Pyrene and its selected 1-substituted derivatives revisited: A combined spectroscopic and computational investigation

Journal of Molecular Structure xxx (2018) 1e9

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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc

Pyrene and its selected 1-substituted derivatives revisited: A combined spectroscopic and computational investigation a € Hümeyra Orücü , Nursel Acar b, * a b

_ Department of Physics, Faculty of Science, Ege University, 35100 Bornova / Izmir, Turkey _ Department of Chemistry, Faculty of Science, Ege University, 35100 Bornova / Izmir, Turkey

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 September 2017 Received in revised form 6 February 2018 Accepted 23 March 2018 Available online xxx

Pyrene and its eOH, eNH2, and eCN substituted derivatives were investigated experimentally by using UV/Vis absorption spectroscopy, steady state and time-resolved fluorescence spectroscopy in different polarity solvents. The computational part includes investigation of Pyrene and derivatives in excited S1 state in gas phase and in solution. Calculations were carried out with density functional theory (DFT) and time-dependent density functional theory (TDDFT) at B3LYP/6-311þþG(d,p) level. Both ground and excited state geometries were fully optimized in gas phase and in solution. Solution calculations were carried out with Polarizable Continuum Model (PCM). Current results indicate that solvent polarity did not affect Py and its derivatives except PyNH2. Solvent has minor effects on PyCN and PyOH. Although PyNH2 has he smallest HOMO-LUMO energy gap, it is not the best candidate due to its shortest fluorescence lifetime. On the other hand, PyOH and PyCN have longer lifetimes. Therefore, it is concluded that investigated molecules are appropriate candidates for photosensitive applications in the order of PyNH2
Keywords: Pyrene Pyrene derivatives Intramolecular charge transfer Spectroscopy Time-dependent density functional theory

1. Introduction Molecules with conjugated p electronic systems are important as they are potential candidates for use in many different fields of science and technology. The most important group among such molecules includes polycyclic aromatic hydrocarbons (PAHs) which have two or more fused aromatic rings. They have the ability to possess intra- and intermolecular photoinduced charge transfer as a result of p-p interactions. Because of this property, they are preferred in photosensitive material design [1,2]. Pyrene, an optically active polycyclic aromatic hydrocarbon (PAH), has attracted increasing attention because of its intra- and intermolecular photoinduced charge transfer applications [3e9] and its high absorption coefficient, long fluorescence lifetime and high fluorescence quantum yield [10e13]. It is widely used in different optical applications [14e16]. Therefore, investigation of structural, optical and charge transfer properties of pyrene and its derivatives are important. For example, 1-hydroxypyrene (PyOH) (Fig. 1), is a member of molecules classified as photoacids and is widely used to investigate excited state proton transfer systems

* Corresponding author. E-mail address: [email protected] (N. Acar).

[17e20]. Pyrene and its derivatives may also display strong bonded and nonbonded interactions with many different types of molecules. Many studies were reported for the development of organic and inorganic photosensitive materials and sensors by using the interactions of Pyrene with CNTs, porphyrins (p-p stacking interactions), phthalocyanines, liposomes, proteins and nucleic acids [21e32]. Additionally, Pyrene-based blue emitters for OLED devices were developed [33e35] and synthesis studies are still in progress for more efficient blue fluorescence emitters in Pyrene electroluminescence [36e38]. Our group also focused on intermolecular photoinduced charge transfer complexes formed by pyrene derivatives and biologically important molecules (aromatic amino acids, some drug molecules etc.) [39e41]. Formerly, intramolecular charge transfer processes for mimicking natural photosynthesis has been investigated in which pyrene acted as a light harvesting antenna [4e6]. In a recent computational work, we have investigated substituent effects on pyrene and its derivatives in different solvents in ground state [42]. However, molecular geometries may change in excited state and using ground state geometries may be insufficient. Additionally, the influence of substituents upon the behaviour of excited states is also still less known of pyrene derivatives.

https://doi.org/10.1016/j.molstruc.2018.03.098 0022-2860/© 2018 Elsevier B.V. All rights reserved.

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Fig. 1. Molecular structures of Pyrene (Py), 1-Hydroxypyrene (PyOH), 1-Aminopyrene (PyNH2) and 1-Cyanopyrene (PyCN).

Therefore, excited state geometries of pyrene and its derivatives were optimized using Density Functional Theory for a better interpretation of the changes in the spectra. Additionally, ground state geometries from previous study were further optimized at a higher level. UVeVis absorption, steady-state and time-resolved fluorescence measurements were performed experimentally to compare with computed results. This study reports spectroscopic and computational investigation of pyrene and its selected derivatives in ground and excited states in solvents with different polarities. Experimental and computational results were compared. Amino and Hydroxy substituents display electron donating character, and cyano substituent has electron accepting character. 2. Experimental and computational details The properties of the molecules are as follows: Pyrene, Py, (99%, Sigma), 1-Hydroxypyrene, PyOH, (98%, Aldrich), 1-Aminopyrene, PyNH2, (97%, Aldrich), and 1-cyanopyrene, PyCN, (synthesized in a former study [43] with the method given by Tintel et al. [44]). The solvents used are methylcyclohexane (99%, Aldrich, spectroscopic grade), tetrahydrofuran (99.9%, Merck), and acetonitrile (99.8%, Merck). Perkin Elmer Lambda 35 UVevis Spectrophotometer was used to record UVevis spectra. A quartz cuvette with a thickness of 1 cm was used. Perkin Elmer LS-55 Spectrofluorophotometer was used for steady-state measurements. Nitrogen gas was applied during fluorescence and lifetime measurements to minimize the

quenching effect of the dissolved oxygen. Time resolved fluorescence lifetime values were calculated according to the time-correlated single-photon counting (TCSPC) luminescence by using a SPEX Fluorolog-3 (Horiba-Jobin Yvon) spectrometer attached to an integrated TCSPC software. 295 nm Laserdiode was used as the excitation wavelength source for the lifetime measurements (295 nm nanoled, TAC (time-to-amplitude converter) range: 2 ms, repitation rate: 500 KHz). The temporal evolution of the fluorescence decay was monitored with a microchannel plate (Hamamatsu, R3809U-58) which provides a time resolution of up to 30 ps Emission maximum values of studied compounds were used for measuring fluorescence lifetimes: Py, PyOH, PyNH2 and PyCN (392 nm, 400 nm, 406 nm, and 402 nm in MCH, 392 nm, 400 nm, 426 nm, and 406 nm in ACN respectively) at room temperature. All molecules presented a monoexponential decay curves and fitted with formula A þ Bexp(t/t) using DAS6 program by HORIBA. B is the preexponential factor, A is the offset for the fitting, c2 (chi2) is the quality of fitting and t is the calculated lifetime in sec. Initial structures for the investigated molecules were prepared using Spartan 08 [45]. Computational investigation was carried out with Gaussian09 [46], and Gaussview5.0 [47] programs. All ground and excited state geometries of the molecules were optimized by density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods at B3LYP/6-311þþG(d,p) level both in gas phase and in solution. Frequency analysis was performed to check the optimized geometries. TDDFT at the above mentioned level was used to investigate the electronic transitions. 40 first singlet excited states were calculated and used to obtain the spectra for each molecule. Gaussview was used to visualize molecular orbitals and the UVeVis measurements of the investigated molecules. The total electron density surface of investigated molecules were calculated based on the electrostatic potential values in the gas state and in solution for the excited state optimized geometry. Solvation effect was taken into account by using the Polarizable Continuum Model (PCM) [48,49] and calculations were carried out in cyclohexane (CH, nonpolar), tetrahydrofuran (THF, medium polar), acetonitrile and water (ACN, and H2O, polar solvents). 3. Results and discussion 3.1. Experimental part: UVeVis absorption, fluorescence and lifetime measurements Ground and excited state properties of pyrene (Py), 1hydroxypyrene (PyOH), 1-aminopyrene (PyNH2), and 1cyanopyrene (PyCN) were investigated by spectroscopic and computational methods. UVeVis absorption and fluorescence spectra of studied molecules were recorded in solvents with different polarities. Experimental and computational results for 00 transitions were displayed on the same graph. Lifetime measurements were given for the solvents MCH and ACN. Molecular structures of investigated molecules are shown in Fig. 1. The UVeVis absorption spectra of investigated molecules in different solvents are displayed in Fig. 2. During the measurements, the concentrations of pyrene, hydroxypyrene, aminopyrene and cyanopyrene were used as 1.2  105 M, 1.5  105 M, 1.17  105 M, and 1.14  105 M respectively to prevent dimer formation. For Pyrene, the 0-0 transition was observed as a very small peak around 371 nm and was accompanied by other very weak vibronic peaks; on the other hand, the strong 0-0 peak corresponding to the second excited state was observed around 334 nm. It was clear that the 0-0 peak became more significant and the vibrational structure shifts by the addition of substituent

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Fig. 2. Normalized UVeVis absorption spectra of Py, PyOH, PyNH2, and PyCN in (a) methylcyclohexane (b) tetrahydrofuran and (c) acetonitrile.

groups. Although different solvents were used in measurements, no significant shifts were observed for 0-0 peaks for Py (371 nm) and PyCN (380e381.6 nm); thus, 0-0 peaks were not sensitive to the nature of the used solvent for two systems. On the other hand, 2e3 nm red shifts were observed for PyOH in MCH at 382 nm, in THF at 385 nm and in ACN at 384 nm, respectively. The most significant changes in UVevis absorption spectra were observed for PyNH2. These changes were measured at 397 nm in MCH, at 404 nm

3

Fig. 3. Normalized fluorescence spectra of Py, PyOH, PyNH2, and PyCN in (a) methylcyclohexane (b) tetrahydrofuran and (c) acetonitrile.

in THF and at 401 nm in ACN. In brief, it can be concluded that PyNH2 was affected most from the changes in the solvent polarity. In general, Py derivatives with electron donating groups like eOH and eNH2 were affected from solvent polarity. However, electron withdrawing eCN substituent resembles unsubstituted Py by using the ring electrons to form a charge balance to avoid the solvent effect. The fluorescence spectra of molecules in MCH, THF and ACN are

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seen in Fig. 3. Excitation wavelengths of molecules were determined by investigating single absorption spectra of them (Fig. S1). The peak at 372 nm refers to 0-0 transition in MCH for Py. It was very weak in the absorption spectra. This peak position did not change with solvent polarity, similar to the UVeVis absorption spectrum. However, the intensity of 0-0 peak (also called band I) changed [10,50]. PyOH displays a fluorescence spectrum with a regular vibrational structure in which intensity reduces with increasing wavelength at 340 nm excitation wavelength. The 00 transition at 381 nm in MCH for PyOH was observed in fluorescence spectra at the same wavelength. It shifted to longer wavelengths as 386 nm and 384 nm in THF and ACN, respectively. Fluorescence spectrum of PyCN which carries an electron withdrawing group (EWG) has three characteristic vibration peaks at 380, 401 and 424 nm in methylcyclohexane (MCH). The fluorescence spectrum has a similar structure to the spectrum of PyOH. First vibrational peaks for PyCN were 382 in THF and ACN. Similar to UVeVis spectrum, it is not much affected by the solvents (approximately 2 nm shift), highest peaks at 380 and 382 nm support the 0-0 transition which was seen from the UVeVis absorption spectra. The excitation wavelength is 365 nm for this spectrum. Similarity of Stokes shifts both in nonpolar and polar solvents may be explained by the fact that the geometries do not change in excited state. Fluorescence spectrum of PyNH2 shows also three vibrational peaks as 399 nm, 405 nm and 420 nm in nonpolar MCH. 0-0 peak was 397 nm in UVeVis spectra in same solvent. A small 2 nm stokes shift was observed for this system, too. Because of changes in vibrational structure with increasing solvent polarity, 0-0 peak (band I) cannot be clearly observed in polar solvents. Significant red shifts were seen in the S1/S0 fluorescence in more polar THF and ACN, at nearly 423 nm. Change in the fluorescence spectra and shift to longer wavelength with changing solvent polarity were observed only for PyNH2 among all studied systems. This may be explained by the changes in excited state geometry and a more effective solute-solvent interaction. In order to display these differences more clearly, fluorescence spectra recorded in different solvents are also given as supplementary information (Fig. S1). Additionally, experimental and computational UVeVis absorption and fluorescence spectra in MCH were displayed in Fig. 4; both were normalized for a better comparison. Computational 0-0 transitions were obtained from the intersection of overlapping S0/S1 and S1/S0 transitions which were drawn on the same graph and these values were given in Table 4 in computational section. S0/S1 transition of Py is not clearly seen on the spectrum as it is symmetry-forbidden; on the other hand, its S0/S2 transition is very intense. The fluorescence is defined as S1/S0 transition; as a result, fluorescence spectrum of Py does not resemble its absorption spectra although its geometry and vibrational structure did not change. Table 1 shows the measured fluorescence lifetimes of investigated molecules in MCH and in ACN. Detection wavelength (ldet) is determined from emission regions of the monomers. B is the preexponential factor, A is the offset for the fitting, c2 (chi2) is the quality of fitting and t is the calculated lifetime in sec. Fluorescence lifetime of Pyrene is very long (t, 362 ns in nonpolar solvent, MCH and 293 ns in polar solvent, ACN). Replacement of hydrogen atom at 1-position with different substituents significantly reduces the fluorescence lifetime. We calculated lifetimes in MCH about 37.8 ns, 26.8 ns and 7.16 ns for PyCN, PyOH and PyNH2, respectively (Fig. S4). A similar decrease is observed in ACN, too. The values measured in ACN are smaller than the ones in MCH. These results indicate that system immediately reaches the S1 relaxed state and then turns back to the ground state with a very fast process because of polar driving force. The fluorescence decay time of PyCN and

PyOH are close to each other; but, PyNH2 has the smallest values. This may result from the geometry change of NH2 substituent on PyNH2 in excited state and/or from fluorescence quenching. PyOH and PyCN have emissions in visible region; therefore, they may form energy processes with dye molecules which have absorption in the same region. 3.2. Computational part: geometry optimization Molecules were optimized at B3LYP/6-311þþG(d,p) level for ground state and excited S1 state in the gas phase and in solution by using Gaussian 09. Fig. 5 shows the molecular structures and some selected geometrical parameters of the optimized compounds. Geometrical parameters in parantheses are shown for S1 excited state. The distance between H and C atom for Py is close to a typical CeH bond, 1.1 Å. For Py derivatives distances between atoms 2-3 showing the distance between the substituents and pyrene ring are given on right side of each molecule. With change of H atom with different substituents, this distance increases approximately by 0.3 Å in the order PyOH < PyNH2 PyCN > PyNH2. Table 3 summarizes the solvation energies for the studied molecules calculated as the gas phase-solution energy differences for both in ground and excited states in different solvents. As seen from the table, the values are negative for both ground and excited states indicating stabilization of the studied systems by solvent. This stabilization is more significant in excited state. It is observed that solvation energy of the molecule is affected from increase of dipole moment, charge on the substituent and increase of the solvent polarity in excited state. The stability order of the molecules changes as Py < PyOH < PyNH2
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Fig. 4. Normalized experimental and calculated UVeVis absorption and fluorescence spectra of Py, PyOH, PyNH2, and PyCN in MCH (methylcyclohexane) (calculated spectra in cyclohexane).

Table 1 Fluorescence lifetimes (t), detection wavelengths (ldet), preexponential factors (B) and chi-square values (c2) of investigated molecules in MCH and in ACN.

lex ¼ 295 nm nanodiod

Pyrene

PyOH

PyNH2

PyCN

MCH, t (ns)

362

26.8

7.16

37.8

ldet, (nm) c2

392 1.66 9.31  102

400 1.24 3.16  102

406 1.27 1.7  102

402 1.05 2.99  102

B (preexp. fac.) ACN, t (ns)

293

22.4

5.70

21.0

ldet, (nm) c2

392 1.09 9.11  102

400 1.28 3.14  102

426 1.31 1.81  102

406 1.09 3.72  102

B (preexp. fac.)

at B3LYP/6-311þþG(d,p) level in ground and excited states by using TDDFT. Excitation energies of all molecules from S0 to S40 states have been calculated at the same level. Transitions, namely S0/S1 and S1/S0, are discussed in the text in gas phase and in solution (Table 4). S0/S1 electronic transitions calculated at B3LYP/6311G(d,p) and 6-311þþG(d,p) levels using the first 10 and 40 excited singlet states are given in Supplementary material for comparison in Table S1 and Fig S5. Additionally, experimental S0/S1 electronic transitions, 0-0 energies and fluorescence lifetime were given in the same table. The energies for frontier highest occupied molecular orbital (HOMO) and lowest unoccupied

molecular orbital (LUMO), are displayed for all investigated systems in gas phase and in different solvents. Electron densities for HOMO and LUMO are shown for the molecules at the excited state equilibrium geometry. Table 4 shows S0/S1 electronic transitions (labs), oscillator strengths (f), excitation character, and molecular orbitals with percentage contributions, experimental S0/S2 electronic transitions, S1/S0 emission wavelengths (lem), zero-zero electronic transition (l0-0), corresponding to excitation energies (E0-0), and measured fluorescence lifetime (texp) of Py and derivatives in different medium. Calculated S0/S1 values show a red shift between 7 and 12 nm both in the gas phase and in solution for all studied systems. There are no shift changes observed for this transition related to solvent polarity. However, 1, 2 and 5 nm shifts are observed for PyOH, PyNH2 and PyCN with respect to Py upon substituent change. S0/S1 electronic transition for Py is between HOMO (bonding porbitals on Py ring) and LUMO (antibonding p*-orbitals on Py ring) and is called locally excited (LE) transition. This p-p* transition includes minor amount of charge transfer transition in addition to LE. The transitions responsible for these shifts occur between H1/Lþ1 orbitals in the form of CT1 (charge transfer from substituent to Py ring for PyOH and PyNH2) and CT2 (from Py ring to substituent for PyCN) (Fig S8). These transitions may include n-p* transitions as substituent groups carry unshared electrons. The

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Fig. 5. Optimized molecular structures and some selected geometrical parameters of the optimized compounds (values in parantheses are for the S1 excited state.).

Table 2 Dipole moments (m), total electronic energies including zero point correction energies (Eelec þ ZPE) and the energy differences of Pyrene and the investigated Pyrene derivatives in the gas phase and in different solvents (Cyclohexane (CH), ε ¼ 2.02, Tetrahydrofuran (THF), ε ¼ 7.5; Acetonitrile (ACN), ε ¼ 37.5, Water (H2O), ε ¼ 78.5, GS: ground state, ES: excited state) calculated at B3LYP/6-311þþG(d,p) level. compound

solvent

mGS, D

mEX, D

Eelec þ ZPEGS (Hartree)

Eelec þ ZPEES (Hartree)

(Eelec þ ZPE)ES e(Eelec þ ZPE)GS (kcal)

Py

gas CH THF ACN H2O gas CH THF ACN H2O gas CH THF ACN H2O gas CH THF ACN H2O

0.0000 0.0000 0.0000 0.0000 0.0000 1.6180 1.8659 2.1630 2.2856 2.3050 2.0254 2.3629 2.7842 2.9621 2.9902 5.5220 6.3017 7.1308 7.4442 7.4923

0.0007 0.0008 0.0009 0.0010 0.0010 2.4202 2.9268 3.5225 3.7709 3.8105 3.5018 4.2875 5.3314 5.7988 5.8740 6.1737 7.3537 8.7288 9.2871 9.3750

615.709945 615.711825 615.714265 615.715351 615.715527 690.952978 690.956212 690.960204 690.961918 690.962194 671.068541 671.071921 671.076167 671.077987 671.078276 707.978743 707.982570 707.986756 707.988407 707.988666

615.585527 615.590577 615.597243 615.600205 615.600685 690.833339 690.839683 690.847895 690.851492 690.852073 670.956708 670.963267 670.971923 670.975723 670.976345 707.861115 707.868835 707.877938 707.881680 707.882271

78.07 76.08 73.43 72.25 72.06 75.07 73.12 70.47 69.29 69.10 70.18 68.18 65.41 64.17 63.96 73.81 71.37 68.28 66.97 66.76

PyOH

PyNH2

PyCN

shifts to longer wavelengths compared to experimental UVeVis absorption spectra support this conclusion. Molecular orbitals in gas phase, in THF and in H2O are given in Fig. S6. As seen in Table 4,

computational S0/S1 transition maximum wavelength is closer to experimental S0/S2 transition maximum wavelength compared to experimental S0/S1 transition wavelength (difference is 10 nm for

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Table 3 Solvation energies (ESOLV, kcal/mol) of the investigated molecules in ground and excited state calculated at B3LYP/6-311þþG(d,p) level. compound

solvent

GS

ESOLV ¼ (E þ ZPE)S-(E þ ZPE)G (kcal/mol)

ES

Py

CH THF ACN H2O CH THF ACN H2O CH THF ACN H2O CH THF ACN H2O

1.18 2.71 3.39 3.50 2.03 4.53 5.61 5.78 2.12 4.78 5.93 6.11 2.40 5.03 6.06 6.23

3.16 7.35 9.21 9.51 3.98 9.13 11.39 11.75 4.11 9.54 11.93 12.32 4.84 10.55 12.90 13.27

PyOH

PyNH2

PyCN

ESOLV ¼ (E þ ZPE)S-(E þ ZPE)G (kcal/mol)

Table 4 S0/S1 electronic transitions (labs), oscillator strengths (f), molecular orbitals, excitation character with % contributions, experimentally determined S0/S2 electronic transitions, S1/S0 emission wavelengths (lem), 0-0 electronic transition (l0-0), corresponding to excitation energies (E0-0), and measured fluorescence lifetime (texp) for the investigated molecules in different medium. exp.

S1/S0

l0-0,

Configuration :character

Transitions %

S0/S2 (lmax)

lem,

nm

S0/S1

labs, nm Py

PyOH

PyNH2

PyCN

f

Gas CH

339 346

0.266 0.411

H/L: LE H/L: LE

67 68

335

THF

346

0.404

H/L: LE

68

336

ACN

345

0.385

H/L: LE

68

334

H2O

345

0.381

H/L: LE

68

Gas

354

0.255

CH

361

0.408

THF

361

0.402

ACN

361

0.383

H2O

361

0.379

Gas

374

0.279

CH

383

0.420

THF

384

0.414

ACN

384

0.397

H2O

384

0.394

Gas

361

0.349

CH

371

0.514

THF

373

0.507

ACN

373

0.488

H2O

373

0.484

H/L: LE H/Lþ1: LE þ CT1 H/L: LE H-1/Lþ1: LE þ CT1 H/L: LE H-1/Lþ1: LE þ CT1 H/L: LE H-1/Lþ1: LE þ CT1 H/L: LE H-1/Lþ1: LE þ CT1 H/L: LE H-1/Lþ1: LE þ CT1 H/L: LE H-1/Lþ1: LE þ CT1 H/L: LE H-1/Lþ1: LE þ CT1 H/L: LE H-1/Lþ1: LE þ CT1 H/L: LE H-1/Lþ1: LE þ CT1 H/L: LE H-1/Lþ1: LE þ CT2 H/L: LE H-1/Lþ1: LE þ CT2 H/L: LE H-1/Lþ1: LE þ CT2 H/L: LE H-1/Lþ1: LE þ CT2 H/L: LE H-1/Lþ1: LE þ CT2

64 18 67 15 67 16 67 16 67 16 67 16 67 16 67 16 67 16 67 16 67 18 69 14 69 14 68 15 68 15

345 348 347

358 365 361

353 354 353

E0-0, eV

texp, ns

nm 368 381 (370)a 398 (371) 407 (371) 408

e 362

382

353 363 (370)a 370 (371) 388 (371) 373 (372) [51] 365

3.51 3.42 (3.35)a 3.35 (3.34) 3.20 (3.34) 3.32 (3.33) 3.40

396 (381)a 415 (386) 424 (384) 425

376 (382)a 384 (386) 387 (384) 388

3.30 (3.25)a 3.23 (3.21) 3.20 (3.23) 3.20

414

388

3.20

e

427 (400)a 446 (424) 456 (423) 457

401 (397)a 410 (405) 414 (400) 413

3.09 (3.12)a 3.02 (3.06) 3.00 (3.10) 3.00

7.16a

390

373

3.32

e

406 (379)a 410 (381) 438 (381) 440

387 (380)a 390 (381) 402 (381) 403

3.20 (3.26)a 3.18 (3.25) 3.08 (3.25) 3.08

37.8a

343 300 [52] 293 290 [53] e e 26.8a e 22.4 e

5.7 e

21.0 e

LE: local excited; CT1: intramolecular charge transfer from electron-donating group to Py moiety; CT2: intramolecular charge transfer from Py moiety to electron-withdrawing group. a In MCH; Experimental results are given in parenthesis. Computational results by B3LYP/6311þþG(d,p).

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Py). HOMO-LUMO energy gap decreases in the order of Py > PyOH > PyCN > PyNH2, which indicates that electron donating or electron withdrawing nature of the substituent is not very effective. Presence of lone pair electrons on N atom in PyNH2 and increase of the conjugation may cause this decrease. Experimental and calculated bathochromic shift of absorption maxima are observed in the same order (Table 4). Energy levels of HOMO and LUMO of PyCN appear at lower positions than those of others. This means CN group act as electron-withdrawing group. HOMO and LUMO levels of PyOH and PyNH2 are higher than those of Py, indicating that OH ve NH2 groups behave as electron-donating groups. This is also supported by calculated positive charge density on eOH and eNH2 groups (Fig S9). Calculated S1/S0 values show red shifts between 13 and 43 nm in the gas phase and in solvents for studied molecules except PyCN for which this range becomes 16e50 nm depending on the increase in solvent polarity. Since the vibrational states are not seen in computational spectra, deviations occur compared to the experimental spectra. Additionally, it is 20 nm closer to S0/S2 here, too. Computational and experimental S0/S2 transitions are in aggreement (Table S2). For a better understanding, 0-0 electronic transitions are used for comparison. Computational value is obtained by using the intersection of S0/S1 and S1/S0 values (Fig. 4, Fig. S2 and Fig. S3). When E0-0 values are calculated, it is seen that experimental and computational results agree quite well for Py in THF and H2O, for PyOH in THF and ACN, and for PyCN in nonpolar solvent. This may be attributed to the fact that PyNH2 and PyCN are affected more by the solvent due to the geometry change of PyNH2 in excited state and presence of electron withdrawing CN group on PyCN. Total electron density based on electrostatic potential surface for ground and excited state geometries of the studied molecules are obtained in gas phase and in solvents (Fig. S8). Molecular Electrostatic Potential (MEP) is used to investigate the charge distribution. Especially, intramolecular charge transfer can be determined by looking at the excited state charge distribution. The molecular electrostatic potential surface color scheme depends on the electron density: red, high electron density, partial negative charge; blue, highly electron deficient, partial positive charge; light blue, slightly electron deficient; yellow, low electron density; green, neutral. Gross orbital population based on Mulliken charges is used to obtain the electron density. It was observed that electrostatic potentials of molecules were changed significantly in medium and polar media. Total electron density colors representing the maximum and minimum values for the studied molecules are shown in Fig. S9. The charge density is localized on the electron withdrawing eCN (red) substituent in PyCN. On the contrary, the charges is mostly on the Py ring in molecules with electron donor groups like PyOH and PyNH2. Generally, the charge distribution increases with substituent change (Py < PyNH2
excited states. The comparison was based on computational geometries as well as computational and experimental spectroscopic data. The most significant geometry change was observed for PyNH2 in excited state compared to ground state structure. The tetrahedral structure of NH2 substituent in ground state changed to a planar structure in excited state. This geometry change may cause differences in the shapes and wavelengths observed in the spectra measured in different solvents. It is highly possible that the change in excited state geometry affects the nature of the electron withdrawing nitrogen atom and changes the reactivity of the pyrene ring. Solvent polarity did not affect properties of Py and PyCN. Although PyNH2 has the smallest HOMO-LUMO energy difference, it also has the shortest fluorescence lifetime. The parent compound Py was not affected from the solvent and has a long fluorescence lifetime which classifies it as one of the best candidates for photosensitive material studies. Our results show that substituents at 1-position significantly alter the electronic nature of the Py ring and such modifications can be used in molecular design. Current results indicate that investigated molecules in the order of PyNH2
€ Please cite this article in press as: H. Orücü, N. Acar, Pyrene and its selected 1-substituted derivatives revisited: A combined spectroscopic and computational investigation, Journal of Molecular Structure (2018), https://doi.org/10.1016/j.molstruc.2018.03.098

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€ Please cite this article in press as: H. Orücü, N. Acar, Pyrene and its selected 1-substituted derivatives revisited: A combined spectroscopic and computational investigation, Journal of Molecular Structure (2018), https://doi.org/10.1016/j.molstruc.2018.03.098