ELECTRONIC ABSORPTION SPECTRA OF PROTONATED ANTHRACENES AND PHENANTHRENES, AND THEIR NEUTRALS IN NEON MATRICES

Deposition of m/z = 179 cations produced from 9,10DHAn into a neon matrix resulted in a number of moderately strong absorption systems in the visible range (Figure 1, trace (a)). Known absorptions of the anthracene cation (An⁺) in the near infrared and UV (Szczepanski et al. 1993), and weak ones of the phenanthrene cation (Ph⁺) in the near IR (Brechignac & Pino 1999; Salama et al. 1994) are also present in the spectrum. The reason why bands of An⁺ appear when H-An⁺ cations are deposited could be the contamination of the precursor sample (9,10DHAn) with An or decomposition of the precursor in the ion source. Of 100 anthracene molecules about 15 contain one ¹³C atom. Parent ions of the latter isotopologues have the same m/z as H-An⁺ ions. Fragmentation of H-An⁺ during growth of the matrix could be another reason for the presence of An⁺. Weak bands of Ph⁺ in the near IR are likely caused by the contamination of 9,10DHAn with 9,10DHPh, though isomerization of H-An⁺ in the ion source resulting in H-Ph⁺ cannot be ruled out.

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In order to test whether the new absorptions seen in Figure 1 (trace (a)) are unique for HAn⁺, mass-selected H-Ph⁺ generated from 9,10DHPh were deposited; the obtained spectrum is shown in trace (c). Comparison of the two spectra reveals some similarity. The strongest bands of H-Ph⁺ are also seen, although weakly, in the spectrum of H-An⁺, indicating that in the H-An⁺ experiment H-Ph⁺ cations were also formed.

To separate absorptions of H-Ph⁺ from those of H-An⁺, the spectrum of the former was subtracted from that of the latter after scaling by an appropriate factor so that the distinct, intense bands of H-Ph⁺ would have the same intensity in both spectra. The result is trace (b). Comparison of traces (a)–(c) indicates that the electronic spectrum of H-Ph⁺ also contains the absorptions of H-An⁺. They can again be removed by subtraction, after scaling the H-An⁺ trace by a factor such that the distinct band at 453 nm has the same intensity in both spectra. The absorption of H-Ph⁺ is the bottom trace of Figure 1.

The "pure" spectrum of H-An⁺ is redrawn in Figure 2. The intensities of the bands decrease after UV irradiation of the matrix, confirming its ionic origin. Three band systems are distinguished. The first one with onset at 493.8 nm shows a long progression built on the ∼200 cm⁻¹ mode. Each band of this system has a sharp long-wavelength shoulder, which is spaced from the maximum of the band by ∼50 cm⁻¹. It is the zero phonon line, often observed in spectra of matrix-isolated species. The strongest band at 453.5 nm forms the second system; the third system is a group of broad absorptions around 410 nm. The base line was not corrected in this region and may contain a broad bump underneath the absorptions, which could be artificial, due to light interference upon probing the matrix. Wavelengths of the band maxima are collected in Table 1.

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Table 1. Observed Band Maxima and Their Assignments of Protonated Anthracenes and Phenanthrenes, and Their Neutrals in a Neon Matrix

λ (nm)	$\tilde \nu$ (cm−1)	$\Delta \tilde \nu$ (cm−1)	Assignment
Protonated anthracenes
493.8	20 251	0	000 (2)1A'← $\tilde{X}\, ^{1}$A' 1H-An+
488.2	20 483	232	ν46
484.0	20 661	410	2ν46
479.4	20 859	608	3ν46
474.2	21 088	837	ν38
466.1	21 455	1204	ν28–31
462.0	21 645	1394	ν20–23
449.8	22 232	1981	ν38 + ν28–31
442.7	22 589	2338	2ν28–31
453.5	22 051	0	000 (2)1B2← $\tilde{X}$ 1A1 9H-An+
442.7	22 589	538	ν20
435.4	22 967	917	ν17
431.4	23 180	1130	ν15
427.4	23 397	1347	ν11
424.5	23 557	1506	ν20 + ν17
420.8	23 764	0	000 (2)1A'← $\tilde{X}$ 1A' 2H-An+
416.5	24 010	246	ν43
413.3	24 195	431	ν41 or ν42
407.2	24 558	794	ν35
Hydro-anthracenyl radical
326.4	30 637	0	000 H-An
320.7	31 182	545
313.2	31 928	1291
305.7	32 712	2075
Protonated phenanthrenes
596.6	16 762	0	000 (1)1A'← $\tilde{X}$ 1A' 9H-Ph+
578.2	17 295	533	ν42
561.4	17 813	1051	2ν42
543.7	18 392	1631	ν12
530.3	18 857	0	000 (2)1A'← $\tilde{X}$ 1A' 9H-Ph+
518.6	19 283	425	ν44
506.1	19 759	902	ν36 or ν37
495.5	20 182	1324	ν22–24
465.9	21 464	2607	2ν22–24
583.3	17 144	0	000 (1)1A'← $\tilde{X}$ 1A' 2H-Ph+
570.1	17 541	397	ν45
564.7	17 709	565	ν41
417.4	23 958	0	000 (2) A'← $\tilde{X}$ 1A' 2H-Ph+
410.5	24 361	403	ν44
404.0	24 752	795	2ν44
397.4	25 164	1206	3ν44
391.3	25 556	1598	4ν44
385.5	25 940	1983	5ν44
532.8	18 769	0	000 (1)1A'← $\tilde{X}$ 1A' 1H-Ph+
518.6	19 283	514	ν42
507.9	19 689	920	ν36
484.5	20 640	0	000 (2)1A'← $\tilde{X}$ 1A' 3H-Ph+
474.6	21 070	431	ν44
472.3	21 173	533	ν42
465.9	21 464	824	2ν44
433.1	23 089	0	000 (2)1A'← $\tilde{X}$ 1A' 4H-Ph+
425.8	23 485	396	ν45
551.8	18 123	Not assigned
444.7	22 487	Not assigned
Hydro-phenanthrenyl radical
375.1	26 660	0	000 9H-Ph

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Several decades ago H-An⁺ was studied in strong acidic solutions (Aalbersberg et al. 1959; Dallinga et al. 1958; Gold & Tye 1952; Reid 1954) and melted salts (Brigodiot & Lebas 1976; Zingg et al. 1984), and its electronic spectrum was reported. The anthracenium ion H-An⁺ exhibits a strong, broad absorption with maximum around 410–430 nm in these environments. Because these spectra were measured in strongly perturbing media, it is meaningless to compare them with the present neon matrix data.

In order to assign the observed band systems of H-An⁺, DFT calculations using the B3LYP functional and the 6–311G(d, p) basis set were carried out for three isomers of H-An⁺ differing in the proton position on the anthracene skeleton. All computations were done using the Gaussian 03 program suite (Frisch et al. 2004). The lowest energy was found for the isomer which has the proton attached at position 9 of anthracene (9H-An⁺) (Figure 3). Two other isomers, 1H-An⁺ and 2H-An⁺, lie 36.4 and 49.3 kJ mol⁻¹ higher. The values are corrected for zero-point vibrational energies. Excitation energies of these three isomers of H-An⁺ were calculated using the TD DFT method at the B3LYP/6–311G(d, p) level of theory and are collected in Table 2. The strongest transition for all considered isomers of H-An⁺ was predicted in the UV and its excitation energy lies within the 3.2–3.6 eV range. Usually, TD DFT overestimates the excitation energy; this can be by as much as 0.5 eV. Taking this into account, the excitation energies of the three isomers are close enough to the onsets of three band systems of H-An⁺. The match is even better if a shift of 0.65 eV is used. Corrected electronic transition energies of the three isomers of H-An⁺ and the corresponding oscillator strengths are displayed in Figure 2.

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Table 2. Excited State Symmetries, Calculated Excitation Energies and Oscillator Strengths for Protonated Anthracenes and Phenanthrenes^a,^b

Protonated Anthracenes			Protonated Phenanthrenes
Exc. St. Symm.	ΔE (eV)	f	Exc. St. Symm.	ΔE (eV)	f
9H-An+, $\tilde{X}$ 1A1, C2v (0)			1H-Ph+, $\tilde{X}$ 1A', Cs (0)
1A1	2.78	0.02	1A'	2.39	0.15
1B2	3.13	0.0002	1A'	2.64	0.02
1B2	3.39	0.5	1A'	3.95	0.2
1B1	4.78	0.0003	1A'	4.33	0.005
1A2	4.83	0	1A''	4.66	0
1H-An+, $\tilde{X}$ 1A', Cs (36.4)			9H-Ph+, $\tilde{X}$ 1A', Cs (0.6)
1A'	1.92	0.05	1A'	2.36	0.06
1A'	3.17	0.3	1A'	2.60	0.09
1A'	3.69	0.006	1A'	3.89	0.002
1A'	4.35	0.04	1A'	4.37	0.06
1A''	4.56	0.0003	1A''	4.56	0.0001
2H-An+, $\tilde{X}$ 1A', Cs (49.3)			3H-Ph+, $\tilde{X}$ 1A', Cs (2.4)
1A'	1.91	0.07	1A'	2.63	0.04
1A'	3.42	0.006	1A'	2.70	0.2
1A'	3.58	0.07	1A'	4.13	0.03
1A'	4.57	0.04	1A'	4.56	0.009
1A''	4.60	0	1A'	4.64	0.06
			4H-Ph+, $\tilde{X}$ 1A', Cs (4.4)
			1A'	2.01	0.03
			1A'	2.96	0.2
			1A'	3.74	0.03
			1A'	4.43	0.2
			1A''	4.49	0.0002
			2H-Ph+, $\tilde{X}$ 1A', Cs (9.0)
			1A'	2.24	0.04
			1A'	3.26	0.2
			1A'	3.80	0.03
			1A''	4.58	0
			1A'	4.63	0.5

Notes. ^aExcitation energies were calculated with TD DFT at the B3LYP/6–311G(d, p) level of theory. ^bRelative ground-state energies (kJ mol⁻¹) with respect to the most stable isomer are given in parentheses.

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The onset of the first band system at 493.8 nm lies close to the calculated transition energy of 1H-An⁺. It agrees also well with recent gas-phase photofragmentation data, where the S₁←S₀ transition of protonated anthracene was observed at 491.43 nm (Alata et al. 2010a). The origin of the second system at 453.5 nm coincides with the calculated excitation energy of 9H-An⁺. The broadband system located around 410 nm has energy close to the predicted transition of isomer 2H-An⁺. Therefore, we tentatively assign each band system of H-An⁺ to its nearest-in-energy transition of different isomers. The assignment of individual vibrational bands that form these systems is given in Table 1. It is based on calculated ground-state frequencies of the species considered. In some cases the assignment is not unambiguous, because several modes have frequencies falling within ± 30 cm⁻¹ of the observed ones.

Besides the visible absorptions of H-An⁺ discussed above, the UV range is also relevant for these species. The 295–385 nm section of the spectrum measured after deposition of H-An⁺ into a neon matrix is depicted in Figure 4 (trace (b)). The strongest bands are the absorptions of anthracene cation (trace (a)) and neutral anthracene (trace (e)). The reason why An⁺ is present in the matrix when H-An⁺ was deposited was discussed in the first paragraph of this section. Neutral anthracene appears in the matrix as a result of the neutralization of An⁺. After stepwise UV irradiation of the matrix new absorptions in the 300–340 nm range appeared and grew in intensity (traces (c) and (d)). They are better seen in the inset of Figure 4 where the bands of An⁺, which remained after UV irradiation, were removed. The system, which has appeared after UV photobleaching of H-An⁺ cations, is of hydro-anthracenyl radical (H-An).

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According to TD DFT calculations, two isomers of neutral H-An, 9H-An and 1H-An, have strong transition in the UV range around 342 and 337 nm respectively (J. Fulara et al. 2010, electronic excitation energies calculated with TD DFT at the B3LYP/6-311G(d, p) level of theory, unpublished). These are close to the observed band system with onset at 326.4 nm. It is difficult to find out from the present experimental data which H-An isomer is responsible for the absorptions in this region. It is also possible that both 9H-An and 1H-An contribute to the spectrum in Figure 4, as the bands have a peculiar intensity distribution and may belong to two overlapping systems.

The electronic absorption spectrum obtained after mass-selective deposition of m/z = 179 cations produced from 9,10DHPh is presented in Figure 1 (trace (c)), and after removing the absorptions of H-An⁺ in trace (d). The spectrum is rich in absorption bands, which probably belong to several electronic systems. Thus, a number of UV irradiation experiments were carried out on a matrix containing H-Ph⁺ ions. These differed in exposure time and wavelength domain of the UV radiation. Spectra recorded after deposition and some irradiation steps are compared in Figure 5. The absorption measured after deposition of the cations is trace (a) and those detected after subsequent UV irradiation are traces (b)–(e). To indicate the changes in intensity of the bands, two spectra measured after different irradiation schemes were subtracted from each other after normalizing them to the same intensity of the specific band (traces (f) and (g)).

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The spectra in Figure 5 reveal several electronic band systems, which behave differently under irradiation conditions; this indicates their different origin. Eight band systems of H-Ph⁺ could be distinguished after deposition in a neon matrix. The spectrum is redrawn in Figure 6 and the absorptions belonging to the same isomer of H-Ph⁺ are marked with a specific color.

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Seven of the eight systems decrease in intensity after prolonged irradiation with a medium-pressure Hg lamp (Figure 5, trace (e)). Only the absorption at 375 nm behaves in an opposite way. Therefore, it belongs to neutral species. Most likely it is the hydro-phenanthrenyl radical (H-Ph) which should appear in the matrix as a result of the neutralization of H-Ph⁺ cations. The most stable neutral fragment (phenanthrene molecule), which could be formed from the H-Ph⁺ ions, can be excluded as a carrier of this band, because it absorbs at shorter wavelengths (Salama et al. 1994) and its systems were also detected in this experiment.

In the past, 9-hydro-phenanthrenyl radical (9H-Ph) was studied in organic solutions. It was produced by hydrogen subtraction from 9,10DHPh (Koizumi et al. 1997) or by hydrogen addition to phenanthrene (Grodkowski et al. 2003; Kawakubo 1980) following pulse radiolysis. The spectrum consists of a broad band with maximum at 395 nm. This is not too far from the observed absorption at 375 nm for the neutral product in the present experiment. Therefore, the absorption is assigned to the 9H-Ph radical.

The other seven systems have ionic origin, because they decrease upon UV irradiation. They belong to different isomers of H-Ph⁺. Protonated phenanthrene has been studied in strong acidic solutions and the spectrum exhibits two broad absorptions with maxima at 410 and 520 nm (Dallinga et al. 1958; Reid 1954). These electronic systems were tentatively assigned to two isomers: 1H-Ph⁺ and 9H-Ph⁺. The spectrum of H-Ph⁺ in solution covers the same spectral range as in Figure 6. Interaction of solvent molecules with dissolved ionic species is much stronger than with neutrals. Therefore, detailed comparison of these two spectra is of no relevance, in contrast to the neutral hydro-phenanthrenyl radical.

In order to deduce which isomers of H-Ph⁺ are responsible for the spectrum in Figure 6 the ground-state energy of the isomers 1H-, 2H-, 3H-, 4H- and 9H-Ph⁺ have been calculated using the DFT method with the B3LYP functional and the 6–311G(d, p) basis set. All considered isomers have similar energies spanning only 9 kJ mol⁻¹ (Table 2). The calculations predict 1H-Ph⁺ to be the most stable isomer, but 9H-Ph⁺ lies only 0.6 kJ mol⁻¹ higher. The highest in energy was found to be 2H-Ph⁺, located 9 kJ mol⁻¹ above 1H-Ph⁺. Because the energy difference of the five isomers is so small, their relative order may change with higher-level calculations.

To assign the spectrum in Figure 6 to a specific isomer of H-Ph⁺, excitation energies are needed. TD DFT calculations were carried out for the five isomers considered above. Computed excitation energies and their oscillator strengths are collected in Table 2 and displayed in Figure 6. All H-Ph⁺ isomers have two allowed electronic transitions in the visible, where the absorptions of protonated phenanthrenes have been detected. We tentatively assign the observed band systems to specific isomers of H-Ph⁺ by comparing the excitation energy of its strongest transition with the onset of the nearest system. Dotted lines indicate which isomer is responsible for a certain band system. Individual bands of a given system are assigned using calculated vibrational frequencies of the H-Ph⁺ isomers in their ground state (Table 1).