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Substituent Effects on the Absorption and Fluorescence Properties of Anthracene Salsabil Abou-Hatab, Vincent A. Spata, and Spiridoula Matsika J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b12031 • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 20, 2017

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The Journal of Physical Chemistry

Substituent Effects on the Absorption and Fluorescence Properties of Anthracene Salsabil Abou-Hatab,† Vincent A. Spata,‡ and Spiridoula Matsika∗,† Department of Chemistry, Temple University, Philadelphia, PA 19122 USA, and Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544-5263 E-mail: [email protected],Phone:215-204-7703

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To whom correspondence should be addressed Department of Chemistry, Temple University, Philadelphia, PA 19122 USA ‡ Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544-5263 †

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Abstract Substitution can be used to efficiently tune the photophysical properties of chromophores. In this study, we examine the effect of substituents on the absorption and fluorescence properties of anthracene. The effects of mono-, di-, and quad- substitution of electron donating and withdrawing functional groups were explored. In addition, the influence of donor-acceptor substituent pair and the position of substitution were investigated. Eleven functional groups were varied on positions 1, 2 and 9 of anthracene, and on position 6 of 2-methoxyanthracene and 2-carboxyanthracene. Moreover, the donor-acceptor pair NH2 /CO2 H was added on different positions of anthracene for additional studies of doubly substituted anthracenes. Finally, we looked into quadruple substitutions on positions 1,4,5,8 and 2,3,6,7. Vertical excitation energies and oscillator strengths were computed using density functional theory with the hybrid CAM-B3LYP functional and 6-311G(d) basis set. Correlations between the excitation energies or oscillator strengths of the low-lying bright La state and the Hammett sigma parameter, σp+ , of the substituents were examined. The energy is redshifted for all cases of substitution. Oscillator strengths increase when substituents are placed along the direction of the transition dipole moment of the bright La excited state. Substitution of long chain conjugated groups significantly increases the oscillator strength in comparison to other substituents. In addition, the results of quadruply-substituted geometries reveal symmetric substitution at the 1,4,5,8 positions significantly increases the oscillator strength and can lower the band gap compared to the unsubstituted anthracene molecule by up to 0.5 eV.

Introduction Semiconducting, π-conjugated organic materials illustrate a variety of potential applications and are of interest for use in applications ranging from electronic and optoelectronic devices 1,2 to uses in photovoltaic devices, 3,4 and much more. Anthracene, the three-ringed member of the acene family, is an organic chromophore which exhibits moderate fluorescence. The flu2


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orescence properties in anthracene and other conjugated π-systems are of particular interest for the development of new technologies, one of which is the development of aqueous fluorescent chemical sensors. 5 Anthracene derivatives are good candidates for fluorescent probes because they exhibit good emission properties with moderate to high quantum yields. 5,6 Tuning the fluorescence of these molecules based on the addition of small organic functional groups is then extremely desirable in order to aid our advancement of related technologies. The photochemical properties of anthracene have been studied for years. 7 It has two low-lying excited states, denoted La and Lb , which exhibit π-π ∗ character. 7–9 The bright La state has a transverse transition dipole moment along the short molecular axis, whereas Lb is a dim excited state with a longitudinal transition dipole moment along the long molecular axis, as illustrated in Figure 1a. The natural orbitals describing the two states are shown in Figure 1b. In anthracene the La state lies below the Lb state resulting in the molecule’s emissive properties. A triplet state has very similar energy to La resulting in a high yield of intersystem crossing (0.7). Previous studies have suggested that the addition of substituents on molecules including anthracene have increased the intensity of fluorescence. 7–17 Factors that have been found to influence the substituent effect on the fluorescence intensity include the dipole moment of the molecule, position of substitution, and donating or withdrawing characteristics of the substituent. 7–17 Likewise, a previous study suggests that the double substitution of electron donor-acceptor pairs would enhance the intensity. 12 Substitution can also affect the intersystem crossing and the resulting fluorescence quantum yield. 7 For example, in recent work substitution of thiophene containing derivatives on positions 9 and 10 has been shown to decrease the fluorescence quantum yield via intersystem crossing without changing the absorption properties of anthracene. 18 In this work we attempt a systematic computational investigation on how the addition of Electron Withdrawing (EWG) and Electron Donating (EDG) functional groups on anthracene affects the behavior of absorption and fluorescence. The properties of anthracene

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Table 1: Table of Hammett values for the substituents used in this work (taken from Ref. 20 ). N 1 2 3 4 5 6 7 8 9 10 11 12

Name Hydrogen Phenyl Bromo Cyano Methyl ester Carboxyl Methyl Methoxy Amino Hydroxyl Amide 1,3-Pentadienyl

X H Phenyl Br CN CO2 CH3 CO2 H Me MeO NH2 OH CONH2 C 5 H7

σp+ 0.00 -0.18 0.15 0.66 0.49 0.42 -0.31 -0.78 -1.30 -0.92

will be studied through examining the first two electronically excited singlet states predicted by Time-Dependent Density Functional Theory (TD-DFT). We are only focusing on singlet states in this work. Initial calculations are performed on mono-substituted anthracenes to determine the influence of substituent position and of the donating or withdrawing strengths of the substituent based on the Hammett parameter (σp+ ), an empirically derived parameter used to describe the extent of electron donation and withdrawal of substituents in terms of influence on reaction rates in aromatic systems. 19 In addition, the combined effects from the simultaneous addition of multiple functional groups such as in double- and quadruplesubstituted structures are also studied. The electron donor-acceptor interplay (based on Hammett value) is assessed through the double- and quadruple-substitution of functional groups in donor-acceptor pairs, opposing the EWG and EDG character, based on a variety of symmetric and asymmetric positions.

Methodology Mono- and double- substituted anthracene molecules were constructed by varying substituents X, consisting of Electron Withdrawing (EWG) and Electron Donating functional 5



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Groups (EDG) on anthracene. The donating or withdrawing strength of each group can be quantized through the use of the Hammett parameter, σp+ . The substituents used in this work, along with their σp+ value, are shown in Table 1. The substituent parameter developed by Louis Hammett is a constant which represents the relative effect of donating or withdrawing strength of a functional group on the reaction rates of addition reactions in substituted benzene derivatives. 19,20 In this work, we utilize σp+ to quantitatively monitor the extent of electron donating or accepting character of the substituent or multiple substituents on the nature of photobehavior in anthracene derivatives. Mono-substituted anthracenes were constructed by varying X on positions 1, 2, and 9 (labeling of atoms shown in Figure 1). Double substituted anthracenes were constructed by varying X on position 6 of 2-methoxyanthracene and 2-carboxyanthracene. This set of molecules was used to examine the effect of the nature of the substituents in the electron donor-acceptor substitute pair while the positions were kept fixed (2 and 6). In addition, in order to investigate the effect of the position of substitution of an electron donor-acceptor substitute pair, the pair of carboxy-amino was used and the position of substitution was varied. The carboxy functional group was substituted on position 1 of anthracene while varying the electron donating amine group on all positions of the outer rings. In addition, carboxy was substituted on position 2 and held constant while varying the addition of the amino group on positions 3, 6, and 7. Finally, the carboxy-amino pair was also put on positions 9 and 10 of the central ring. Quadruple substitution of donor-acceptor pairs was also studied by varying substitution patterns at the 2,3,6,7 and 1,4,5,8 positions on anthracene (see Figure 2). For all structures studied, two excited states were computed using TD-DFT with the CAM-B3LYP functional and the 6-311G(d) basis set in aqueous solution using the Integrated Equation Formalism variant of the Polarizable Continuum Model (IEFPCM). 21 All of the ground state geometries were optimized using B3LYP and the cc-pVDZ basis set in the gas phase, and the minimum was confirmed using vibrational frequencies. In the cases that

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of anthracene in the gas phase, reported in Table 2. Results from the literature using the approximation to CCSD (CC2) and CCSD with the addition of perturbative triples (CREOM-CCSD(T)), as well as experimental values, are also shown in the table for comparison. Experimentally, the energy of the bright, La , state is reported to be 3.60 eV and that of the Lb state is 3.64 eV. 25 These values have actually been extrapolated from the 0-0 origin transition as outlined below, and some uncertainty may be involved in them. The 0-0 transitions for the La and Lb states in the gas phase were reported to be 3.38 and 3.57 eV, respectively. 26,27 The 0-0 transition for the La state can easily be obtained from absorption spectra which exhibit clear vibrational progressions, 10 while it is more complicated to obtain the transition for the dark Lb transition. 26–30 The 0-0 transitions were used by Grimme and Parac, 25 after they were corrected for solvent effects. In their work the theoretical shift between vertical and adiabatic (0-0) transition energies was used to shift the experimental adiabatic energies in order to obtain the experimental vertical transition energies. Using TD-CAM-B3LYP/6-311G(d) the first excited state, S1 , corresponds to the La excited state and the second excited state, S2 , to Lb . As shown in Table 2 the energy of La at this level of theory is predicted to be 3.54 eV, very close to the experimental value. The energy of Lb however is overestimated by about 0.4 eV. Including diffuse functions with the 6-311+G(d) has a small effect on the energies and the oscillator strengths f . On the other hand, EOM-CCSD/6-311G(d) predicts the energies of La and Lb to be 4.05 eV and 3.83 eV, respectively. So, a switch occurs between S1 and S2 when using EOM-CCSD, which predicts that Lb is lower in energy. The energies of La and Lb at the CC2/cc-pVTZ level taken from previous work 25 are 3.69 eV and 3.89 eV, with the correct ordering. CR-EOMCCSD(T) is the most accurate method presented in the table and is expected to give the best results. Indeed the energies for both states are very close to experimental values, although the ordering is switched compared to experiment. This is a consequence of the very small energy gap predicted experimentally, 0.04 eV. This gap is smaller than the errors expected from the methods used, so a switching of the ordering of states can easily occur. Focusing on the

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Table 2: Energies of the first two excited states of anthracene in gas-phase using EOM-CCSD/6-311G(d), TD-CAM-B3LYP/6-311G(d) and TD-CAM-B3LYP/6311G+(d) and their corresponding oscillator strengths f in parenthesis. Computational results of the La and Lb excited states of anthracene using CREOMCCSD(T) with basis set POL1 9 and CC2/cc-pVTZ 25 from previous work are also shown. Experimental values are taken from Grimme et al. 25 La TD-CAM-B3LYP/6-311G(d) 3.54 TD-CAM-B3LYP/6-311G+(d) 3.52 EOM-CCSD/6-311G(d) 4.05 25 CC2/cc-pVTZ 3.69 CR-EOMCCSD(T)/POL1 9 3.69 Exp. 25 3.60

(0.083) (0.082) (0.104) (bright) (bright)

Lb 4.03 4.00 3.83 3.89 3.59 3.64

(7 × 10−4 ) (1 × 10−4 ) (6 × 10−4 ) (dim) (dim)

gap, CR-EOMCCSD(T) gives the most accurate value of 0.1 eV. The energy gap between the La and Lb state is overestimated by TD-CAM-B3LYP by 0.49 eV due to the incorrect prediction of the energy of the Lb state. The difficulty in the description of the two states arises from the nature of their excited state character. Anthracene has D2h symmetry, and the states obey the corresponding irreducible representations. The La state has B2u symmetry and has been described within a valence-bond description as exhibiting ionic character evident as a lack of overlap between the charged regions of the ground and excited state, 9 and has been suggested to behave like a charge transfer state in disguise. 31 The Lb state illustrates B3u symmetry and is described as more covalent in character with an increase in the overlap between charged regions in the ground and excited state. 9 A benchmark study on polyacenes of varying size has been performed by Lopata et al. to thoroughly compare TD-DFT methods capable of describing the relationship between the low lying La and Lb excited states relative to length of conjugation among varying acene family members. 9 The challenging charge-transfer like La state is described very accurately by the TD-CAM-B3LYP method but this results in a compromise which decreases the accuracy of the description of the Lb state. 9 TD-CAM-B3LYP predicts the La excited state with a higher level of accuracy than EOM-

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CCSD at a cheaper computational cost. Additionally, it is qualitatively able to describe the correct ordering of the La and Lb states based on their respective energy predicted by experimental results. Since our focus is on the first bright excited state, TD-CAM-B3LYP will be utilized throughout the remainder of the study. Furthermore, we will use the 6311G(d) basis set without diffuse functions since they do have a minimal effect, and, as we are studying a large number of systems, efficiency is very important. The Supporting Information (SI, Table S1) shows energies of several systems with both basis sets, and this comparison further verifies that for our comparative work the diffuse functions are not essential.

Solvatochromic Effect The energy and oscillator strength f of the La excited state for anthracene and a series of mono- and double-substituted anthracene derivatives were computed using TD-CAM-B3LYP both in the gas phase and in water solvent as modeled through IEFPCM. Comparison of the vertical excitation energies and oscillator strengths of the La state for molecules in gas phase and solution are presented in SI (Figure S1). Solvatochromic shifts of spectral bands are caused by the relative polarization of the states involved in the transition in the solute by the solvent electric field. The excitation energy of the La state is redshifted in aqueous phase consistently for a series of mono- and double-substituted molecules with electron withdrawing and electron donating properties. The average shift is 0.050±0.019 eV. These shifts agree with the expected solvatochromic shifts of anthracene in polar solvents. 28 Comparison between gas phase and solvated calculations also reveals the oscillator strength increases by 0.036 ± 0.026 in solution. All results of energies and oscillator strengths in the remainder of the paper are obtained using TD-CAM-B3LYP and IEFPCM. Anthracene is not very soluble in water, while it is soluble in ethanol and other polar solvents. Some of the other derivatives we have studied are more likely to be soluble in water. We chose water as a solvent because it is the most common solvent and most well studied with continuum solvation models, and we wanted a uniform solvent for all the systems we 10


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studied. So, even if this may not be the most directly related to experimental conditions solvent for all systems, it provides a more solid theoretical description. Our work further shows that solvation effects do not vary a lot with the various derivatives, so our conclusions should be valid in most of the solvents.

Absorption Mono-Substitution on Position 1, 2, and 9 of Anthracene The absorption energies and oscillator strengths of mono-substitution of anthracene are first examined. Because of the high symmetry of anthracene there are only three positions that are unique, positions 1, 2 and 9. Position 9 is on the central ring, while positions 1 and 2 are on the side ring, with 1 being closer to the central ring. The position of substitution relative to the transition dipole moment of the La excited state (along the short axis) is expected to be important. The excitation energies of the La excited state of the mono-substitution of EWG and EDG on positions 1, 2, and 9 of anthracene are shown in Figure 3a plotted as a function of the Hammett parameter σp+ . The energy is redshifted with the addition of substituents of both EWG and EDG character with the exception of the methyl substituent when it is substituted at position 2. The redshift of the absorption for various substituents agrees with experimental observations. 10 The data were analyzed to determine whether a correlation between the absorption and σp+ exists. A linear fitting of the curves shown in Figure 3a shows some correlation with the magnitude of σp+ . The linear fitting equations for all plots discussed in this work are shown in SI (Table S5). The values of R2 calculated from linear fits for EWG are 0.789, 0.878, and 0.848, for positions 1, 2, 9, respectively, and 0.668, 0.544, and 0.838 for EDG at the same positions, respectively. Substitution on position 9 in general causes a stronger redshift to the excitation energy compared to substitution on positions 2 or 1. Substitution of the phenyl group on position 9 lead to distortion of the planar structure of anthracene, and for this reason we excluded this result from the plots. 11




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The oscillator strength of the mono-substituted anthracenes is shown in Figure 3b. Substitution on position 2 does not affect the oscillator strength in almost all cases which remains the same as in anthracene, while the oscillator strength increases by 20-50% for the other two positions compared to anthracene. In exception to this trend, substitution of the long chain pentadienyl on all three positions is found to increase the oscillator strength significantly. For this substituent, the change in the magnitude of the oscillator strengths as a consequence of position is negligible with an average value of 0.34 (vs. 0.1 in anthracene). A correlation plot between the oscillator strength and σp+ is shown in Figure S2 and demonstrates that there is no meaningful correlation except for a moderate effect when EWG are substituted on positions 1 and 9. The substituent C5 H7 shows much higher oscillator strength than any other substituent. One may notice here that such substitution changes the length of conjugation which changes the character of states even qualitative. For this reason this substituent and other related conjugated substituents will be discussed in more detail in a separate section below. The oscillator strength of a transition is determined by the strength of the transition dipole moment. It is worth remembering that the transition dipole moment is a vector, and consequently, besides the magnitude, the direction is also important for certain photophysical properties of a state, such as the exciton coupling and energy transfer. The substituents affect the direction in addition to the magnitude. In order to illustrate this variation we calculated the angle between the short axis of anthracene derivatives (defined by the line through C9 and C10) and the transition dipole moments of La and Lb for the mono-substituted derivatives on position 2. These values are shown in SI (Table S3). In anthracene, La is parallel to the short axis so the angle is 0o , but this value can increase significantly depending on the substituent. The properties affected by the direction of the transition dipole moment are not the focus of the current study, so we will not explore the variation in the direction any further. The components of the vectors, however, are reported in SI, for anyone interested in the direction of the vectors.

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on energies or intensities compared to single substitution. 2-Methoxyanthracene derivatives have been studied as potential pH-sensitive fluorescence probes, and these studies have also demonstrated a dual fluorescence in these species due to the two possibilities of orientation of OH. 32 The excitation energies of the La state for these two groups of doubly substituted structures versus the Hammett parameters of the substituents are shown in Figure 4a,b. The energies of the La excited state for the double substituted systems are all redshifted compared to 2-methoxyanthracene or 2-carboxyanthracene, respectively. There is some correlation between the energy and the strength of the Hammett parameter (R2 = 0.6 − 0.9 for linear fitting, see Table S5) when examining separately the EWG and EDG. The strongest correlation is for 2-methoxyanthracene when the substituent on position 6 is EWG. The oscillator strengths of La are also plotted against the Hammett σp+ of the substituents in Figure 4c,d. In most cases the oscillator strength increases with the addition of the second substituent. For both systems some correlation between oscillator strength and σp+ is observed for EWG substitution (positive σp+ ). No correlation between the Hammett value and the oscillator strength is observed from substitution of EDGs to 2-methoxyanthracene (Figure 4c), while there is small correlation in 2-carboxyanthracene (Figure 4d). These results suggest that a pair of EDG-EWG substitutions does not always lead to increased oscillator strength. 6-Methoxyanthracene-2-carboxylic acid derivatives have been studied experimentally. 12 A previous experimental study suggested that the substitution of this electron donor-acceptor substituent pair on anthracene enhanced the fluorescence intensity. 12 This is in agreement with our calculated oscillator strength for this molecule which is 0.13 compared to 0.10 for anthracene. Experimentally, 6-methoxyanthracene-2-carboxylic acid (2-carboxy, 6-methoxyanthracene) has an absorption maximum at 3.31 eV, 12,33 which also compares well with our calculated value of 3.22 eV.

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Varying the position of EWG-EDG pairs: The effect of double substitution on different positions was also examined by using two fixed substituents, NH2 (EDG) and COOH (EWG), and placing them at different places on the two side rings of anthracene. This pair generated the highest oscillator strength in the previous section, so we can further explore how the position affects these values. It should be noted that in aqueous solution the NH2 /COOH derivatives may exist as zwitterions, and in that case their photophysical properties will be different than what has been calculated here. We report values for the standard forms here though, since we are mainly interested in the general effects of EWG-EDG pairs in various positions, rather than the specific properties of the NH2 /COOH pair. We use this pair as a test case. The excitation energies and oscillator strengths for the various positions are shown in Figure 5a,b in order of increasing value of f . Once again, the energy is always redshifted, with the strongest effect occurring when the two groups are not adjacent to each other, with maximum stabilization energy of ∼0.5 eV. The oscillator strength tends to increase when the two substituents are aligned along the short axis and decreases when the two substituents are along the long axis. The largest values for the oscillator strength are for substitution on (9,10) and (1,4). Overall using the NH2 -COOH pair of substituents on anthracene can lead to higher oscillator strengths which may lead to increased absorption and fluorescence intensity. The best combination is when the two substituents are in positions (9,10) and (1,4) leading to doubling of the oscillator strength compared to anthracene. Quadruple substitution We next examine the effect of four substituents on anthracene. Figure 2 shows the cases examined. Initially all four substituents were identical and were placed either on positions 1,4,5,8 (along the short axis) or 2,3,6,7 (along the long axis). Additional calculations were performed for quadruply-substituted anthracene where the substituents were added in pairs

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of two EDG and two EWG in differing symmetries on the molecular frame representing inversion, and transverse and longitudinal mirror symmetries (see Figure 2). Figure 6a shows the excitation energies of the first and second excited states for all cases of four substituted anthracenes, while Figure 6b shows the oscillator strengths in order of increasing value of f for S1 . The molecules with substituents on the long axis are shown to the left of anthracene while molecules with substituents on the short axis are shown on the right side of anthracene. In the discussion so far we only focused on S1 , but here we will look at both S1 and S2 for reasons that will become apparent in this section. The energy of both states is redshifted in all cases compared to unsubstituted anthracene. In general the energies are lower for substitution along the short axis, while the redshift is much smaller for substitution along the long axis. The effect is the strongest for C5 H7 substitution along the short axis. This case will de discussed separately below. Among the other substituents 1,4,5,8-NH2 substitution leads to the lowest excitation energy for S1 . The absolute value of the Hammett parameter for NH2 is the largest in magnitude compared to the other substituents considered in this analysis, so, as in mono-substitution, the Hammett parameter is important. Substitution with electron donor-pairs at various locations does not seem to be more effective compared to using the same substituent in all four positions. Figure 6b shows the oscillator strengths for the first two excited states for the same set of molecules. The oscillator strength shows a dramatic effect with substitution, much stronger than all other cases studied here. f of S1 increases by as much as five times when the substituents are along the short axis and decreases by the same factor for substitution along the long axis. The direction of the effect is in agreement with all previous systems where the oscillator strength increases when substituents are placed along the short axis, but the magnitude is much stronger here. The effect is stronger for EDG-EWG double pairs compared to all four substituents being the same. The substituent C5 H7 shows much higher oscillator strength when placed along the short axis. Quite interestingly for many of the 2,3,6,7 derivatives the La state has much smaller oscillator strength compared to Lb . This

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strength by 20%. In 1,4,5,8-NH2 substitution the energy is redshifted by 0.5 eV and the oscillator strength increases almost 50%. So the effect is much stronger for the quadruple substitution compared to single substitution, as one may expect.

Energy Gap between the La and Lb Excited States The energy gap between the brighter La first excited state and the dim Lb state may play an important role in the fluorescence behavior of these molecules. Particularly if the Lb state becomes stabilized relative to the bright state internal conversion from the bright to the dark state can take place leading to fluorescence quenching. For this reason we examine here how the gap changes with substitution. The La and Lb states have perpendicular transition dipole moments so it is expected that the substituents will have different effects on each state. The energy gap between the La and Lb excited states of mono-substituted anthracenes on positions 1, 2 and 9 is plotted in Figure 7a. The energy gap between the La and Lb states varies upon substitution of the different functional groups. The average energy gap when substitution occurs on position 9 is 0.61 ± 0.06 eV, while it is 0.60 ± 0.06 eV for position 1 and 0.53 ± 0.06 eV when substitution occurs on position 2 of anthracene. Substitution at a position closer to the short axis increases the gap. The gap increases more when the NH2 substituent is used, which has the most negative Hammett parameter. In general, there is modest correlation between the gap and the magnitude of σp+ . The increase of the gap going from position 2 to 1 is in agreement with an experimental study on the substitution of hydroxyl on positions 1 (α) and 2 (β). 16 In 2,6 doubly-substituted anthracenes the energy gap between the La and Lb excited states was found to have a strong correlation (R2 =0.866) for substitution of EWG and EDGs on position 6 of 2-methoxyanthracene as portrayed in Figure 7b. The gap decreases as σp+ increases. On the other hand, Figure 7c shows a moderate correlation (R2 =0.411) for the addition of substituents on position 6 of 2-carboxyanthracene, and the gap increases as σp+ increases. The gap decreases in both of these cases when substituents are electron 20


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donor-acceptor pairs. TD-DFT predicts the energy gap in anthracene to be 0.53 eV. Experimentally the gap is thought to be 0.04 eV, so we estimate that TDDFT overestimates the error by 0.49 eV. Using this error we can estimate whether the gap will become negative for any substituent. In mono and double substituted systems the only clear case where switching is predicted is for C5 H7 substituent on position 2. Figure S4 in SI shows the gap along with the error so that it can be seen clearer whether a switching should be expected. The gap in quadruple substituted systems is shown in SI (Figure S5). Quadruple systems behave differently once again. The gap for substitution on positions 2,3,6,7 is much smaller (average 0.25 eV) while the gap for substitution on positions 1,4,5,8 is larger (average 0.67 eV). The former systems have an S1 state with weaker oscillator strength as discussed above, so they are not good fluorescent probes for more than one reason.

Fluorescence Up to now we have focused on the vertical absorption energies in anthracene and its derivatives. There derivatives can be more helpful in fluorescence applications however. An important question is whether the substituent effects observed for absorption are also seen in fluorescence. The most important requirement is that the fluorescent state is the same as the absorption state. Based on the previous discussion on the energy gap between the S1 and S2 states this seems to be true for most of the derivatives studied here (some of the quadruple substituted derivatives are exceptions). Experimentally, the absorption spectrum of anthracene is a mirror image of the fluorescence spectrum which is also a signature of both absorption and fluorescence attributed to the same state. Furthermore, if the S1 relaxation from the Franck-Condon region to the S1 minimum is similar for all systems then the trends discussed so far will likely apply to both absorption and fluorescence. If however the S1 minimum varies a lot depending on the substituent then fluorescence trends can be different from absorption. To address this question, geometry minimization of the excited La state 22


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that relaxation remains similar upon substitution. The Stokes shift (absorption maximum fluorescence maximum) was found to be 0.58 ± 0.03 eV, showing very small variance upon substitution. The geometries of the minimum of the ground state and the La state are shown in Figure 8b,c. The bond lengths of the outer rings change the most while bond lengths between atoms that contribute to the formation of the center ring are not distorted significantly. The largest distortion occurs for bonds C2-C3 and C6-C7 which decrease by 0.05 Å. The CC bonds adjacent to them increase by 0.04 Å, indicating a bond alternation pattern.

Conjugated Substituents Out of all substituents discussed in this study, pentadienyl substituents have the largest effect on both the energy of the La and Lb excited states and on the oscillator strength. They especially show a dramatic increase in the oscillator strength of La , and they are thus the most promising substituted systems if one wants to increase the intensity of the fluorescence in anthracene derivatives. The oscillator strength is 0.3 in all mono-substituted pentadienylanthracenes, regardless of the position of substitution. This value is almost three times as large as of the other substituents studied. In double-substituted systems where C5 H7 is one of the substituents the oscillator strength is 0.3 for 2-methoxy-6-pentadienyl-anthracene and 0.5 for 2-carboxy,6-pentadienyl-anthracene. In quadruple-substituted structures where C5 H7 is placed along the short axis the oscillator strength is even higher, 0.8, while the excitation energy has the smallest value, 2.7 eV. In summary, in all systems with at least one substituent being pentadienyl the oscillator strength is dramatically increased (except 2,3,6,7 quadruple systems). The addition of pentadienyl is also redshifting the excitation energy of the La state, which can be explained by the Particle in a Box model. 17 The gap between La and Lb increases as well, except in mono-substitution on position 2. We explored this idea further by examining other conjugated substituents with a varied number of carbons and double bonds placed on position 1. Table 3 shows the results for 24


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systems with 1, 2, and 3 double bonds for 1-propyl, 1,3-pentadienyl and 1,3,5-heptatrienyl substituents, respectively. The results show a monotonic behavior as the number of double bonds increases. The oscillator strength almost doubles with the addition of a double bond, while the excitation energy of state S1 decreases by 0.1 eV. The energy of S2 also decreases but much slower, so the net effect is an increase in the gap between La and Lb . The excited states for these systems are delocalized in both anthracene and the conjugated substituent. As a demonstration we show the natural orbitals describing the first two excited states for 1-heptatrienyl-anthracene in SI (Figure S7). Table 3: Energies in eV and oscillator strengths of anthracenes substituted with an enyl group on position 1. No. Double Bond 1 2 3

No. Carbons E(S1 ) f (S1 ) 3 3.352 0.1879 5 3.242 0.3476 7 3.130 0.6652

E(S2 ) f (S2 ) 3.973 0.0028 3.948 0.0142 3.853 0.4760

S2 -S1 Gap (eV) 0.621 0.706 0.723

The consideration of conjugated groups as substituents on anthracene may have direct application in furthering the development of fluorescent sensors. Our results show that pentadienyl-substituted anthracenes should exhibit stronger fluorescence with higher quantum yield due to the increase in oscillator strength, decrease in the energy of absorption, and the additional increase in the energetic gap between the La and Lb states. Similar effects should also be expected to result in other members of the acene family such as tetracene or pentacene and could be applicable to a variety of technologies. The possibility of photochemical reactions competing with the desirable fluorescence is possible for all anthracene derivatives. 7 In these derivatives additional reactions based on the conjugated substituent are possible. For example, pentadienyl could be involved in electrocyclic ring closure reactions similar to the one in butadiene. So, the photochemistry should be considered carefully if one wants to develop the suggested fluorescent sensors further.

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Comparisons with experiment More detailed comparison between the systems studied here and available experimental data is given here. The UV spectrum of anthracene shows vibrational structure, with peaks equally spaced around 1400 cm−1 . These spectra can provide the 0-0 adiabatic excitation energy for anthracene and the other substituted systems studied. 10 Substituent effects on anthracene were considered as early as 1947 by Jones, 10 where a number of substituents were studied. Among them there are two common derivatives with our study, 9-cyano-anthracene and 2-amino-anthracene. The redshift for 9-cyano-anthracene according to Jones is 0.23 eV for almost all vibronic peaks, in very good agreement with our value of 0.21 eV. The shift for 2-amino-anthracene is more complicated experimentally since it is not constant for all vibronic bands (this may be because of mixing between La and Lb ). The shift for the 0-0 band is 0.28 eV while our value is 0.17 eV. Another early study focused on hydroxy substituted anthracene in positions 1 and 2. 16 The experimental redshift for 1-hydroxy-anthracene (α-anthrol) is 0.11 eV while the value we calculated is 0.06 eV. Again, the vibronic structure for 2-hydroxy-anthracene is more complicated for comparisons due to mixing with Lb . These experiments also reveal an increase of the La -Lb gap going from position 2 to 1, in agreement with our work. A more recent study focused on 2,6 doubly substituted anthracenes, and specifically 6-methoxyanthracene-2-carboxylic acid. 33 The absorption maximum of this compound in methanol is 3.31 eV, while our calculated value in aqueous solution is 3.22 eV, in quite good agreement. In a different study, 9-bromo-anthracene in CCl4 has a first peak at 3.16 eV while anthracene in the same solvent has a first peak at 3.27 eV, resulting in a redshift of 0.11 eV for 9-bromoanthracene. 34 The same redshift has been predicted by our calculations. The excited state dynamics of 9-carboxy-anthracene (anthracene-9-carboxylic acid) has been studied experimentally in more detail including time resolution experiments. 35 The experimental redshift for this system in acetone solution is 0.05 eV while we have calculated it to be 0.12 eV. The Stokes shift for 9-carboxy-anthracene is 0.56 eV, in excellent agreement with 26


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our value of 0.58 eV. Finally, last year a large series of functional 9-substituted anthracenes was synthesized and evaluated for the ability to undergo [4+4]-cycloadditions. Even though the substituents were mostly different from ours their conclusion was similar to ours, finding that the absorption and emission maxima of the prepared monomeric anthracenes red-shift with increasing electron-donating power of the 9-substituent. 36 Overall, although there are not experimental data available for all the systems presented here, our results agree well with the available experimental results.

Conclusions In this work we have examined how the addition of functional groups in anthracene affects its photophysical properties. The substituent effects on the absorption and fluorescence properties of anthracene were computationally examined using TD-DFT and the hybrid long-range corrected functional, CAM-B3LYP. We have considered changes in the behavior based on the position of mono-substitution, double-substitution and quadruple-substitution, and the donating/accepting character of substituents in double-substituted and quadruplesubstituted anthracenes. Substitution of EWG and EDG on anthracene was found to redshift the energy in almost all cases. The position of substitution has a great impact on the excitation energies. Mono-substitution on position 2 of anthracene has no significant effect on the oscillator strengths f , however addition of substituents on positions 1 and 9 enhances f significantly. Likewise, double- and quadruple- substitution increases the oscillator strength significantly. Substitution of electron donor-acceptor pairs enhances fluorescence in double-substituted anthracenes, however, in quadruple-substituted anthracenes the differences between donoracceptor pairs and all donor or all acceptor is comparable. While quadruple substitution along the short axis (positions 1,4,5,8) increases the intensity significantly, substitution along the long axis makes the first excited state mostly dark, making these derivatives inappropri-

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ate fluorescent probes or chemosensors. This work predicts that the most promising systems for fluorescent chemosensors are enyl-substituted anthracenes due to dramatic increases in oscillator strength, smaller band gap, and increased La and Lb energetic separation. Minimization of the excited state of anthracene shows that bonds with more antibonding character elongate and those with bonding character shrink as the excited anthracene relaxes. The relaxation energy is 0.4 eV while the overall Stokes shift is 0.6 eV. The fluorescence energy and intensity of anthracene and double substituted 2-carboxyanthracenes were examined in order to compare substituent effects in absorption and fluorescence. These results show that the effects are very similar and one may use the absorption properties to predict the properties of fluorescence. Supporting Information: Additional figures and tables of energies and oscillator strengths; Cartesian coordinates for all molecules; x,y,z components of transition dipole moments,

Acknowledgements This material is based upon work supported by the National Science Foundation under grant CHE-1465138.

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