioxane (DOX), cyclohexane (CYC), sulfuric acid (H2SO4), and triethylamine (TEA) (E Merck, spectroscopic grade) were used after checking emission in the required wavelength range. Millipore water was used for preparation of the aqueous solutions. The room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium tetrafluoroborate [bmin][BF4] was obtained from Aldrich Chemical Co. (USA) and was used as supplied. To record UV−vis absorption and fluorescence spectra of Dapsone in pure solvents, a primary solution of Dapsone (5.203 × 10−3 M) was prepared in pure ethanol. Freshly prepared solutions of Dapsone with a final concentration of ∼8 μM were used for all emission measurements in order to avoid aggregation. The absorption spectra at 298 K were recorded using a Shimadzu absorption spectrophotometer model UV-2401 PC, and the fluorescence measurements were performed using a Hitachi F-4500 fluorescence spectrophotometer or Horiba Jobin Yvon Fluoromax 4 along with a temperature controller. The quantum yields were determined by using the secondary standard method, with recrystallized β-naphthol in cyclohexane (ϕn = 0.23) as referenced32 in the usual way (Supporting Information). The steady state fluorescence anisotropy measurements24 were performed with the same steady state spectrophotometer fitted with a polarizer attachment. All measurements other than the study of temperature effect were done at room temperature (25 °C). The fluorescence lifetimes of the probe molecule in different solvents were determined from time-resolved intensity decay by the method of time correlated single photon counting (TCSPC) technique in a HORIBA JOBIN YVON instrument. By using a picoseconds diode laser (IBN Nanoled-07) the system was excited at 295 nm. The Hamamatsu MCP plate photomultiplier (R3809U) detector was used. An Ortec 9327 discriminator (CFD, Tenelec TC 454) and Fluoro Hub Single Photon Counting Controller were used in the single photon counting technique. The data were collected using a DAQ card as a multichannel analyzer. The instrument response function (IRF) of the setup is ∼120 ps. The obtained spectra were analyzed with the software DAS6 at data station v2.3 through exponential fitting.8 The quality of the fit was determined in terms of a Durbin-Watson (DW) parameter, weighted residuals and reduced χ2 values. The time-resolved emission spectra (TRES) and time-resolved area normalized emission spectra

3. RESULTS AND DISCUSSION 3.1. Steady State Emission and Absorption Spectra. The absorption spectra (Figure 1) of the probe molecule

Figure 1. Absorption spectra of Dapsone (5 × 10−6 M) in methanol (MeOH), acetonitrile (ACN), water, chloroform (CHCl3), and methyl cyclohexane (MCH) at room temperature.

Chart 1. Structure of Dapsone

(Chart 1) show the band maxima at 279 nm, 284 nm, 296 nm, 291 nm, 297 nm, and 300 nm in cyclohexane, MCH, MeOH, water, glycerol, and DMSO, respectively (Table 1). Bathochromic shift in all solvents except water evinces progressively diminishing energy gap between the ground and excited states due to greater stabilization of excited state with increasing solvent polarity. Contrary to this, the absorption band gets blue-shifted in water, which is possibly due to less stabilization of hydrophobic Dapsone molecule in the water. The absorption 2506

dx.doi.org/10.1021/ie201113b | Ind. Eng.Chem. Res. 2012, 51, 2505−2514


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Table 1. Observed Absorption Band Maxima (λabs) and Emission Band Maxima (λems)a florescence band (nm)

quantum yields

solvent

absorption maxima (nm)

λLE

λCT

ϕLE

ϕCT

ν̅a (cm−1)

ν̅f (cm−1)

water DMSO glycerol ACN DMF MeOH EtOH i-PrOH i-BuOH 1-chloro butane DOX CYC MCH

291 300 297 292 299 296 295 288 296 284 294 279 284

327 344 340 340 349 341 342 340 346 334 341 328 332

459 414 418 440 406 417 404 394 396 --------------------358 -----------

0.0001 0.0126 0.0277 0.2944 0.1224 0.0034 0.0115 0.0116 0.0432 0.1349 0.0350 0.0119 0.0191

0.0057 0.1505 0.2741 0.2534 0.0956 0.0510 0.1093 0.0874 0.1738 --------------0.0089 --------

34364 33333 33670 34246 33445 33783 33898 34722 33784 35211 34013 35842 35211

21787 24155 23923 22727 24591 23981 24752 25381 25252 29940 29325 27960 30120

(νa̅ − ν̅f) (cm−1) 12577 9178 9747 11519 8854 9802 9146 9341 8532 5271 4688 8011 5091

a The quantum yields of emission from locally excited state (ϕLE) and from intramolecular charge transfer state (ϕCT) of Dapsone in different solvents at room temperature.

formed, and from this possibly another state ‘B’ with a larger dipole moment is originated due to reorientation of solvent molecules. It should be noted that the absorption spectra in solvents of different polarity do not show a large shift of band position (Figure S2 Supporting Information). As the ground and excited state dipole moments are collinear the LippertMataga relation43 has been used to determine the change in dipole moment (Figure S3 Supporting Information) from ground to excited state using the relation

spectra of Dapsone in mixed water-RTIL solvent do not change significantly upon addition of RTIL. In MCH Dapsone shows only one fluorescence band, whereas in polar solvents the spectrum possesses two bands (Figure 2) which are independent of concentration over a wide

Δν̅ = νa̅ − ν̅f = [2|μe − μg |2 /hcρ3] × [{(εr − 1)/(2εr + 1)} − {(n2 − 1)/(2n2 + 1)}]

(1)

where va̅ and vf̅ are the absorption and emission maxima wavenumber in cm−1, respectively. The terms h, c, r ε, and n correspond to Planck’s constant (6.626 × 10−34 Js), velocity of light in vacuum (2.9979 × 108 ms−1), dielectric constant, and refractive index44 of the solvent, respectively. Onsager cavity radius (ρ) have been computed 5.3 Å for the minimum structure of the molecule at the DFT level (B3LYP/6-31G +(d,p)). The estimated change in dipole moment, caused by the redistribution of atomic charges in the excited state, may be computed as Δμ⃗ (μ⃗ e−μ⃗g) = 7.6 D̅ by using the Lippert-Mataga relation. The above results points to the charge transfer characteristic of the probe molecule. The quantum yields for the hydrogen-bonding solvents were found to decrease (Table 1), and in water it is very small due to active nonradiative hydrogen bonding channel.45 It seems that the hydrogen bonding ability46 of the solvent plays a crucial role in the stabilization of ‘B’ state. Addition of water in ethanol causes a decrease in fluorescence intensity which obviously confirms the nonradiative hydrogen bonding effect and the ‘B’ band shifts toward red. The plot (Figure S4, Supporting Information) of Stokes shift vs solvent polarity parameter ET(30)47 clearly indicates that two types of interactions are present in the system, one is dipolar interaction and another is H-bonding interaction (Table T1 Supporting Information). On addition of RTIL to an aqueous solution of Dapsone the emission intensity of locally excited (A) band increases more

Figure 2. Emission spectra of probe molecule Dapsone in methyl cyclohexane, chloroform (CHCl3), acetonitrile (ACN), methanol (MeOH), and water at room temperature (λext = 290 nm).

range (10−7−10−4 M). The dual fluorescence38,39 in high polarity solvents generally comprises of a normal emission from the Franck−Condon (A) state and an anomalous emission from the ‘charge transfer’ (B) state. Both of these states arise from the same S1 potential energy surface. Generally a donor and an acceptor connected through an electron bridge in a molecule emit dual fluorescence40 from S1 singlet excited state. The excitation spectra (Figure S1, Supporting Information) of Dapsone in MCH (monitored at 332 nm) and also in ethanol (monitored at 342 and 404 nm, respectively) match well with the absorption spectra which indicates that all the emissions originated from the ground state species and rules out any involvement of impurity. Distinctive bathochromic shift of the second anomalous band from 358 nm (in cyclohexane) to 459 nm (in water) indicates that the emitting species must have a larger dipole moment compared to ground state.41,42 After photoexcitation, ‘A’ state is 2507



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than the charge transfer (B) band with a little hypsochromic shift compared to that in bulk water (Figure 3). This indicates

Figure 4. Emission spectra (λext = 290 nm) of Dapsone in (1) pure ethanol, (2) in the presence of dilute base (TEA), and (3) in the presence of dilute acid (HCl). Figure 3. Emission spectra of probe molecule Dapsone in water-RTIL mixed solvent at room temperature (λext = 290 nm) (1) 0% RTIL, (2) 3% RTIL, (3) 6% RTIL and 10% RTIL.

and as a result formation of both the locally excited state and charge transfer state gets decreased. 3.2. Emission at 77 K. Dapsone shows similar spectral signature (Figure 5) in ethanol glass matrix (77 K) as that of

that the water-RTIL mixed solvent is less polar than that of bulk water. In the water-RTIL mixed solvent two main factors are polarity and viscosity. The ET(30) values for neat RTIL [bmin][BF4] and water are 48.9 and 63.10,48,49 respectively, and the corresponding coefficient of viscosities (η) are 154 and 1.002 cP.48 As we add RTIL to the aqueous solution of Dapsone the polarity decreases and viscosity increases. Due to the reduction of polarity the charge transfer emission should decrease, like other organic solvents, and due to the increment of viscosity of solvent the rotational movement of aniline group gets hindered, i.e. formation of TICT state gets hindered which in turn would decrease the charge transfer emission, if its origin is set to be TICT state. But we observe an overall increase in emission in mixed solvent. This can only be explained considering many elements which contribute to the enhancement of emission of both LE and charge transfer band. An increase in RTIL in mixed solvent reduces the nonradiative hydrogen bonding channel45,50 along with an increase in viscosity driven rotational hindrance of aniline groups and the associated reduction of a nonradiative path through TICT dark state. The addition of RTIL in water also causes upshift of excited states and related reduction of a nonradiative path due to decreased polarity. In addition the another important fact is that the transition from the Franck−Condon state (A) to the charge transfer state (B) decreases in mixed RTIL-water solvent, and as a result the intensity of A state increases about 30%, whereas the charge transfer band increases by ∼15%. Addition of acid or base to an ethanolic solution of Dapsone fails to show any appreciable change in the absorption spectrum, which rules out the possibility of ground state protonation of the molecule. No measurable change in the emission spectrum could be observed by adding base to the ethanolic solution of Dapsone, but addition of the very low concentration of acid in MCH and ethanol solution of Dapsone results in a decrease in fluorescence intensity (Figure 4) of both ‘A’ and ‘B’ states without any shift of the band. Probably the H+ ion of acid binds to the lone pair of one or both nitrogen atoms of the amino group (−NH2) of the probe increasing the nonradiative channels and after photoexcitation charge migration from nitrogen to the sulfur atom gets hindered,

Figure 5. Spectral signature of Dapsone in ethanol at different low temperatures up to 77 K.

fluorescence in room temperature with sharp structures and massive intensification (∼X5), but no phosphorescence could be observed. In MCH glass matrix Dapsone shows a broad redshifted single peak at 406 nm compared to a room temperature band at 332 nm, and here also the phosphorescence is absent. An increase in effective solvent polarity in MCH at low temperature51 and that stabilizes the excited ‘B’ state at lower energy than that in ethanol glass matrix (Figure S5 Supporting Information). We have taken steady state temperature-dependent emission in a wide range (from ambient to 77 K) which shows a gradual increase in emission intensity (both bands) as we decrease the temperature from ambient temperature to 77 K (Figure 5). At a glance the intensification of both the bands in ethanol glass matrix is primarily due to a decrease in all types of nonradiative processes. To explain the enhancement of a charge transfer (B) state in frozen matrix, one would be tempted to think of a nonemissive state (C) acting as an energy wasting funnel in all environments other than frozen matrix, where the rotation is ceased. A decrease of fluorescence quantum yield (Figure 6) on increasing the environment temperature has also been observed due to the enhanced movement of the molecule and opening of 2508



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change is not necessary to achieve CT configuration like we found in a water-RTIL mixed solvent (vide supra). In order to get more insight about the rotational motion of an aniline group of the probe, the steady state emission anisotropy was measured as a function of wavelength. A very high anisotropy (0.27) in the initial part of the emission spectrum (Figure 8) was found in all the solvents measured. In

Figure 6. Emission spectra (λext = 290 nm) of Dapsone at (i) 283 K, (ii) 293 K, (iii) 303 K, (iv) 313 K, (v) 323 K, and (vi) 333 K in ethanol.

more and more nonradiative channels.52 Normally an increase in temperature causes a decrease in local viscosity of the solvent, which facilitates rotation and increases the rotation dependent nonradiative decay. This fact also vindicates the presence of an energy-wasting funnel which draws energy through it. 3.3. Viscosity Effect on Fluorescence Spectra. The basic idea of measuring fluorescence in an ethanol-glycerol mixture is to control the local viscosity of the solvent without greatly altering the polarity of the solution. Figure 7 clearly shows that

Figure 8. Steady-state emission anisotropy of Dapsone as a function of wavelength in glycerol, chloroform, and methanol.

glycerol there was a little decrease (from 0.27 to 0.20) of anisotropy with increasing wavelength indicating considerable decrease in nonradiative channels due to hindrance in rotational motion, while unconstrained rotation in chloroform is reflected from a considerable decrease (from 0.20 to 0.05) of anisotropy. This again raises concern about the nature of the charge transfer state not being molecular geometry dependent.

4. FLUORESCENCE LIFETIME The fluorescence life times (Figure 9) have been measured by exciting the probe at 295 nm and monitored at the two steady state emission maxima of ‘A’ (∼330 nm) and ‘B’ (∼400 nm). The lifetime of Dapsone, regardless of collecting the emission at ‘A’ or at ‘B’ bands, is found to fit well with a biexponential function as24

I(t ) = D1 exp( − t / τ1) + D2 exp( − t / τ2)

(2)

where τ1 and τ2 are the fluorescence lifetime associated with the high energy species (A) and the low energy species (B), and D1 and D2 are the pre-exponential factors. The biexponentially fitted lifetime of Dapsone in different solvents, the contributions of different species in terms of pre-exponential factors and χ2 are also mentioned in Table 2. Analysis of the decay profile by monitoring at the low energy band maxima indicates the formation of two distinct species in all solvents. The component of lifetime which changes much with the environmental polarity and viscosity may be assigned to the lifetime of the charge transfer state (B) and the other as the lifetime of the normal excited state (A). The slow decay of the charge transfer species in viscous environments supports the restricted orientational reorganization of solvent molecule. In chloroform, one species having a lifetime of 0.17 ns (relative abundance 67.03%) and the other is 0.68 ns (relative abundance 32.97%), in ethanol, they are 0.16 ns (11.83%) and 0.74 ns (88.17%), while in water the relative abundance of the longest lifetime increases to 96.98% with a lifetime value

Figure 7. Viscosity dependent emission spectra at room temperature of Dapsone with (C0) pure ethanol, (C1) 20% glycerol, (C2) 40% glycerol, (C3) 60% glycerol, and (C4) 100% glycerol.

the intensity of the emission spectra increases (∼6 times) as the concentration of the glycerol in ethanolic solution increases, and the peak position is shifted toward higher wavelength. Glycerol being more polar than ethanol it stabilizes the excited state to a greater extent, and hence a red shift of charge transfer band ensues. It is natural that the rotational motion of the aniline group53,54 should be hindered in viscous medium. The observation of massive intensity enhancement in viscous solvent similar to that in frozen matrix (at 77 K) only vindicates the consideration of the presence of an energy wasting channel (vide supra) in other solvents, where the rotation is not hindered. This result confirms that geometry 2509



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maxima corroborates the above results nicely. The above observations suggest that the lifetime of the charge transfer state increases as we go from lower dielectric constant to the higher dielectric constant of the medium. A possible explanation for the increase in τf may be due to the rearrangement of electronic excited states with increasing solvent polarity. So it is clear that viscosity and the dielectric constant of the local environment affect the excited species formation. From the values of quantum yield (ϕf) and lifetime (τf) of Dapsone in all solvents, the radiative and nonradiative rate constants of both the ‘A’ and ‘B’ states were calculated using the equation φf Kr = τf (3a) and

1 = K r + K nr τf

(3b)

where Kr and Knr are the radiative and nonradiative rate constant, respectively, and ϕf is the fluorescence quantum yield. The values of radiative and nonradiative rate constants for ICT and LE were computed from the respective contributions of LE and ICT lifetime and steady state emission (LE and ICT) data CT (Table 2) where it is seen that the magnitude of Knr is about CT two orders higher than Knr , and low fluorescence quantum yield indicates that the nonradiative process being dominant in the excited state photophysics. The radiative rate of the first band in water is very low (0.62 × 106 s−1) than that of the second band (6.48 × 106 s−1), probably due to the high polarity. Opposite of this found in chloroform, the radiative rate for the first band is higher than that of the second band, which again is due to extremely low polarity (Table 2). In a restricted environment like glycerol the radiative rate is much higher than the other solvents indicating the inactivity of the nonemissive energy wasting funnel.

Figure 9. Time-resolved decay profile of the probe molecule in chloroform, ethanol, and glycerol at room temperature monitoring at their corresponding steady state emission wavelength. (IRF = instrument response function.)

0.88 ns. The increase in amplitude of the decay of the second band from chloroform to water reflects the efficient conversion from the Franck−Condon state (A) to the charge transfer state (B). The equality of the decay times measured at LE and CT bands (Table 2) is quite indicative of the process of transition of LE to ICT being barrierless. The lifetime of the species formed in glycerol is 0.21 ns (∼28%) and 1.92 ns (∼72%). In water-RTIL mixed solvent the lifetimes of Dapsone are 0.25 ns (93%) and 3.98 ns (∼7%) for LE and ICT bands, respectively. The interesting observation here is an almost reversal of amplitude of the two lifetimes compared to that in neat water, which possibly indicates a huge reduction in LE → ICT rate. The increase in ICT lifetime 0.88 ns in water to 3.98 ns in water-RTIL mixed solvent is due to a combination of increased viscosity and decreased nonradiative process in mixed solvent. The measured lifetime also by monitoring at high energy band

5. TIME-RESOLVED EMISSION SPECTRA (TRES) Time-resolved emission spectra (TRES) of Dapsone were constructed and fitted to log-normal function as described by Maroncelli and Fleming.55 This is the measurement of the evolution of the emission spectrum with time. Normalized fluorescence emission spectra in ethanol with delay times of 0.828 ns, 1.66 ns, 2.48 ns, and 3.31 ns are shown in Figure 10. There is a noticeable change in the spectral peak of the charge

Table 2. Lifetime of Both the Bands and the Radiative and Nonradiative Rate Constants of the Probe Molecule in Different Solventsa lifetimes (λmon = 327−342 nm) solvent (dielectric constant) chloroform (4.81) ethanol (24.50) glycerol (42.5) water (80.01)

lifetimes (λmon = 373−459 nm)

ϕfLE× 10−3

ϕfCT × 10−3

τ1 (ns) (amp)

τ2 (ns) (amp)

KrLE (× 106 s−1)

KnrLE (× 109 s−1)

τ1 (ns) (amp)

τ2 (ns) (amp)

Kr CT (× 106 s−1)

KnrCT (× 109 s−1)

8.8

9.3

0.14 (95.24)

0.62 (4.76)

51.76

7.09

0.17 (67.03)

0.68 (32.97)

13.67

1.45

11.5 27.7 0.1

109.3 274.1 5.7 6.25

0.18 (69.55) 0.20 (69.44) 0.17 (87.24)

0.72 (30.45) 1.48 (30.56) 0.93 (22.76)

63.88 138.50 00.62

5.49 4.87

0.16 (11.83) 0.23 (24.64) 0.11 (3.02)

0.74 (88.17) 1.42 (75.36) 0.88 (69.88)

147.70 193.03 06.48

1.20 0.51 1.13

a

The sample was monitored at higher energy band (LE) maxima (λmon = 327−342 nm), the charge transfer band maxima, and low energy band (ICT) maxima (λmon = 373−459 nm) in these solvents. (λexe = 295 nm.) 2510



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the optimized ground state geometry. In the minimum energy ground state structure Dapsone has both phenyl rings perpendicular to the N−S−N exactly similar to the earlier report.25 The ground state dipole-moment of Dapsone in vacuum is found to be 6.2 D which indicates an asymmetrical charge distribution in the ground state. As the Dapsone molecule contains 29 atoms with several heavy atoms a detail geometry optimization in excited state involves huge computer time so we have restricted ourselves only to low-lying excited states. We have tried to interpret the excited state phenomena qualitatively by theoretical calculation using both the HF-CIS method and TDDFT with 6-31G+(d,p) basis set. Tomasi’s Polarizable Continuum Model (PCM), as implemented in Gaussian 03, is used to incorporate the solvent effect in transition energies.59 The theoretically computed dipole moment change from ground to excited state using the HF-CIS method for Dapsone without distorting the optimized geometry is 6.8D which is very close to the earlier reported result,26 and it matches fairly well with the experimental result (7.6D). The net positive charge on S decreases in the excited state from 3.239 to 2.287 (in au), and the net negative charge on N atom decreases from −0.506 to 0.287 (in au), i.e., charge migrates from N to S atom in excited state. Our calculation in the gas phase as well as in the solvent indicates that the maximum absorption observed at ∼290 nm corresponds to the S1 (π π*) state. The transition energies to this state in vacuum, acetonitrile, and ethanol are found to be 4.39 eV (282.36 nm), 4.28 eV (289.87 nm), and 4.24 eV (292.94 nm), respectively. The symmetry of the molecule changes from C2v in the ground state to C1 in the excited state. The change in dihedral angle O13−S11−C2−C1 from −21.980 to −26.510 indicates that the SO bond inclines toward one phenyl ring compared to symmetrically placed in the ground state. In contrast, the transition from HOMO (Highest Occupied Molecular Orbital) to LUMO (Lowest Unoccupied Molecular Orbital) has also been computed employing TDDFT-with B3LYP functional and 6-31G+(d, p) basis set. The TDDFT wave function calculation predicts that the S1 state has energies 4.44 eV, 4.33 eV, and 4.11 eV in vacuum, acetonitrile, and ethanol, respectively, as mentioned in Table 3. Interestingly, a decrease in positive charge density over the S11 atom and the negative charge density over O12, O13, N24, and N27 atom could also be observed as we add polar solvent (Table 4). From the results using HF/CIS and TDDFT it is noted that HF/CIS results are closer to the experimental results. To verify the applicability of the quantum chemical calculation as described before, the oscillator strength was calculated for both the cases from the integrated absorption intensity using the following relation

Figure 10. Time-resolved emission spectra of Dapsone in ethanol.

transfer band as time evolved. The evolution phenomenon of the emission spectra with time can indicate two distinct excited states. At the initial delay times the spectral signature visible in the short-wavelength side is the indication of fluorescence from the ‘A’ state. In more polar solvents, the charge transfer state becomes the low-energy state, and emission occurs at longer wavelengths. Time-resolved area normalized emission spectra (TRANES)56,57 are more useful than the more conventional peak normalized time-resolved emission spectra (TRES). The spectra in TRANES can be resolved into two components from isoemissive point at ∼375 nm (Figure 11), the redder one of

Figure 11. Time-resolved area normalized emission spectra of the probe molecule in ethanol between time zero and 2.48 ns at intervals of 0.828 ns.

which can be assigned to the charge transfer emission and the other can be designated as normal emission. The TRANES plot of Dapsone in ethanol clearly shows the dynamics of two excited state species - the gradual decrement of ‘A’ state emission (342 nm) with concomitant increase in ‘B’ state emission (400 nm) after photoexcitation.

f=

(4.39 × 10−9) n

∫ ε(ν)d(ν)

(4)

where n is the refractive index of the medium. We have found that state arising from HOMO to LUMO transition (S0―S1) has the maximum oscillator strength (∼0.42) vis-à-vis experimental result ∼0.4 (Table 3). It is to be noted here that the theoretically calculated value for the HOMO-1 to LUMO (S2) transition energy is ∼4.8 eV (260 nm). We also observe that with an increase in solvent polarity, the S1 state stabilizes significantly by dipole−dipole interaction corroborating experimental results. The charge migration from nitrogen atom to sulfur atom could clearly be seen from the Kohn−

6. QUANTUM CHEMICAL CALCULATIONS It is known that in the case of DMABN and other dimethylamino compounds that a minimum of 6-31G* basis set in either Hartree−Fock (HF) or Density Functional Theory (DFT) method is necessary to reproduce the experimental geometrical parameters.58 For Dapsone we have chosen 6-31G +(d, p) basis sets in B3LYP density functional methods to get 2511



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Table 3. Computed Transition Energy (eV) and Oscillator Strengths of Different Transition of the Probe in Different Solvents Using DFT and TDDFT Methods and the HF-CIS Methoda oscillator strength

transition energy (eV) (wavelength in nm)

theoretical

theoretical

medium

transitions

TD-DFT

HF-CIS

exp.

vacuum (methyl cyclohexane)

HOMO-1 to LUMO HOMO to LUMO HOMO-1 to LUMO HOMO to LUMO HOMO-1 to LUMO HOMO to LUMO HOMO-1 to LUMO HOMO to LUMO HOMO-1 to LUMO HOMO to LUMO

0.0351 0.4251 0.0397 0.4306 0.0435 0.4681 0.0381 0.4586 0.0724 0.4868

0.0325 0.4412

0.3976

chloroform acetonitrile ethanol water

0.3581 0.0395 0.4726 0.0394 0.4745

0.3913 0.4354 0.3876

TD-DFT

HF-CIS

exp.

4.90 (252.73) (254.35) 4.44 (279.47) 4.88 (253.92) 4.37 (283.44) 4.85(255.69) 4.33 (286.24) 4.78 (259.43) 4.11 (301.55) 4.65 (266.56) 4.18 (296.28)

4.88 4.39 (282.36)

4.37 (284) 4.34 (286)

4.72 4.28 4.67 4.24

(262.97) (289.87) (265.79) (292.94)

4.26 (291) 4.20 (295) 4.25 (292)

a

PCM model has been incorporated for the solvent inclusion for all the above mentioned methods. Experimental absorption energy (eV) and oscillator strength of the molecule in solvents of different polarity also reported in this table.

Table 4. Mullicken Charges in Atomic Unit (au) on the Atoms of the Probe in Gas Phase and in Water (Tomasi’s Polarizable Continuum Model (PCM)) Using TDDFT and the HF CIS Method and 6-31G+(d,p) Basis Set functional used Dapsone TDDFT-B3LYP HF-CIS Dapsone + Water TDDFT-B3LYP HF-CIS

O13

O12

S11

C2

C3

C4

C5

N27

−0.989 −1.005

−0.994 −1.001

3.375 3.242

−1.034 −1.039

−0.019 −0.023

−0.288 −0.283

0.153 0.149

−0.496 −0.507

−0.968 −0.972

−0.968 −0.981

2.988 3.011

−0.953 −0.989

−0.058 −0.049

−0.252 −0.168

0.131 0.132

−0.398 −0.407

7. CONCLUSION The observed results in this investigation like gradual emission enhancement of both bands in low temperature rigid matrix and massive intensity enhancement of charge transfer band in viscous media vis-à-vis that in solvents, theoretically optimized geometry in ground and excited state and excited state energy destabilization in orthogonal or twisted position of aniline ring, dipole moment change, increase of intensity of low energy band in water-ionic liquid mixture, decrease of charge transfer band with higher temperature, and rapid fall of anisotropy value in nonviscous solvent vis-à-vis steady anisotropy value in viscous solvent confirm the origin of charge transfer state is not due to geometry distortion (TICT) but an intramolecular charge transfer (ICT). Intensity decrement of the ICT band in a nonviscous and nonrigid environment necessitated us to conceive of a rotation involved dark state (possibly TICT) might be acting to bleed the excitation, and it plays a significant role in the dynamics of barrierless excited state (LF and ICT). Theoretical calculations predict that the excited state charge transfer system in S1 potential energy surface which has clearly been demonstrated in the TRANES picture as evolution of one species (LE) to the other (ICT).

Sham orbital picture (Figure 12) of HOMO and LUMO. From the observed experimental and theoretical results we expect

■

Figure 12. The HOMO (a) and LUMO (b) picture of Dapsone calculated by DFT using B3LYP functional.

ASSOCIATED CONTENT

S Supporting Information *

intramolecular charge transfer in the probe molecule upon excitation. As we rotate the NH2 group about C−N bonds or the phenyl ring about S−C bonds an increment in energy of

Definitions of quantum yield, steady state anisotropy and polarity function. The excitation spectra. Plot of (a) absorption maxima vs polarity function and (b) emission maxima vs polarity function. Lippert-Mataga plot. Plot of Stokes shift vs ET(30) parameter. Plot of fluorescence emission spectra of Dapsone at 77 K in (A) ethanol and (B) methyl cyclohexane. Table of the dielectric constant (εr), solvent polarity parameter

Dapsone was observed which possibly rules out the geometrically twisted charge transfer emission in gas phase as well as in acetonitrile solvent using the HF-CIS PCM method. 2512



Article

ET(30), solvent polarity function Δf, and hydrogen bond donor ability (β) of different solvents. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected].

ACKNOWLEDGMENTS The authors express thanks to Professor Subrata Ray and Professor P. K. Mukherjee for their kind help in the computational facility, and the authors also thank Mr. Subrata Das, Department of Spectroscopy, I.A.C.S, for taking picoseconds time-resolved data.

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